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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