Martijn

há 11 anosdiscuss

You're in the wrong thread. :-)

But to answer your question anyway, this is the responsible line of code: https://github.com/marekdlugos/TakeMeAsIntern/blob/master/as...

Martijn

há 12 anosdiscuss

It's a combination of avoiding potential bugs and just modelling things differently altogether:

If I had to leave out the answer type of questions, I'd have to come up with an entirely different way of modelling answers. For example, I might have had to write a datatype like:

  data Answer = AnswerBool Bool | AnswerInt Int | ...

and then processing the answer (through pattern matching) would have to discard any answer it didn't like, which is awkward. Or I would have had to use a type class to keep everything type-safe which would have resulted in more complicated type signatures throughout the code.

For the TargetList, the current setup allows me to write functions that consume a target list of exactly the right size, for example:

  \(target1, target2) -> ...

rather than a normal list:

  \[target1, target2] -> ...

which in this case results in a pattern match exception if you pass it a list of any length other than 2.

Martijn

há 12 anosdiscuss

My Magic: the Gathering implementation uses a few GADTs. Interestingly they use GADTs in very different ways. Source code is here: https://github.com/MedeaMelana/Magic/blob/master/Magic/src/M...

Let me briefly highlight three of them:

1) Type Question models questions that the game might ask of players. Its constructors are tagged with the type of the answer to that question:

  data Question a where
    AskKeepHand :: Question Bool
    AskTarget   :: [EntityRef] -> Question EntityRef
    ...

This allows me to write cards that ask questions and later on plug in various interpreters of these questions (command line interface, JSON webserver, AI, test framework).

2) The ZoneRef datatype enumerates the various zones in a game of Magic (battlefield, hand, etc) but I've chosen to tag each ZoneRef constructor with a type that indicates what the type of a card is when you look up a reference to a card ("object" in Magic jargon):

  type ObjectRef ty = (ZoneRef ty, Id)
  lookupObject :: ObjectRef ty -> Magic (ObjectOfType ty)

Most functions that take object references as an argument like to constrain the type of the object referenced, and using a GADT allows them to do so. For example, dealing damage to an object (creature or planeswalker):

  data SimpleOneShotEffect = DamageObject Object (ObjectRef TyPermanent) Int ...

3) Type TargetList models zero or more targets a spell on the stack can target. Even though in essence it is just a list of targets, it is a GADT so that it can have a "return type" which is re-evaluated when one of more of the targets change (for example, by a card like Redirect or Reverberate).

Hope this helps!

Martijn

há 13 anosdiscuss

If I remember correctly the automatically sorting through your pictures and picking the best is exactly what Google announced for Google+ at their last I/O keynote.

Martijn

há 13 anosdiscuss

You don't really need to understand all these things in order to build "real" things with it. Just build something without caring too much whether an experienced Haskell programmer would find it elegant enough.

Most of the "clicks" I've had is when I'd already built something in a naive way (that worked perfectly) and someone showed me an alternative, more elegant way in which it could be written.

Martijn

há 13 anosdiscuss

I wonder if this is also true if you are a foreigner visiting the US.

And I wonder if this also works for the Dutch police.

Martijn

há 13 anosdiscuss

What company in Portugal do you work for?

Martijn

há 14 anosdiscuss

For some reason this reminds me of Quine Central[1]. :-)

[1] http://blog.sigfpe.com/2011/01/quine-central.html

Martijn

há 14 anosdiscuss

Not really. There's a really nice paper by Graham Hutton, "A tutorial on the universality and expressiveness of fold" which goes into a bit more depth than OP does. Your question is answered directly in section 3.2, "The fusion property of fold". Basically, a sequence of multiple folds can be fused to a single fold.

See http://www.cs.nott.ac.uk/~gmh/fold.pdf

Martijn

há 14 anosdiscuss

> > boo :: Map Integer String -> String -> Integer

> This syntax is clean but it doesn't tell me anything that

> def boo(map: Map[Integer, String], key: String) : Integer

> doesn't tell me.

OP's point was not so much that the Haskell syntax is clean, but more that there is a huge difference between

Map Integer String -> String -> Integer

and

Map Integer String -> String -> IO Integer

The latter is allowed to do IO, while the former is not. There are very few languages that allow you to express this difference and have the compiler enforce it (Haskell being one of them). So "Map Integer String -> String -> Integer" tells you a lot more than "def boo(map: Map[Integer, String], key: String) : Integer" does.

For more information about what exactly types tell you, look up parametricity or read Philip Wadler's "Theorems for free".

Martijn

há 14 anosdiscuss

In Haskell variables are called variables because they potentially have a different value each time the function is called.

See also Variable (mathematics) [1] vs. Variable (computer science) [2].

[1] https://secure.wikimedia.org/wikipedia/en/wiki/Variable_%28m... [2] https://secure.wikimedia.org/wikipedia/en/wiki/Variable_%28c...

Martijn

há 14 anosdiscuss

Can you tell us more about the book? Would you recommend it? Is it easily accessible?

Martijn

há 14 anosdiscuss

Small nitpick, the type of forever is actually:

  forever :: Monad m => m a -> m b

Since it never returns a value, the resulting monadic action can be polymorphic in its return type.

Martijn

há 14 anosdiscuss

Is the color->note map arbitrary or is there some logic behind it? I imagine it greatly affects the associations (including emotions) he has built up over the years.

Martijn

há 15 anosdiscuss

Also: