Why nullable types?(medium.com)
medium.com
Why nullable types?
https://medium.com/dartlang/why-nullable-types-7dd93c28c87a
118 comments
> You can map, filter, reduce, chain, compose functions that all work with optionals, and lift functions that work on numbers (or any other type) to be functions that work on Optional<number> but none of that is even mentioned in this piece (likely cause then its harder to justify picking nullable types instead)
Maybe I'm missing something, but I don't see how that's special to option types. Here's a set of extension methods in Dart that provide the operations you describe:
Of course, this is not entirely as expressive as option types because of the inability to nest, but the article is pretty clear that nesting is the major advantage of option types.
Maybe I'm missing something, but I don't see how that's special to option types. Here's a set of extension methods in Dart that provide the operations you describe:
extension NullableExtensions<T> on T? {
R? map<R>(R Function(T) transform) => this == null ? null : transform(this);
T? filter(bool Function(T) predicate) {
if (this == null) return null;
if (predicate(this)) return this;
return null;
}
}
extension FunctionExtensions<T, R> on R Function(T) {
R? Function(T?) lift() => (T? param) => param == null ? null : this(param);
}
extension NullableIterableExtensions<T> on Iterable<T?> {
Iterable<R?> map<R>(R Function(T) transform) =>
this.map((e) => e == null ? null : transform(e));
Iterable<T?> filter(bool Function(T) predicate) =>
where((e) => e == null || predicate(e));
}
int? maybeInt = ...
print(maybeInt?.isEven.toString());
Of course, this is not entirely as expressive as option types because of the inability to nest, but the article is pretty clear that nesting is the major advantage of option types.
You missed out the most interesting one, which is 'reduce':
Of course this is all a bit easier to read if we use some syntax specially built for Monads such as Haskell's do syntax:
extension NullableIteratorExtensions<T> on Iterator<T?> {
T? reduce<T>(T? Function(T,T) f, T? x0)
=> this.moveNext()
? (this.reduce(f, (x0 == null || this.current == null) ? null : f(x0,current)))
: x0;
}
Of course this is all a bit easier to read if we use some syntax specially built for Monads such as Haskell's do syntax:
reduce f (head:tail) x0 = do { x <- x0
; y <- head
; reduce f tail (f x y) }
reduce f [] x0 = x0
> I'm not sure what you mean by "compose".
The simplest illustration is the ability to have multiple levels of semantically meaningful optionality:
Option<Option<int>> is a meaningful type, and None, Some(None), and Some(Some(1)) are all meaningfully-distinct values. int? doesn't compose to int?? and has no good way to differentiate what Option<Option<int>> represents as None and Some(None).
The simplest illustration is the ability to have multiple levels of semantically meaningful optionality:
Option<Option<int>> is a meaningful type, and None, Some(None), and Some(Some(1)) are all meaningfully-distinct values. int? doesn't compose to int?? and has no good way to differentiate what Option<Option<int>> represents as None and Some(None).
Ironically the main advantage of Monads is that Option<Option<int>> has a natural transformation to Option<int>.
The reason Option<Option<int>> isn't a problem is because it's a monad, so people will naturally transform it into Option<int> while writing their programs.
There is no irony there. If it wasn't a monad, the GGP would be incorrect, and ad-hock solutions would be very valuable.
There is no irony there. If it wasn't a monad, the GGP would be incorrect, and ad-hock solutions would be very valuable.
In a language with Haskell's guarantees, you can mechanically get from the standard "bind" function to prove that "m (m a)" can be converted to "m a" [1]. That function is called "join", and is a lesser-well known way to write monad implementations which is equivalent to the more famous bind function.
(IMHO, join is a better way to understand the typeclass intuitively. The standard bind is better to program with in general use, but much harder to grok.)
So, in Haskell, it's more than just "the APIs would tend to encourage not nesting the types"... having a valid monad implementation means they really are equivalent.
[1]: https://stackoverflow.com/questions/3382210/monad-join-funct...
(IMHO, join is a better way to understand the typeclass intuitively. The standard bind is better to program with in general use, but much harder to grok.)
So, in Haskell, it's more than just "the APIs would tend to encourage not nesting the types"... having a valid monad implementation means they really are equivalent.
[1]: https://stackoverflow.com/questions/3382210/monad-join-funct...
They are not equivalent. Join can throw away information - and certainly does for (Maybe (Maybe a) -> Maybe a) and ([[a]] -> [a]).
They would be equivalent if there was an isomorphism, but of course there isn't.
They would be equivalent if there was an isomorphism, but of course there isn't.
By compose I quite literally mean the compose function.
Nowhere in any of the functions I wrote above do I as a user have to worry about handling the case where no number exists, that's handled by the option type and signified by the type (The value in the option may or may not exist) and I can write my functions without ever having to worry about handling that case
const add3 = val => val + 3;
const multiplyBy10 = val => val * 10;
const subtract5 = val => val - 5;
const doABunchOfMath = compose(add3, subtract5, multiplyBy10);
const ninetyEight = doABunchOfMath(10);
const optionallyNinetyEight = Some(10).map(doABunchOfMath)
Nowhere in any of the functions I wrote above do I as a user have to worry about handling the case where no number exists, that's handled by the option type and signified by the type (The value in the option may or may not exist) and I can write my functions without ever having to worry about handling that case
Compose can also be implemented easily:
In effect, int? is almost nearly Option<int>, except you cannot represent Option<Option<int>> with Dart's nullable syntax.
Check out https://nullsafety.dartpad.dev/ to play around with the possibilities!
extension NullableExtensions<T> on T? {
R? map<R>(R Function(T) transform) => this == null ? null : transform(this!);
}
V Function(V) compose<V>(Iterable<V Function(V)> functions) =>
functions.reduce((composedFunction, function) {
return (V value) => composedFunction(function(value));
});
num add3(num val) => val + 3;
num multiplyBy10(num val) => val * 10;
num subtract5(num val) => val - 5;
final doABunchOfMath = compose([add3, subtract5, multiplyBy10]);
final optionallyDoABunchOfMath = (num? value) => value.map(doABunchOfMath);
doABunchOfMath(10); // 98
optionallyDoABunchOfMath(10); // 98
optionallyDoABunchOfMath(null); // null
In effect, int? is almost nearly Option<int>, except you cannot represent Option<Option<int>> with Dart's nullable syntax.
Check out https://nullsafety.dartpad.dev/ to play around with the possibilities!
>By compose I quite literally mean the compose function.
Outside Lisp/Scheme has anybody used those for anything other than examples in FP tutorials?
Every case I ever seen is non-production tutorial thing like "add3".
Outside Lisp/Scheme has anybody used those for anything other than examples in FP tutorials?
Every case I ever seen is non-production tutorial thing like "add3".
I do on a regular basis (front end dev). For an example of how, I found this article[1] to be a nice demonstration of how it can be useful at a basic level for ReactJS. Than you just begin applying the same concepts one level higher for your use case and keep going.
Even if you don't use compose directly, the mindset of working to build systems in a composable manner is invaluable.
Its a mental shift that leads you to thinking how a system can be represented as a series of inputs being piped from one function to the next between boundries of my code (server response data --> client side caller --> transformation functions --> local data store --> UI).
Achieving these pipelines requires expressing intent with functions and with a self imposed constraint of writing pure functions, you begin needing functors and applicatives and monads to help store stateful information.
If at any point you need a new capability or need to add a new code path, you just modify the relevant pipeline(s), write any new (pure) functions along with any adapters you might need to inject it in your pipeline, and you're good to go. If everything is a pure function, testing and debugging instantly become easier. All this from wanting to build composable functions.
Some languages also support piping (runs the functions in the reverse order that compose does) which can help visually since functions are invoked in left to right order which is how we read too.
[1] https://hackernoon.com/forms-of-composition-in-javascript-an...
Even if you don't use compose directly, the mindset of working to build systems in a composable manner is invaluable.
Its a mental shift that leads you to thinking how a system can be represented as a series of inputs being piped from one function to the next between boundries of my code (server response data --> client side caller --> transformation functions --> local data store --> UI).
Achieving these pipelines requires expressing intent with functions and with a self imposed constraint of writing pure functions, you begin needing functors and applicatives and monads to help store stateful information.
If at any point you need a new capability or need to add a new code path, you just modify the relevant pipeline(s), write any new (pure) functions along with any adapters you might need to inject it in your pipeline, and you're good to go. If everything is a pure function, testing and debugging instantly become easier. All this from wanting to build composable functions.
Some languages also support piping (runs the functions in the reverse order that compose does) which can help visually since functions are invoked in left to right order which is how we read too.
[1] https://hackernoon.com/forms-of-composition-in-javascript-an...
Threading syntax is a giant compose. OK, but that's an operator.
In the TXR internals, I have a C function called chain, which composes N functions together (left to right, not right to left like typical compose functions). It is variadic: the end of the arguments is signaled by nao: a not-an-object constant:
In the TXR internals, I have a C function called chain, which composes N functions together (left to right, not right to left like typical compose functions). It is variadic: the end of the arguments is signaled by nao: a not-an-object constant:
git grep '\<chain('
eval.c: chain(car_f, eq_to_list_f, nao),
eval.c: chain(cdr_f, consp_f, nao),
eval.c: chain(cdr_f, cdr_f, null_f, nao),
eval.c: chain(car_f, eq_to_quote_f, nao),
eval.c: chain(car_f, eq_to_list_f, nao),
eval.c: chain(cdr_f, consp_f, nao),
eval.c: chain(cdr_f, cdr_f, null_f, nao),
eval.c: chain(cdr_f, car_f, consp_f, nao),
eval.c: chain(cdr_f, car_f, car_f, eq_to_quote_f, nao),
eval.c: chain(cdr_f, car_f, cdr_f,
eval.c: return chain(juxt_fun, apf_fun, nao);
eval.c: iter_from_binding_f = chain(cdr_f, iter_begin_f, nao);
lib.c: val pred_key = chain(default_arg(key, identity_f), pred, nao);
lib.c: val pred_key = chain(default_arg(key, identity_f), pred, null_f, nao);
lib.c:val chain(val first_fun, ...)
lib.c: chain(car_f, keyfun_in, nao));
lib.h:val chain(val first_fun, ...);
match.c: chain(func_n1(cdr),
match.c: chain(func_n1(cdr),
match.c: chain(func_n1(length_list),
match.c: chain(func_n1(rest),
Adding an `Option` type to an existing language doesnt really solve many problems unless you can also somehow remove it's existing support for `null.
Java added `Optional` and while it makes interacting with newer API's clearer, there is nothing preventing you from passing a null Optional.
I think if you are designing a language from scratch, you should avoid null, but nullable types are a good feature for existing languages.
Having written a fair bit of kotlin, I have found the ergonomics of it's nullable type system to be really nice.
Java added `Optional` and while it makes interacting with newer API's clearer, there is nothing preventing you from passing a null Optional.
I think if you are designing a language from scratch, you should avoid null, but nullable types are a good feature for existing languages.
Having written a fair bit of kotlin, I have found the ergonomics of it's nullable type system to be really nice.
Language culture matters. Scala has both `Option` and `null`, but there's a strong culture of "`null` is only for Java interop", so you very rarely see a `null` in the wild from libraries, or wonder whether it'd be valid to pass it in.
Interestingly, Scala 3 is adding union types as well as explicit `null`s:
https://dotty.epfl.ch/docs/reference/other-new-features/expl...
I think that the expectation is that idiomatic Scala code will mostly still use `Option`, while `null` will be used for interoperability with Java, JavaScript or Scala libraries that happen to use `null`.
In any case, I am very much looking forward to these additions.
https://dotty.epfl.ch/docs/reference/other-new-features/expl...
I think that the expectation is that idiomatic Scala code will mostly still use `Option`, while `null` will be used for interoperability with Java, JavaScript or Scala libraries that happen to use `null`.
In any case, I am very much looking forward to these additions.
Your username is so incredibly relevant here that I had to check if this was a novelty account.
I generally agree with you but the one point they make in this article that really matters for Dart is it does not have pattern matching. Dart’s nominative type system makes much of standard FP much harder than you’d imagine. I know Rust is nominal too but Rust has a lot of complexity to support these things.
It is very simple to add a Maybe and Either type to Dart (my code base has variants of these to deal with null issues) but imperative destructing is a pain. You can get most of the benefits of those techniques in user land without the language knowing about it, but I agree it still misses out on some stuff.
If Dart already had pattern matching or was willing to add it first then the ADT version could have been chosen, but without that prerequisite nullable types was the only ergonomic choice.
It is very simple to add a Maybe and Either type to Dart (my code base has variants of these to deal with null issues) but imperative destructing is a pain. You can get most of the benefits of those techniques in user land without the language knowing about it, but I agree it still misses out on some stuff.
If Dart already had pattern matching or was willing to add it first then the ADT version could have been chosen, but without that prerequisite nullable types was the only ergonomic choice.
I don't buy the pattern matching argument. I've been using Vavr's Option type in Java a lot lately (having banned the use of null from my code), and I don't have any issues with it. I avoid Vavr's pattern matching because I find the syntax to be clunky (not a criticism of the Vavr developers; I think what they've done is impressive within Java's syntax limitations). I do miss pattern matching in general, but I don't find using Option to be any more difficult without it. And no, I never use Option.get(); I have an ArchUnit test set up that will fail the build if anyone tries.
On the flip side, I do find Option a pain to use in Rust sometimes, which does have pattern matching. I think that's a function of the borrow checker making some things harder to express. So perhaps it's not Dart's missing pattern matching that would make Option hard to use there, but some other feature (or missing feature) of the language or type system?
On the flip side, I do find Option a pain to use in Rust sometimes, which does have pattern matching. I think that's a function of the borrow checker making some things harder to express. So perhaps it's not Dart's missing pattern matching that would make Option hard to use there, but some other feature (or missing feature) of the language or type system?
That's an interesting counter point, though another comment in a different post for this same article[1] pointed out that Dart is considering adding pattern matching too.
Though I have trouble understanding how a nominal type system affects this? Does dart not have generics? That's all you need to support these patterns? I'm having trouble understanding how the type of type system plays a role here.
Also, what's 'imperative destructing'? Haven't heard that term before, is that just pulling out a property from an object like this from JS?
Though I have trouble understanding how a nominal type system affects this? Does dart not have generics? That's all you need to support these patterns? I'm having trouble understanding how the type of type system plays a role here.
Also, what's 'imperative destructing'? Haven't heard that term before, is that just pulling out a property from an object like this from JS?
const { foo } = objectWithPropertyFoo;
Yes Dart has generics which is how you would implement optional types yourself. The type of type system doesn't play a role here you are right. I was thinking of the really good pattern matching I have experienced usually being present with structural subtyping.
Dart is considering adding pattern matching (and real tuples!) and I think they will eventually get there. I just think that would have had to come first to make the case of optionals over nullable types more compelling.
I would call that example destructing in the normal sense and it is basically a form of pattern matching. To do that in an imperative language without pattern matching (i.e. what I am calling imperative destructuring), you end up with code like
Dart is considering adding pattern matching (and real tuples!) and I think they will eventually get there. I just think that would have had to come first to make the case of optionals over nullable types more compelling.
I would call that example destructing in the normal sense and it is basically a form of pattern matching. To do that in an imperative language without pattern matching (i.e. what I am calling imperative destructuring), you end up with code like
if (maybeFoo.isSome) {
final foo = maybeFoo.asValue;
...
}
if let Some(foo) = maybeFoo {...}You may prefer
for foo in maybeFoo { ... }
maybeFoo.forEach((foo) { ... })Dart is looking at adding pattern matching and "record" types: https://github.com/dart-lang/language/tree/master/working/05...
If you are starting from scratch and designing a brand new language then ADT's are a clearly better choice than allowing null. However Dart is not a brand new language. It's a preexisting language that already has null. Given that context nullable types are the only option that doesn't break compatibility with previous languages and allows code to be made null safe. If you already have to have null values because of prior decisions then ADT's don't add null safety. There may be other reasons to want them that aren't about handling null safety. But you'll still have the problem that values can be null and you won't have given developers a tool to handle those potentially null values safely.
This comment is answering post’s title more honestly than the author.
Less words too.
Less words too.
> I also found the Some(Some(3)) example just plain wrong. In those scenarios, typically you just use chain (aka flatMap) instead of map
I think you misunderstood. The point here was that you can represent different things using nested Options:
1. Some(Some(3)) = I checked it and the value is 3
2. Some(None) = I checked it and there is no value
3. None = I haven't checked it
It's very useful when building a cache, and also for things like JSON parsing (property was not present, property present but null, property present and has non-null value).
You just can't represent that using Dart's nullable types system, because "int?" and "int??" are the same, indistinguishable type.
Anyway...
One thing not mentioned in the article is that the way they've implemented it in dart is via "magic syntax", which I consider to be a big negative. Option<T> is something anyone could implement without language support (assuming the type system already supports it). Anyone could build their own Option type and use it. (Sure, it's best to have this sort of thing in the stdlib so the stdlib will use it itself.) Nullable types with the "?" syntax makes the language itself bigger, and is something a regular user of the language couldn't implement if they wanted to.
(Yes, I'm aware that the "?" suffix is just a shortcut for the more generalized union type syntax, but I think my argument holds.)
The use of these types in regular code also requires what I consider a sort of special unintuitive requirement on how you structure your code. I have to do explicit null checks and then the compiler/interpreter just "knows" that code in certain places is null-safe through flow analysis. That kind of magic really turns me off.
I also don't buy the argument that Option types are a pain to use if your language doesn't have pattern matching. I've been doing a bit of Java lately and have banned null from my code, instead using Option from the excellent Vavr library (I find java.util.Optional to be lacking, ditto for Java's built-in collections library). I tend not to use Vavr's pattern matching, as I find it clunky. But I also have no trouble using Vavr's Option.
I guess I just prefer to use languages that have strong type systems where I can express constraints and safety through the types themselves, and not have to rely on language/compiler features to implement those constraints. I think the latter also gives you less flexibility as well.
I think you misunderstood. The point here was that you can represent different things using nested Options:
1. Some(Some(3)) = I checked it and the value is 3
2. Some(None) = I checked it and there is no value
3. None = I haven't checked it
It's very useful when building a cache, and also for things like JSON parsing (property was not present, property present but null, property present and has non-null value).
You just can't represent that using Dart's nullable types system, because "int?" and "int??" are the same, indistinguishable type.
Anyway...
One thing not mentioned in the article is that the way they've implemented it in dart is via "magic syntax", which I consider to be a big negative. Option<T> is something anyone could implement without language support (assuming the type system already supports it). Anyone could build their own Option type and use it. (Sure, it's best to have this sort of thing in the stdlib so the stdlib will use it itself.) Nullable types with the "?" syntax makes the language itself bigger, and is something a regular user of the language couldn't implement if they wanted to.
(Yes, I'm aware that the "?" suffix is just a shortcut for the more generalized union type syntax, but I think my argument holds.)
The use of these types in regular code also requires what I consider a sort of special unintuitive requirement on how you structure your code. I have to do explicit null checks and then the compiler/interpreter just "knows" that code in certain places is null-safe through flow analysis. That kind of magic really turns me off.
I also don't buy the argument that Option types are a pain to use if your language doesn't have pattern matching. I've been doing a bit of Java lately and have banned null from my code, instead using Option from the excellent Vavr library (I find java.util.Optional to be lacking, ditto for Java's built-in collections library). I tend not to use Vavr's pattern matching, as I find it clunky. But I also have no trouble using Vavr's Option.
I guess I just prefer to use languages that have strong type systems where I can express constraints and safety through the types themselves, and not have to rely on language/compiler features to implement those constraints. I think the latter also gives you less flexibility as well.
> I have to do explicit null checks and then the compiler/interpreter just "knows" that code in certain places is null-safe through flow analysis.
You need to do it anyway or you got a crash. Because remove null from a existing language completely is nearly impossible.
And now the compiler is able to know something went wrong in your code instead of happily see you use `null` as a object and crash the whole program.
Besides that, that also benefit the library users that the compiler/ide will tell them `you can't pass null here`
You need to do it anyway or you got a crash. Because remove null from a existing language completely is nearly impossible.
And now the compiler is able to know something went wrong in your code instead of happily see you use `null` as a object and crash the whole program.
Besides that, that also benefit the library users that the compiler/ide will tell them `you can't pass null here`
> I also found the Some(Some(3)) example just plain wrong.
I don't see any such example in the article, and searching for that string yields nothing. What part are you talking about?
I don't see any such example in the article, and searching for that string yields nothing. What part are you talking about?
It was the first bullet point in one of the bullet point sections. The text from the article was
A Some(Some(string)) means the resource did exist on the server, and we have it in the cache now.
> You can map, filter, reduce, chain, compose functions that all work with optionals
Maybe they didn't mention it because both approaches can do that, or maybe I'm misunderstanding?
Maybe they didn't mention it because both approaches can do that, or maybe I'm misunderstanding?
The difference I see is, when you work with a nullable type, each function has to do the check
so
so
const add3 = (val) => val ? val + 3 : null;
const multiplyBy10 = (val) => val ? val * 10 : null
const add3 = val => val + 3;
const multiplyBy10 = val => val * 10;
You could define operations on nullable types to allow this in Dart, like:
extension NullableNumExtensions on num? {
num? operator +(num? other) {
if (this != null && other != null) return this + other;
return null;
}
}
main() {
num? i, j;
print(i + j);
}
int? i;
var b = i?.isEven; // b has type null?
That is where you use optional chaining + extension method. And that is basically identical to Optional.map or whatever method in those language with Optional type
in kotlin
Unfortunately, there is no such method in js currently, so it looks half baked.
in kotlin
func add3 (val: Int?) = val ?.let { it + 3 }
Unfortunately, there is no such method in js currently, so it looks half baked.
Ah, I gotcha, and I had already read another of your comments below. Yep, agree that is nicer in these cases. I still agree with the author that they made the right choice for Dart.
>There are two main solutions: Use an option or maybe type or Use a nullable type
I don't get it. These are literally all exactly the same thing, all slightly varying in ergonomics and compiler support. They may differ slightly but to call them two different categories of solution is just creating a false dichotomy for yourself.
> However, the type system is a little more flexible than with option types. The type system understands that a union type is a supertype of its branches. In other words, int is a subtype of int?. That means we can pass a definitely-present-integer to something that expects a maybe-present-integer since that’s safe to do.
You can do that in Swift, despite being placed as an example of "Solution 1".
> The short answer is that, yes, it is entirely possible to live without null, and languages like Rust do.
If you can call say that Swift has "null", then you can equally say that Rust has "null"--it's called "None". The only way "None" and "null" from other languages differ is in built-in compiler support.
I don't get it. These are literally all exactly the same thing, all slightly varying in ergonomics and compiler support. They may differ slightly but to call them two different categories of solution is just creating a false dichotomy for yourself.
> However, the type system is a little more flexible than with option types. The type system understands that a union type is a supertype of its branches. In other words, int is a subtype of int?. That means we can pass a definitely-present-integer to something that expects a maybe-present-integer since that’s safe to do.
You can do that in Swift, despite being placed as an example of "Solution 1".
> The short answer is that, yes, it is entirely possible to live without null, and languages like Rust do.
If you can call say that Swift has "null", then you can equally say that Rust has "null"--it's called "None". The only way "None" and "null" from other languages differ is in built-in compiler support.
They're not the same. The difference is that what the article calls "nullable types" are based on commutative unions, while option types are based on non-commutative unions.
In haskell, Either a (Either b c) is distinct from Either (Either a b) c. In typescript, a | (b | c) is identical to (a | b) | c. And further, x | x is identical to x in typescript.
The upshot is that Haskell (using Either instead of Maybe for clarity) lets you have Either (Either a ()) () as distinct from Either a (), but the equivalent typescript is (a | null) | null, which is equivalent to a | (null | null) which is further equivalent to a | null.
So nullable types are less powerful than option types. I'm not certain that they're worse, because dealing with nested option types can get confusing, and maybe making them impossible to express would encourage API design that requires less thinking, but unfortunately I think it's more likely that it would lead to API design with more footguns.
In haskell, Either a (Either b c) is distinct from Either (Either a b) c. In typescript, a | (b | c) is identical to (a | b) | c. And further, x | x is identical to x in typescript.
The upshot is that Haskell (using Either instead of Maybe for clarity) lets you have Either (Either a ()) () as distinct from Either a (), but the equivalent typescript is (a | null) | null, which is equivalent to a | (null | null) which is further equivalent to a | null.
So nullable types are less powerful than option types. I'm not certain that they're worse, because dealing with nested option types can get confusing, and maybe making them impossible to express would encourage API design that requires less thinking, but unfortunately I think it's more likely that it would lead to API design with more footguns.
> So nullable types are less powerful than option types.
They are differently powerful. Option types nest, while nullable types flatten. But nullable types subtype while option types do not.
Otherwise, I think your comment is an excellent summary of the differences.
They are differently powerful. Option types nest, while nullable types flatten. But nullable types subtype while option types do not.
Otherwise, I think your comment is an excellent summary of the differences.
They touch on why they're different a bit, but it's kinda subtle.
The author might be wrong about Swift being an example of Solution 1 (looking at Swift's nullability syntax).
But the point is that nullable types provide a bit more power because they're not just a data-structure, but a language feature. If you look at the source of `Option` in Rust, it's an enum like `Result` or (kinda) anything you could write rather than a language feature. Rust has built-in some things like the `?` "try" operator for special enums like `Option` and `Result` to unwrap them (or error), but it isn't quite the same as the null-guards in Dart/Kotlin and you still need to wrap them for functions like `Some(myVar)` rather than being able to pass `myVar` directly for an `Option<T>` parameter.
Again, they are almost the same, but they do talk about reasons for choosing Solution 2 over Solution 1. In Rust, you pattern-match like `match myVar { Some => x(); None => y() }` which is very functional, but they wanted something that felt like traditional null checks with conditionals. They note that nullable types are basically erased at runtime - an `int?` at runtime is either something like `7` or `null`. At compile time, you've checked that you're never assigning a `null` to an `int`, but the runtime doesn't need to know anything about it because the compiler has checked everything. Rust's enums aren't just a single special case, but something that you could make more complicated. Maybe you want a `SchrodingersResult` which could have `Success`, `Err`, or `Huh`. Ultimately, something like `Option` is an algebraic data type that you can compute off of - ex. pattern-match. The runtime needs to know that it's an `Option` because you can write code for that.
It is almost the same. The question is whether you create a one-off language feature for nullability and get advantages like knowing that any `int` can be assigned to an `int?` without needing to wrap it or whether you decide to create a type that works like any other type in your system like Java's `Optional`. Both are reasonable ways to go, but they have subtle differences and the article outlines a lot of those differences.
foo(int? i) {
if (i != null) {
print(i + 1);
}
}
The author might be wrong about Swift being an example of Solution 1 (looking at Swift's nullability syntax).
