Hello,
Thanks for the answer.
I tried the code you sent, but now I'm getting a "type variable not in
score" error. (I added both extensions)
This is the whole code pertaining to this problem (with the extensions
mentioned before):
class Newtype a b where
wrap :: a -> b
unwrap :: b -> a
newtype MyInt = MyInt Int
newtype YourInt = YourInt Int
instance Newtype Int MyInt where
wrap = MyInt
unwrap (MyInt i) = i
instance Newtype Int YourInt where
wrap = YourInt
unwrap (YourInt i) = i
add :: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 -> b1
add x y = wrap @a @b1 $ unwrap @a x + unwrap @a y
For further reference, the exercice to which this code should be a solution
can be found at:
https://github.com/i-am-tom/haskell-exercises/blob/answers/09-MultiParamType...
Finally, the question remains: Is it "normal" that ghci behave differently
depending on whether
the type signature is declared or not? (Remember that the signature is
given by ghci itself)
Thanks again,
Michel :)
On Wed, Apr 10, 2019 at 1:28 PM Sylvain Henry
Hi,
It looks like an effect of ExtendedDefaultRules: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html#ex...
It's hard to tell without the code but maybe something like that will do:
{-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeApplications #-}
add :: forall a b1 b2 b3. (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 -> b1 add x y = wrap @a @b1 $ unwrap @a x + unwrap @a y
-Sylvain On 10/04/2019 12:32, Michel Haber wrote:
Hello Cafe,
I was trying to load a module containing this function in ghci: "add x y = wrap $ unwrap x + unwrap y" with the following extensions activated:
ConstraintKinds DataKinds DeriveFunctor DuplicateRecordFields FlexibleContexts FlexibleInstances GADTs KindSignatures MultiParamTypeClasses PolyKinds TypeFamilies TypeOperators AllowAmbiguousTypes
And it loaded without problem.
So then I tested its type with ":t add", which gave: add :: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 -> b1
Then I added this signature to the function in the module. This caused ghci to refuse to load it and give the following error:
src/Exercises.hs:55:11: error: • Could not deduce (Newtype Integer b1) arising from a use of ‘wrap’ from the context: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) bound by the type signature for: add :: forall a b1 b2 b3. (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 -> b1 at src/Exercises.hs:54:1-74 • In the expression: wrap $ unwrap x + unwrap y In an equation for ‘add’: add x y = wrap $ unwrap x + unwrap y | 55 | add x y = wrap $ unwrap x + unwrap y | ^^^^^^^^^^^^^^^^^^^^^^^^^^
src/Exercises.hs:55:18: error: • Could not deduce (Newtype Integer b2) arising from a use of ‘unwrap’ from the context: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) bound by the type signature for: add :: forall a b1 b2 b3. (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 -> b1 at src/Exercises.hs:54:1-74 • In the first argument of ‘(+)’, namely ‘unwrap x’ In the second argument of ‘($)’, namely ‘unwrap x + unwrap y’ In the expression: wrap $ unwrap x + unwrap y | 55 | add x y = wrap $ unwrap x + unwrap y | ^^^^^^^^
src/Exercises.hs:55:29: error: • Could not deduce (Newtype Integer b3) arising from a use of ‘unwrap’ from the context: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) bound by the type signature for: add :: forall a b1 b2 b3. (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 -> b1 at src/Exercises.hs:54:1-74 • In the second argument of ‘(+)’, namely ‘unwrap y’ In the second argument of ‘($)’, namely ‘unwrap x + unwrap y’ In the expression: wrap $ unwrap x + unwrap y | 55 | add x y = wrap $ unwrap x + unwrap y | ^^^^^^^^ Failed, no modules loaded.
This does not make sense to me, since I only used the signature that ghci itself gave me.
Is this a bug? if not, could someone please explain this behaviour to me?
Thanks, Michel
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