How to call a function taking no arguments from an instance?

Question

Assume, you have a class:

class AClass a where
  func:: Int

instance AClass SomeTree where
  func = 0

instance AClass Double where
  func = 1

How do I call the function func?

neutral :: Int, should work, since information both is gathered from the input type and output type. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jan 8, 2021 at 14:03
There's no such thing as a function that takes no arguments. func here is just an Int. Use it like any other Int. The bigger issue is that you can't define func to have a type that doesn't involve a in some way. — chepner
– chepner, Commented Jan 8, 2021 at 14:13
@TVSuchty: Then you can use TypeApplications: func @Double. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jan 8, 2021 at 14:18
You can add {-# LANGUAGE TypeApplications #-} to your file, which enables a new @ notation for providing type arguments. Then, you can write func @SomeTree, which will yield 0, or func @Double, which will yield 1. See the docs — DDub
– DDub, Commented Jan 8, 2021 at 14:23
Can you provide a more detailed example of what you are trying to do? — chepner
– chepner, Commented Jan 8, 2021 at 14:27

leftaroundabout · Accepted Answer · 2021-01-08 14:31:54Z

7

{-# LANGUAGE AllowAmbiguousTypes, TypeApplications #-}

class AClass a where
  func :: Int

instance AClass SomeTree where
  func = 0

instance AClass Double where
  func = 1

foo :: Int
foo = func @SomeTree + func @Double

{-# LANGUAGE ScopedTypeVariables, UnicodeSyntax #-}

bar :: ∀ a . AClass a => a -> Int
bar _ = func @a

answered Jan 8, 2021 at 14:31

leftaroundabout

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2 Comments

TVSuchty Over a year ago

What magic does the bar Type constraint do? Thanks for the answer!

leftaroundabout Over a year ago

∀ a . is just what you implicitly have in functions like sum :: Num a => [a] -> a, which is actually shorthand for sum :: ∀ a . Num a => [a] -> a. The difference is that the explicit ∀ (aka forall) brings the a into scope in such a way that it can be used in the function body, as here with func @a.

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