But the point is that nullable types provide a bit more power because they're not just a data-structure, but a language feature. If you look at the source of `Option` in Rust, it's an enum like `Result` or (kinda) anything you could write rather than a language feature. Rust has built-in some things like the `?` "try" operator for special enums like `Option` and `Result` to unwrap them (or error), but it isn't quite the same as the null-guards in Dart/Kotlin and you still need to wrap them for functions like `Some(myVar)` rather than being able to pass `myVar` directly for an `Option<T>` parameter.
Again, they are almost the same, but they do talk about reasons for choosing Solution 2 over Solution 1. In Rust, you pattern-match like `match myVar { Some => x(); None => y() }` which is very functional, but they wanted something that felt like traditional null checks with conditionals. They note that nullable types are basically erased at runtime - an `int?` at runtime is either something like `7` or `null`. At compile time, you've checked that you're never assigning a `null` to an `int`, but the runtime doesn't need to know anything about it because the compiler has checked everything. Rust's enums aren't just a single special case, but something that you could make more complicated. Maybe you want a `SchrodingersResult` which could have `Success`, `Err`, or `Huh`. Ultimately, something like `Option` is an algebraic data type that you can compute off of - ex. pattern-match. The runtime needs to know that it's an `Option` because you can write code for that.
It is almost the same. The question is whether you create a one-off language feature for nullability and get advantages like knowing that any `int` can be assigned to an `int?` without needing to wrap it or whether you decide to create a type that works like any other type in your system like Java's `Optional`. Both are reasonable ways to go, but they have subtle differences and the article outlines a lot of those differences.
No, this is valid in languages with nullable types:
String s = null;
But this is not legal in languages without them:
String s = None;
In those languages, in order to have a value of None, it must be of type Option<String> (or whatever the syntax is), and in order to get a value of type String, you must assert its presence.
This is a fundamental difference with many ramifications. It really isn't just "the same but with better compile support".
String s = null;
But this is not legal in languages without them:
String s = None;
In those languages, in order to have a value of None, it must be of type Option<String> (or whatever the syntax is), and in order to get a value of type String, you must assert its presence.
This is a fundamental difference with many ramifications. It really isn't just "the same but with better compile support".
String s = null;
String? s = null;
Option<String> s = None;
That is legal in languages with poor null support. That statement would not be valid in Kotlin for example.
String s = null;
> No, this is valid in languages with nullable types:
> String s = null;
Only in languages with default-nullable types, (where the non-nullable form, if it exists at all, would be something like “String s!”); in languages with explicity billable types, the above is not allowed, because you would need to explictly opt-in to nullability:
> String s = null;
Only in languages with default-nullable types, (where the non-nullable form, if it exists at all, would be something like “String s!”); in languages with explicity billable types, the above is not allowed, because you would need to explictly opt-in to nullability:
String s? = nullThis comment is misleading. They may appear to be the same but they are not because in certain languages (like Rust) you are forced to handle the `Option` case when a value is `None` which guides a programmer's thinking in the direction of what to do in that situation.
Without these higher level types runtime null pointer exceptions are very common, using `Option` or `Maybe` creates a situation where these sorts of errors are a lot less likely.
Without these higher level types runtime null pointer exceptions are very common, using `Option` or `Maybe` creates a situation where these sorts of errors are a lot less likely.
> They may appear to be the same but they are not because in certain languages (like Rust) you are forced to handle the `Option` case when a value is `None` which guides a programmer's thinking in the direction of what to do in that situation.
Languages with nullable types are equally strict:
Languages with nullable types are equally strict:
int? maybeInt;
maybeInt + 3; // <-- Compile error.They're not the same, and the biggest difference is explained in the Map<int, Option<String>> example: union types merge different uses of null that you usually don't want merged.
> I don't get it. These are literally all exactly the same thing, all slightly varying in ergonomics and compiler support. They may differ slightly but to call them two different categories of solution is just creating a false dichotomy for yourself.
Option types can be nested, nullable types cannot.
Option types can be nested, nullable types cannot.
That's why I laugh when I see that a language "doesn't have null."
It's more like newer languages make it hard to accidentally have a null reference error.
Nullable versus Option just seems like a semantics argument. The discussion makes sense when designing a language, but when choosing a language, it's more important to just look for "compiler makes it possible to enforce that a value isn't null."
BTW:
In Rust, calling unwrap() on an Option can panic. It's just that the compiler will prevent you from passing None to a function that expects a value.
C# has nullable scalars: (int? char? float?, ect.) They compile to a Nullable<T> struct, so you can do a lot of generalizing like you can with Option<T>.
It's more like newer languages make it hard to accidentally have a null reference error.
Nullable versus Option just seems like a semantics argument. The discussion makes sense when designing a language, but when choosing a language, it's more important to just look for "compiler makes it possible to enforce that a value isn't null."
BTW:
In Rust, calling unwrap() on an Option can panic. It's just that the compiler will prevent you from passing None to a function that expects a value.
C# has nullable scalars: (int? char? float?, ect.) They compile to a Nullable<T> struct, so you can do a lot of generalizing like you can with Option<T>.
Back when I was really into FP, but didn't use functional languages, I decided that the humble list was the answer to all my problems. Operations over lists behave the same regardless of their length, and a 0 element list is not a special case. Say you want to send mail to someone's mailing addresses:
Of course, it's possible that having two addresses is just as annoying as having 0 addresses, in which case lists don't help you. Someone will add a second address, and then you won't know where to walk to to go see them.
I'll also point out that it's the same number of lines of code as just handling null:
addresses = user.addresses()
for address in addresses {
send_mail(address)
}
Of course, it's possible that having two addresses is just as annoying as having 0 addresses, in which case lists don't help you. Someone will add a second address, and then you won't know where to walk to to go see them.
I'll also point out that it's the same number of lines of code as just handling null:
address = user.address()
if address != null {
send_mail(address)
}
In many cases you'd want your program to blow up in some form when trying to send mail to someone without an email address, because that would be better than silently assuming they received a message you never ended up even trying to send.
While this is true, this kind of flexibility makes the other layers harder.
On the database side, if you use a relational store, it forces you to normal form 5 which few people are used to.
On the front end, allowing everywhere an arbitrary number elements instead of mandatory one, means you need custom widgets everywhere. You need a way of marking the default one. Validation also gets exponentially more complicated.
On the UX side, people are simply not used to this level of flexibility. It will get labeled as impenetrable.
As much as I like the idea, and believe it is elegant, most probably YAGNI.
On the database side, if you use a relational store, it forces you to normal form 5 which few people are used to.
On the front end, allowing everywhere an arbitrary number elements instead of mandatory one, means you need custom widgets everywhere. You need a way of marking the default one. Validation also gets exponentially more complicated.
On the UX side, people are simply not used to this level of flexibility. It will get labeled as impenetrable.
As much as I like the idea, and believe it is elegant, most probably YAGNI.
I know this is just a throwaway example, but feel just looking at the code layer this kind of elegance is comes at the cost of clarity and conveying intent, and I see that happen a lot.
The second method explicitly checking for null is great because it's telling me so many things very plainly:
1. There can be no address for a user 2. There is otherwise one address per one user 3. If there is no address we explicitly do nothing (if clause with no else)
If we do the same analysis for the first method it actually feels like it's conveying false conclusions. It's implying there's multiple addresses per user, for example.
And the fact that if a user has no address we don't do anything is still true, but the intent feels buried in the behavior of the loop.
By default I try to value writing code in a way that conveys the most intent possible over how it feels to write
For example, another language with nullable types (Kotlin) has
The second method explicitly checking for null is great because it's telling me so many things very plainly:
1. There can be no address for a user 2. There is otherwise one address per one user 3. If there is no address we explicitly do nothing (if clause with no else)
If we do the same analysis for the first method it actually feels like it's conveying false conclusions. It's implying there's multiple addresses per user, for example.
And the fact that if a user has no address we don't do anything is still true, but the intent feels buried in the behavior of the loop.
By default I try to value writing code in a way that conveys the most intent possible over how it feels to write
For example, another language with nullable types (Kotlin) has
user.address()?.let { it.sendMail() }
You may have accidentally reinvented Icon:
https://en.wikipedia.org/wiki/Icon_(programming_language)#Go...
https://en.wikipedia.org/wiki/Icon_(programming_language)#Go...
Me and my team prefer Option to nullable types. Option is much more clear and idiomatic FP than nullable types.
Granted, Option.map and flatMap and nullable type ?.let{} are functionally equivalent but the former just reads so much better, and we want not just functor and monad but applicative as well, no?
Eg given a List<Option>, we can use applicative to declaratively and with referential transparency etc etc (you know the usual FP sales pitch) turn it into an empty list if it contains a single non-populated option, or a list of Foo if all options in the list are populated.
With nullable types I don't think you could do it as declaratively and elegantly.
Actually, even if you could, it's kind of beside the point...
To me, the point is, Option has been designed from scratch to be an FP style Maybe type with all that comes with that in terms of it being a functor, applicative functor, monad etc etc whereas while nullable types in some ways are functionally equivalent, a nullable type doesn't implement a Functor interface, or a Monad interface, or an Applicative interface, and when it behaves like it does, it's mostly kind of by accident driven by a pragmatic need, not any real understanding of reusable, lawful FP abstractions.
Granted, Option.map and flatMap and nullable type ?.let{} are functionally equivalent but the former just reads so much better, and we want not just functor and monad but applicative as well, no?
Eg given a List<Option>, we can use applicative to declaratively and with referential transparency etc etc (you know the usual FP sales pitch) turn it into an empty list if it contains a single non-populated option, or a list of Foo if all options in the list are populated.
With nullable types I don't think you could do it as declaratively and elegantly.
Actually, even if you could, it's kind of beside the point...
To me, the point is, Option has been designed from scratch to be an FP style Maybe type with all that comes with that in terms of it being a functor, applicative functor, monad etc etc whereas while nullable types in some ways are functionally equivalent, a nullable type doesn't implement a Functor interface, or a Monad interface, or an Applicative interface, and when it behaves like it does, it's mostly kind of by accident driven by a pragmatic need, not any real understanding of reusable, lawful FP abstractions.
> it is entirely possible to live without null, and languages like Rust do.
It seems as though there is this common misconception that Rust does not have a concept of null. Rust does have null pointers [1]! The reason many people do not see null often is because working with and dereferencing raw pointers is an unsafe operation
1: https://doc.rust-lang.org/std/ptr/fn.null.html
It seems as though there is this common misconception that Rust does not have a concept of null. Rust does have null pointers [1]! The reason many people do not see null often is because working with and dereferencing raw pointers is an unsafe operation
1: https://doc.rust-lang.org/std/ptr/fn.null.html
> It seems as though there is this common misconception that Rust does not have a concept of null.
I don't think that's quite relevant, though. Rust doesn't have a concept of nulls like like Java, Javascript, Python, etc do in which just about any variable might contain a value or null. Rust certainly doesn't have that.
I don't think that's quite relevant, though. Rust doesn't have a concept of nulls like like Java, Javascript, Python, etc do in which just about any variable might contain a value or null. Rust certainly doesn't have that.
I would not say that it is not relevant. Raw pointers are a fundamental part of the language. It is just that Rust abstracts over them so that many users do not have to worry about them.
I see your point though.
I see your point though.
I don't think people mean it doesn't have a raw null pointer. Normally it's about a null object reference that you can assign to (almost) anything like Java's / c's null.
Of course Rust has a null pointer type. Otherwise, it wouldn’t really be a systems programming language :)
But most users would rarely if ever need to use that type. Option<T> is how Rust deals with optional/nullable values.
But most users would rarely if ever need to use that type. Option<T> is how Rust deals with optional/nullable values.
Note that nowadays the preferred way to handle null pointers in APIs is the `Option<NonNull>` type. This unfortunately wasn't available in Rust 1.0.
https://doc.rust-lang.org/std/ptr/struct.NonNull.html
https://doc.rust-lang.org/std/ptr/struct.NonNull.html
I agree that union-typed null was the best solution for Dart, but I think many of the article's criticisms of option types aren't good.
> We can’t perform arithmetic on an Option<int> any more than we could on a List<int>.
Why not? One sensible definition is
This is a very good reason to use union types — lots of programs and lots of people are used to thinking this way, and pulling that out from under their feet wouldn't be very nice.
> Nullable types, since they have no explicit runtime representation, are implicitly flattened.
Yes, flattening things that shouldn't be flattened is the biggest problem with union types. Different people will assign different meanings to null, and it's a real headache when trying to write generic code. Discriminated unions make this a no-brainer.
> With nullable types, since there is no representation difference, you can pass a value of the underlying type directly: takesMaybeInt(3);
That's a very minor difference, but if it's that important, Rust lets you write
> We can’t perform arithmetic on an Option<int> any more than we could on a List<int>.
Why not? One sensible definition is
Some(x) + Some(y) = Some(x + y)
_ + _ = None()
This is a very good reason to use union types — lots of programs and lots of people are used to thinking this way, and pulling that out from under their feet wouldn't be very nice.
> Nullable types, since they have no explicit runtime representation, are implicitly flattened.
Yes, flattening things that shouldn't be flattened is the biggest problem with union types. Different people will assign different meanings to null, and it's a real headache when trying to write generic code. Discriminated unions make this a no-brainer.
> With nullable types, since there is no representation difference, you can pass a value of the underlying type directly: takesMaybeInt(3);
That's a very minor difference, but if it's that important, Rust lets you write
fn takesMaybeInt(x: impl Into<Option<i32>>) { ... }I agree with all your other points but
> > We can’t perform arithmetic on an Option<int> any more than we could on a List<int>.
> Why not? One sensible definition is (...)
It's not the only sensible definition. It would be equally sensible to implement it as:
> > We can’t perform arithmetic on an Option<int> any more than we could on a List<int>.
> Why not? One sensible definition is (...)
It's not the only sensible definition. It would be equally sensible to implement it as:
Some(x) + Some(y) => Some(x+y)
Some(x) + None => Some(x)
None + Some(y) => Some(y)
None + None => NoneIt really depends on what the meaning of None is. OP's definition makes perfect sense if it means "unknown". Yours make sense if it means "zero", but that's far less useful. If it means "missing" - which is what the article says they wanted - then any operation involving None should be an error (but Some can still be handled automatically).
It can be interesting to see what other languages do in this situation. For example, C# got nullable value types in version 2, and had to decide what to do with operators in a similar vein (including pre-existing overloaded operators in user code) - they called it "lifting":
https://docs.microsoft.com/en-us/dotnet/csharp/language-refe... https://docs.microsoft.com/en-us/dotnet/csharp/language-refe...
You might notice that it almost, but not quite, has consistent semantics: null means "unknown", which is particularly obvious from Booleans: (true | null) is true, and (false & null) is false, but (true & null) and (false | null) are both null. However, comparison operators aren't consistent: you'd expect x == null to be null if it meant "unknown", but the language will always give you either true or false; and ditto for relative comparisons. This last one means that it's possible for (x == y), (x < y), and (x > y) to all be false.
What's interesting is that, in practice, it's rare to see reliance on any of this behavior in idiomatic C# code - "null" is usually used as "missing", not as "unknown", so all this magic is, at best, irrelevant, and at worst, actively harmful (because it defers a logic error).
It can be interesting to see what other languages do in this situation. For example, C# got nullable value types in version 2, and had to decide what to do with operators in a similar vein (including pre-existing overloaded operators in user code) - they called it "lifting":
https://docs.microsoft.com/en-us/dotnet/csharp/language-refe... https://docs.microsoft.com/en-us/dotnet/csharp/language-refe...
You might notice that it almost, but not quite, has consistent semantics: null means "unknown", which is particularly obvious from Booleans: (true | null) is true, and (false & null) is false, but (true & null) and (false | null) are both null. However, comparison operators aren't consistent: you'd expect x == null to be null if it meant "unknown", but the language will always give you either true or false; and ditto for relative comparisons. This last one means that it's possible for (x == y), (x < y), and (x > y) to all be false.
What's interesting is that, in practice, it's rare to see reliance on any of this behavior in idiomatic C# code - "null" is usually used as "missing", not as "unknown", so all this magic is, at best, irrelevant, and at worst, actively harmful (because it defers a logic error).
> It really depends on what the meaning of None is.
Exactly. This will generally be application specific so we can't choose The One Right Definition for adding `Option<i32>`s.
> Yours make sense if it means "zero", but that's far less useful.
Not in my experience but I agree there are different situations. Personally, I'd default to `Option<i32>::None` meaning "no number there, so just skip it"[1] and prefer `Result<i32, E>::Err` to denote "there should be a number here but we don't have one", which would behave like a SQL NULL (propagate in all calculations).
1. What's the length of data in a header-only packet? I wouldn't say it's zero, especially when trying to fit a read() like API on top of a framed protocol (read() returning Ok(0) marks the end of file).
Exactly. This will generally be application specific so we can't choose The One Right Definition for adding `Option<i32>`s.
> Yours make sense if it means "zero", but that's far less useful.
Not in my experience but I agree there are different situations. Personally, I'd default to `Option<i32>::None` meaning "no number there, so just skip it"[1] and prefer `Result<i32, E>::Err` to denote "there should be a number here but we don't have one", which would behave like a SQL NULL (propagate in all calculations).
1. What's the length of data in a header-only packet? I wouldn't say it's zero, especially when trying to fit a read() like API on top of a framed protocol (read() returning Ok(0) marks the end of file).
> > > We can’t perform arithmetic on an Option<int> any more than we could on a List<int>.
Is actually kind of a weird point to make. There are number of ways to add together List<int> including:
Is actually kind of a weird point to make. There are number of ways to add together List<int> including:
listA.zip(listB).map(([a, b]) => a + b);
optionalA.zip(optionalB).map(([a, b]) => a + b);
optionalA
.zip(optionalB)
.map(([a, b]) => a + b)
.or(optionalA)
.or(optionalB)> It's not the only sensible definition
Right, which is why I said it's just one sensible definition. :)
Although I guess there's at least two sensible definitions of arithmetic on List<int>, too.
