Generalised plusequals(leontrolski.github.io)
leontrolski.github.io
Generalised plusequals
https://leontrolski.github.io/alt.html
12 comments
The website asks what they do in Haskell. The answer is property modification and reading, as well as very powerful traversal constructs, use lenses (https://hackage.haskell.org/package/lens , tutorial at https://hackage.haskell.org/package/lens-tutorial-1.0.5/docs...).
What would be the equivalent to this in Haskell (with or without lens):
cat = Cat(age=3)
l = [1, [2, cat], 4]
alt l[1][1].age.=9
[1, [2, Cat(age=9)], 4]In Haskell this list is not well-typed
l = [1, [2, cat], 4]
{-# LANGUAGE TemplateHaskell #-}
import Control.Lens
data Cat = Cat { _age :: Int }
deriving Show
makeLenses ''Cat
data Item
= I Int
| L [Item]
| C Cat
deriving Show
makePrisms ''Item
cat :: Cat
cat = Cat 3
l :: [Item]
l = [I 1, L [I 2, C cat], I 4]
l' :: [Item]
l' = set (ix 1 . _L . ix 1 . _C . age) 9 l
ghci> l'
[I 1,L [I 2,C (Cat {_age = 9})],I 4]In Lil[0], this is how ordinary assignment syntax works. Implicitly defining a dictionary stored in a variable named "cat" with a field "age":
cat.age:3
# {"age":3}
l:1,(list 2,list cat),4
# (1,(2,{"age":3}),4)
l[1][1].age:9
# (1,(2,{"age":9}),4)
cat
# {"age":3}
l:l,5
# (1,(2,{"age":9}),4,5)
This is surprising to me:
l[1][1].age:9
# (1,(2,{"age":9}),4)
{"age":9}
l_inner = l[1][1]
l_inner.age:9Amending a slice would amend only the slice:
l_inner:l[1][1]
# {"age":3}
l_inner.age:9
# {"age":9}
l_inner
# {"age":9}
l
# (1,(2,{"age":3}),4)
(1,(list 2,list ().age:5),4)[1][1].age:99
# (1,(2,{"age":99}),4)Yeah this looks like lenses at first glance
It seems like this is proposing syntactic sugar to make mutating and non-mutating operations be on equal footing.
> The more interesting example is reassigning the deeply nested l to make the cat inside older, without mutating the original cat
Isn't that mutating l, though? If you're concerned about mutating cat, shouldn't you be concerned about mutating l?
> The more interesting example is reassigning the deeply nested l to make the cat inside older, without mutating the original cat
Isn't that mutating l, though? If you're concerned about mutating cat, shouldn't you be concerned about mutating l?
It doesn't mutate l exactly, it makes a new list slightly different from the original one and assigns it to l.
That means if someone has a reference to the original l, they do not see the change (because l is immutable. Both of them).
That means if someone has a reference to the original l, they do not see the change (because l is immutable. Both of them).
[deleted]
I think I am misunderstanding the behavior of the alt keyword
q has the concept of amend, which is similar: https://code.kx.com/q4m3/6_Functions/#683-general-form-of-fu...
It's quite handy, though the syntax for it is rather clunky compared to the rest of the language in my opinion.
It's quite handy, though the syntax for it is rather clunky compared to the rest of the language in my opinion.