Right, which is why I said it's just one sensible definition. :)
Although I guess there's at least two sensible definitions of arithmetic on List<int>, too.
> Different people will assign different meanings to null, and it's a real headache when trying to write generic code.
This is a legitimate concern, but my experience has been that it is a surprisingly rare problem in practice. We software engineers are trained to believe that any system that supports 1 of something is obligated to generalize and support N of them. But you'd be surprised how far just one sentinel value gets you.
I mean, one of the key observations underpinning having nullable (and thus non-nullable) types is that most places in your code need zero of these sentinel values. Most type, around 90% the last time I tried to count, don't permit null at all, so needing two distinct absent values is quite rare.
This is a legitimate concern, but my experience has been that it is a surprisingly rare problem in practice. We software engineers are trained to believe that any system that supports 1 of something is obligated to generalize and support N of them. But you'd be surprised how far just one sentinel value gets you.
I mean, one of the key observations underpinning having nullable (and thus non-nullable) types is that most places in your code need zero of these sentinel values. Most type, around 90% the last time I tried to count, don't permit null at all, so needing two distinct absent values is quite rare.
The bigger problem in generic code is that flattening means that you can't distinguish between incoming nullable values (that you don't know are nullable, because it's a type parameter!) that are null, and nulls in your own code.
Yeah, it is a rare problem, but it's not that rare when writing anything generic. For example, look at all the special-casing of null (ctrl-f "null") in the Java HashMap docs.[0] All of that would immediately go away if they didn't have null.
[0]: https://docs.oracle.com/javase/8/docs/api/java/util/HashMap....
[0]: https://docs.oracle.com/javase/8/docs/api/java/util/HashMap....
I sometimes wonder if the problem is that we don't have enough nulls.
null is typically used as a flag value, but the meaning can be ambiguous: maybe it's the absence of a value, maybe an error occured, etc. Sometimes it has more than one meaning for the same type.
Maybe types should be allowed to declare multiple nulls (effectively like an Enum in java) for different flag values. Operations on the different nulls would throw specialized exceptions indicating why that particular null value exists. Equality... hmmm.
null is typically used as a flag value, but the meaning can be ambiguous: maybe it's the absence of a value, maybe an error occured, etc. Sometimes it has more than one meaning for the same type.
Maybe types should be allowed to declare multiple nulls (effectively like an Enum in java) for different flag values. Operations on the different nulls would throw specialized exceptions indicating why that particular null value exists. Equality... hmmm.
> null is typically used as a flag value, but the meaning can be ambiguous
Languages like Haskell or Rust handle this in a straightforward way by removing the null and using sum types to replace them.
> maybe it's the absence of a value
Option<T> contains a possible value, or nothing. https://doc.rust-lang.org/std/option/
> maybe an error occured
Result<T, E> contains either a value or an error. https://doc.rust-lang.org/std/result/index.html
> Sometimes it has more than one meaning for the same type.
Result<Option<T>, E> is a result that might have a value.
Languages like Haskell or Rust handle this in a straightforward way by removing the null and using sum types to replace them.
> maybe it's the absence of a value
Option<T> contains a possible value, or nothing. https://doc.rust-lang.org/std/option/
> maybe an error occured
Result<T, E> contains either a value or an error. https://doc.rust-lang.org/std/result/index.html
> Sometimes it has more than one meaning for the same type.
Result<Option<T>, E> is a result that might have a value.
I'm not 100% sure if you're being serious, but in case you are, Algebraic Data Types exist to accommodate those ideas.
Completely reasonable request, easy to implement along the lines of:
public class Name {
private Name(String name, int flag){...}
public static Name asMissing(){new Name(null, -1);}
public static Name asNotApplicable(){new Name(null, -2);}
public static Name asNotAvailableYet(){new Name(null, -3);}
public static Name asForbidden(){new Name(null, -4);}
public static Name create(String name){new Name(name, 0)}
public boolean isMissing(){flag==-1}
public boolean isNotApplicable(){flag==-2}
public boolean isNotAvailableYet(){flag==-3}
public boolean isForbidden(){flag==-4}
public String get() {if (flag!=0) throw new Exception(flag) else name}
}Yep. I'd prefer enum-like syntactic sugar, something like:
public class Name {
private Name(String name){...}
flags {
MISSING, NOT_APPLICABLE, NOT_AVAILABLE_YET, FORBIDDEN
}
...
}
Most of those meanings don't make sense for the type itself to define, because it's the context where the type is used that determines whether something can be missing, or there can be an error etc. ADTs solve that.
Is it me, or is the real answer: it's hard to retrofit optional types into a language, but you can add nullable types.
Nah, I was in early on both Swift and nullable dart - at the end of the day, Dart is preferable because it's using flow analysis to know when operations are safe, avoiding the noise of if...let and guard... (Yes, those are nice too, Dart is better! It feels much more ObjC-y)
Working with Dart now and it looks like it’s straight from the 90’s in a lot of aspects. It is not s bad as vanilla JS but when given a clean slate to design a new platform so many better choices could have been made...
I use Dart every now and then and for me "straight from the '90s" is a benefit. I learned the language in about a day. IDE works great, no issues with autocomplete whatsoever (which is often an issue in "modern" languages).
Most mobile developers come from Kotlin or Swift, that are pretty similar to each other and Kotlin is the official language for Android anyway. I had no trouble doing Kotlin coming from a Swift background, the differences are related to the fact it's a JVM language running on Android but the ideas are largely the same.
It would make bridging for native plug-ins a lot easier as well. I don't know how Google Flutter interfacing Google Android could look so medieval when they're basically the same company. They managed to make Xamarin look great in comparison.
It would make bridging for native plug-ins a lot easier as well. I don't know how Google Flutter interfacing Google Android could look so medieval when they're basically the same company. They managed to make Xamarin look great in comparison.
I think saying that Option types are different from nullable types is not true. They differ at the semantic level, around how you can interact with them (one requires you to case on whether or not the value is there, the other requires you to do that with an if statement), but at the type level the describe the same construct.
I think saying something is not the same as something else, when evidently the difference can to reduced down to syntactic sugar is a bad way categorizing and differentiating types.
I think saying something is not the same as something else, when evidently the difference can to reduced down to syntactic sugar is a bad way categorizing and differentiating types.
The article mentions unions vs discriminated unions and then mentions nesting nullable being different than nesting optional, but it doesn't really tie the knot.
Imagine (sorry for pseudocode):
Imagine (sorry for pseudocode):
type Option T = Some T | None
type Nullable T = T | null
I think I'd put it even more on the syntax. Option requires you to explicitly deal with the fact that it is an Option. For nullable types, frequently the syntax doesn't require you to acknowledge that it is nullable at all, and will allow you to continue forward on the assumption that the object doesn't contain null.
This is a very readable writeup. From a distance, Dart seems to focus on usability, while other languages focus more on simplicity (Go) or expressive power (most modern languages). I appreciate the focus on usability.
It's not unique to Dart (Python, ...), but many languages that are popular around here tend to favor expressiveness over usability.
It's not unique to Dart (Python, ...), but many languages that are popular around here tend to favor expressiveness over usability.
> Dart seems to focus on usability, while other languages focus more on simplicity (Go) or expressive power (most modern languages).
Author here. I think we try to focus on all three, but the latter are largely constrained by the language as it was designed in Dart 1.0 (which was mostly done by a different set of people than the ones who work on the language now).
I do wish Dart was a simpler language and worry a lot about the added cognitive load of the usability features we add. But it's really hard to take away features. In order to make non-nullable types a non-breaking change, we added a general versioning system to the language [1]. We have to use it carefully, of course, because users don't like migrating code. But I have some hope that we can use that to gradually simplify the language over time. Complexity has a direct usability cost.
We also care a lot about expressiveness, but I think other languages tend to define that in terms of "how many different things can you express in a minimal amount of code"? With Dart, we weight more by how often you want to certain things, and by how you want to express that. We're willing to make some things more verbose if it makes common things shorter or more familiar.
[1]: https://dart.dev/guides/language/evolution#language-versioni...
Author here. I think we try to focus on all three, but the latter are largely constrained by the language as it was designed in Dart 1.0 (which was mostly done by a different set of people than the ones who work on the language now).
I do wish Dart was a simpler language and worry a lot about the added cognitive load of the usability features we add. But it's really hard to take away features. In order to make non-nullable types a non-breaking change, we added a general versioning system to the language [1]. We have to use it carefully, of course, because users don't like migrating code. But I have some hope that we can use that to gradually simplify the language over time. Complexity has a direct usability cost.
We also care a lot about expressiveness, but I think other languages tend to define that in terms of "how many different things can you express in a minimal amount of code"? With Dart, we weight more by how often you want to certain things, and by how you want to express that. We're willing to make some things more verbose if it makes common things shorter or more familiar.
[1]: https://dart.dev/guides/language/evolution#language-versioni...
You don't necessarily have to take away features to reduce cognitive load. You can actually reduce it by adding features in some cases.
For example, I believe that adding Sum Types would actually simplify most class-based languages, because it would mean that you can use a straightforward tool with a 1:1 mapping from real-world concept to language concept for representing "or" (a very fundamentally logical construct!). As-is you have to use an awkward inheritance pattern with classes. Or use a union and keep track of the tag manually. There's an extra feature, but the usages of the feature become straightforward. It's the same principle as writing longer but more straightforward code rather than a gnarly one-liner.
For example, I believe that adding Sum Types would actually simplify most class-based languages, because it would mean that you can use a straightforward tool with a 1:1 mapping from real-world concept to language concept for representing "or" (a very fundamentally logical construct!). As-is you have to use an awkward inheritance pattern with classes. Or use a union and keep track of the tag manually. There's an extra feature, but the usages of the feature become straightforward. It's the same principle as writing longer but more straightforward code rather than a gnarly one-liner.
Wow tough crowd. I appreciate the pragmatic approach taken here, and as long as there is type safety round nulls im happy to use the language.
Nitpick: As of C# 8 they’ve mostly solved nullable types for classes too with nullable reference types (though retrofitting it made it awkward and imperfect).
Nitpick: As of C# 8 they’ve mostly solved nullable types for classes too with nullable reference types (though retrofitting it made it awkward and imperfect).
If you talk about type systems, the static FP folks naturally show up and according to many of them, anything that SML/Haskell doesn't do must be wrong. :)
For anyone interested, there's also an excellent article on how nullable types work in Dart, that's more of a deep dive:
https://dart.dev/null-safety/understanding-null-safety
https://dart.dev/null-safety/understanding-null-safety
I guess a third option would be to use an "Option" but with much more syntactic sugar. So, rather than calling func(some(3)), you'd just call func(3) and the compiler would automatically wrap it up. Func would be declared this way: def func(int?) to indicate that it's an optional type. IMO you get the best of both world. And then, in many places in the code, you'd have to do "r?.foo()" if r is an Option type. It's different than javascript where the "?" isn't enforced by the compiler.
If you have a function that works on numbers:
const add3 = val => val + 3;
const optionally6 = Some(3).map(add3);
const add3 = val => val + 3;
[1, 2, 3].map(add3);
const add3 = val => val + 3;
const add3ToArrayVals = Array.map(add3);
const add3ToOptionals = Optional.map(add3);
const someSix = add3ToOptionals(Some(3)); // Some(6);
const some456 = add3ToArrayVals([1, 2, 3]);// [4, 5, 6];
const staticArrayMap = (fn) => (array) => array.map(fn);
func(some(3))
some(3).map(func);
some(3)
.map(func)
.map(otherFunc)
.map(yetAnotherFunc);> So, rather than calling func(some(3)), you'd just call func(3) and the compiler would automatically wrap it up.
You can do implicit conversions and that helps somewhat, but it's not quite the same as actual subtyping. In many cases, there's no natural or efficient way to insert that conversion so you still run into restrictions. For example, in a language with nullable types you can write:
You can do implicit conversions and that helps somewhat, but it's not quite the same as actual subtyping. In many cases, there's no natural or efficient way to insert that conversion so you still run into restrictions. For example, in a language with nullable types you can write:
int sumPresentValues(Iterable<int?> values) {
var result = 0;
for (var value in values) if (value != null) result += value;
return result;
}
main() {
Iterable<int> values = [1, 2, 3, 4];
print(sumPresentValues(values)); // <--
}
TypeScript and, AFAIK, Kotlin provide exactly that.
The problem with that special syntax sugar is that then there's no monad to compose with. But it does feel nice for many use cases.
The problem with that special syntax sugar is that then there's no monad to compose with. But it does feel nice for many use cases.
TypeScript also uses union types for null & undefined. But then on top of that, it has optionals. So you can have:
But this is mostly because they were trying to come up with a type system to capture existing JavaScript patterns. I doubt it'd look like that if it could be designed from scratch.
function foo(x: number | undefined);
function foo(x?: number);
But this is mostly because they were trying to come up with a type system to capture existing JavaScript patterns. I doubt it'd look like that if it could be designed from scratch.
In terms of compiler comfort I'd much rather the compiler be able to look at func(3) and deduce that "func accepts an int" rather than "maybe func accepts an int and maybe func accepts an Option<Int>"
Previous, with a comment from the author: https://news.ycombinator.com/item?id=25334248
Good article, but there is no "choice". You need both and the article even explains why!
You need union types if you have subtyping to avoid wrapping/unwrapping all the time.
And you also need option/maybe types for the cases where you must not lose information at runtime. These types can implemented by union types even, by simply "tagging" (= wrapping) the runtime values with a class).
A good language gives you both out of the box.
You need union types if you have subtyping to avoid wrapping/unwrapping all the time.
And you also need option/maybe types for the cases where you must not lose information at runtime. These types can implemented by union types even, by simply "tagging" (= wrapping) the runtime values with a class).
A good language gives you both out of the box.
> You need union types if you have subtyping to avoid wrapping/unwrapping all the time.
In my experience, the option types are always more concise. That's mostly because having option types encourages people to build out nice helper functions like these[0] and these[1]. Haskell's do notation and Rust's Try notation also save a lot of space.
[0]: https://doc.rust-lang.org/std/option/enum.Option.html
[1]: https://hackage.haskell.org/package/base-4.14.0.0/docs/Contr...
In my experience, the option types are always more concise. That's mostly because having option types encourages people to build out nice helper functions like these[0] and these[1]. Haskell's do notation and Rust's Try notation also save a lot of space.
[0]: https://doc.rust-lang.org/std/option/enum.Option.html
[1]: https://hackage.haskell.org/package/base-4.14.0.0/docs/Contr...
I was actually not referring to specifically option types but generally sum types. E.g. inside a function I have a result of (union) type "Integer | ErrorA" in one branch and "Integer | ErrorB" in another branch. ErrorA and ErrorB are completely unrelated types (coming from thirdparty libraries, can't change them).
With union types I don't have to do anything and the return type for my function will be inferred as "Integer | ErrorA | ErrorB". Or if you prefer to work with a result type / either type, then it will become "Either Integer (ErrorA | ErrorB)". In most languages I'd have to make it "Either Integer (Either ErrorA ErrorB)" and also change the function code by lifting accordingly. That's verbose and many times I don't want to do that.
With union types I don't have to do anything and the return type for my function will be inferred as "Integer | ErrorA | ErrorB". Or if you prefer to work with a result type / either type, then it will become "Either Integer (ErrorA | ErrorB)". In most languages I'd have to make it "Either Integer (Either ErrorA ErrorB)" and also change the function code by lifting accordingly. That's verbose and many times I don't want to do that.
In Rust, you would implement From<ErrorA> and From<ErrorB> for a custom Error enum defined in your application/library. This will allow you to convert any ErrorA or ErrorB into Error.
Then, you could simply return Result<Integer, Error> from your function, and the conversion would happen for you behind the scenes.
Of course, unless you track the original error types, you will lose some information. But it’s a very clean way to handle multiple error types within your library imo.
An example of how this works: https://doc.rust-lang.org/rust-by-example/error/multiple_err...
Then, you could simply return Result<Integer, Error> from your function, and the conversion would happen for you behind the scenes.
Of course, unless you track the original error types, you will lose some information. But it’s a very clean way to handle multiple error types within your library imo.
An example of how this works: https://doc.rust-lang.org/rust-by-example/error/multiple_err...
> This will allow you to convert any ErrorA or ErrorB into Error.
I chose my example for cases where I want to keep track of the different error cases. But even if I don't, I still have a _lot_ of overhead for converting them.
Unfortunately Rust isn't a language that offers a solution to this problem so far. But there is an open RFC for it: https://github.com/rust-lang/rfcs/issues/294
I chose my example for cases where I want to keep track of the different error cases. But even if I don't, I still have a _lot_ of overhead for converting them.
Unfortunately Rust isn't a language that offers a solution to this problem so far. But there is an open RFC for it: https://github.com/rust-lang/rfcs/issues/294
I see. Yeah, this approach breaks down a bit if the error conversion logic is non-trivial.
Actually, I'm wrong here, it's tagged union types not plain ones. But some commentary in there explains it better than me, so still helpful.
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I have this funny question all the time: why dont we just use a bit boolean for representing option/nullable/unknown instead? (one bit of hasValue/isEmpty like C# HasValue, but like struct uncertain_value<T> { T value; bool dont_care: 1; } and uninitialized value f<T>() => uncertain_value<T> { dont_care = 1 })
What do you see as being the benefits of this approach?
Given a 64-bit variable, you can fill it with 64 bits of value, or 64 bits of pointer to some value elsewhere.
With your scheme that would be 63 (which I'm not objecting to).
Pointers are already special-cased to only contain 0 already. There's no use-case for having 63 bits of pointer and an extra bit saying "ignore the other 63 bits".
As for the value case, I think the argument is pretty similar. Why store 63 bits of value if the 64th bit says to ignore them?
If you're not talking memory layouts and are looking at adding an extra field to a struct/class. Take you value field - if it's not initialised yet, and null doesn't exist, then what is its state?
With your scheme that would be 63 (which I'm not objecting to).
Pointers are already special-cased to only contain 0 already. There's no use-case for having 63 bits of pointer and an extra bit saying "ignore the other 63 bits".
As for the value case, I think the argument is pretty similar. Why store 63 bits of value if the 64th bit says to ignore them?
If you're not talking memory layouts and are looking at adding an extra field to a struct/class. Take you value field - if it's not initialised yet, and null doesn't exist, then what is its state?
if we extend the value to (value, is_null: bit<1>) we can have operated on the value field directly while also knowing whether the value is loaded or not. but you are right, maybe the compiler will outright "optimize" this to (value: align_pow2(sizeof T), is_null: byte) since the compiler will have to align value anyway, so that it will always have to expand the "sentinel".
My original idea exactly contrary to your thinking: rather than making 0 NULL and give it the sentinel we always had, why don't we just avoid using it like using some data structure magic? You know, the null is infamous for causing the special case in integral constant expression that C++ inherited from C, and is confusing too, because then 0 is can be considered as (void ) and thus can cause an overload fiasco, that consider f(void) and f(int), what do you think f(0) would be? So the gist is I always think using x=0 to be able to represent NULL/false and !(x=0) to represent true is really really confusing (at least in C/C++). This also caused other languages to have some probable cases of negative number being cast to true so like `if (-1) console.log('lol')` it will print, wat?
Also, with my configuration, not only we can always assume the value was intended to have a place to stay without indirection load, but the optional/null/don't-care/noninit will always have a stable state between 0 and 1 -- so we can clamp down without having to deal with extra states like I shown above, which is good from theoretical (because we can express its definite intent rather than guessing the type of the wrapped value) & logical standpoint (that we should not think in 3VL for null, rather, judge it by its concrete properties), not quite so in practice however (again, optimization kicks in nullifying all the good works), this might not entirely escape the pointer-not-null-then-consider-valid problem (what if we loaded a garbage value that might have a random 0/1? nowhere to tell also).
In fact, this kind of configuration is not rare to be found: an open addressing hash table can use this kind of setting to represent "tombstone"/deleted field, but in a more optimized manner that we "bit set" because we are going to allocate a contiguous block of memory anyway so like the current universe can have 32 entries then we use lg(32) bytes bit-set at the beginning/end, if insert or delete value at entry x then toggle bit-tombstone of x, if search value y then find first i, x in entries if compare(x, y) = exact and !bit-tombstone[i] then (yes, i) else no...so on.
But at the end of the day even that I don't like using 3VL to represent NULL, and my configuration is still like mapping 3VL into double binary anyway, iirc its like if value = U then don't care (either true and false; neither true nor false) else value.
An analogy for NULL: if addr = NULL then value is no where to be found/random garbage value else load memory from addr.
My original idea exactly contrary to your thinking: rather than making 0 NULL and give it the sentinel we always had, why don't we just avoid using it like using some data structure magic? You know, the null is infamous for causing the special case in integral constant expression that C++ inherited from C, and is confusing too, because then 0 is can be considered as (void ) and thus can cause an overload fiasco, that consider f(void) and f(int), what do you think f(0) would be? So the gist is I always think using x=0 to be able to represent NULL/false and !(x=0) to represent true is really really confusing (at least in C/C++). This also caused other languages to have some probable cases of negative number being cast to true so like `if (-1) console.log('lol')` it will print, wat?
Also, with my configuration, not only we can always assume the value was intended to have a place to stay without indirection load, but the optional/null/don't-care/noninit will always have a stable state between 0 and 1 -- so we can clamp down without having to deal with extra states like I shown above, which is good from theoretical (because we can express its definite intent rather than guessing the type of the wrapped value) & logical standpoint (that we should not think in 3VL for null, rather, judge it by its concrete properties), not quite so in practice however (again, optimization kicks in nullifying all the good works), this might not entirely escape the pointer-not-null-then-consider-valid problem (what if we loaded a garbage value that might have a random 0/1? nowhere to tell also).
In fact, this kind of configuration is not rare to be found: an open addressing hash table can use this kind of setting to represent "tombstone"/deleted field, but in a more optimized manner that we "bit set" because we are going to allocate a contiguous block of memory anyway so like the current universe can have 32 entries then we use lg(32) bytes bit-set at the beginning/end, if insert or delete value at entry x then toggle bit-tombstone of x, if search value y then find first i, x in entries if compare(x, y) = exact and !bit-tombstone[i] then (yes, i) else no...so on.
But at the end of the day even that I don't like using 3VL to represent NULL, and my configuration is still like mapping 3VL into double binary anyway, iirc its like if value = U then don't care (either true and false; neither true nor false) else value.
An analogy for NULL: if addr = NULL then value is no where to be found/random garbage value else load memory from addr.
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This is not intended as an attack, but a genuine question: what does Dart offer over any other language? Why would I choose Dart?
Had wondered this for a bit myself - I'm a bimobile developer, ObjC, Swift, to Java, to Kotlin over the last decade, now on Dart.
Dart is by far the most fun I've had in a language because it has an extremely simple surface area, and they really optimize for that, ex. here. But, Flutter is the revolution - utterly paradigm-shifting tool. Much like ObjC, you probably use Dart because it's what Flutter does, but not like ObjC, the simplicity, ease, and clarity of thought it allows is a benefit, as well as Swiss he army knife way the language can be deployed (Android app? Web app? Linux CLI tool? Windows 10 app?)
Dart is by far the most fun I've had in a language because it has an extremely simple surface area, and they really optimize for that, ex. here. But, Flutter is the revolution - utterly paradigm-shifting tool. Much like ObjC, you probably use Dart because it's what Flutter does, but not like ObjC, the simplicity, ease, and clarity of thought it allows is a benefit, as well as Swiss he army knife way the language can be deployed (Android app? Web app? Linux CLI tool? Windows 10 app?)
Nullable type is the biggest mistake in all the "modern" languages
It should be a type Optional<T>, not built into a language, it make no sense memory wise
It's like NULL, it should stay, it is not an issue, it is how computers work!!!!!!!
It should be a type Optional<T>, not built into a language, it make no sense memory wise
It's like NULL, it should stay, it is not an issue, it is how computers work!!!!!!!
You seem to have a very strong opinion about the issue, but I cannot tell whether you prefer a missing value to be represented by null or by Option.empty.
If we look at set theory the NULL set is a valid set. The problem with NULL is often it's just treated as zero or if passed to a language that does not have NULL it's pretty often to just treat it as Zero. This bit equivalence between zero and NULL zero is often what you see lead to problems.
It's why for instance with GPS coordinates NULL island exists.
However, errors aside NULL is a perfectly logical value to encounter or need.
It's why for instance with GPS coordinates NULL island exists.
However, errors aside NULL is a perfectly logical value to encounter or need.
> NULL is a perfectly logical value to encounter or need.
Great! Assign to it the type Null.
This is legal:
Great! Assign to it the type Null.
This is legal:
Null myNull = null;
Integer i = null;This is essentially what Dart does:
https://api.dart.dev/stable/2.10.4/dart-core/Null-class.html
The main change with adding nullable and non-nullable types to the language is that we no longer treat Null as a subtype of all types. Instead, you have to opt in using "?", which is basically syntactic sugar for "| Null".
https://api.dart.dev/stable/2.10.4/dart-core/Null-class.html
The main change with adding nullable and non-nullable types to the language is that we no longer treat Null as a subtype of all types. Instead, you have to opt in using "?", which is basically syntactic sugar for "| Null".
NULL can be its own type, but the union between the set of integers and NULL is equally valid. However, that new set created by this union is not just integers.
To represent a type that can be NULL or an integer it should be a different type.
Like I said the problem again is mostly your data is not always contained to your language. If you export GPS coordinates some systems may treat this composite type incorrectly. So even if you have a strong type system people will just be lazy.
To represent a type that can be NULL or an integer it should be a different type.
Like I said the problem again is mostly your data is not always contained to your language. If you export GPS coordinates some systems may treat this composite type incorrectly. So even if you have a strong type system people will just be lazy.
I give the author the benefit of the doubt but Optionals are so much more powerful than what this article covers. You can map, filter, reduce, chain, compose functions that all work with optionals, and lift functions that work on numbers (or any other type) to be functions that work on Optional<number> but none of that is even mentioned in this piece (likely cause then its harder to justify picking nullable types instead)
I also found the Some(Some(3)) example just plain wrong. In those scenarios, typically you just use chain (aka flatMap) instead of map... Seems odd this was even included as a motivating reason when this is very much a solved problem.
I'm certainly biased, as I think FP is an incredible mental model for coding and it seems to be catching on in lots of places too.
I understand language designers have to make choices that benefit their users now and be as simple as possible to opt into without any overhead to account for changes, but man, what a missed opportunity to give developers better tools and mechanisms in addition to opening up the doorway for many people to functional programming. But even Java has support for optionals.
Overall, this appears to be like a missed opportunity and the justification doesn't seem to really reflect an understanding of the benefits optionals provide, and misconstrues their use and capability.