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  • Glasgow Haskell CompilerGlasgow Haskell Compiler
  • GHCGHC
  • Issues
  • #4492

Type families regression

Here is a small program:

type family F a b
type instance F (Maybe a) b = b -> F a b

class C a where
  go :: (forall a. Maybe a -> b -> a) -> a -> F a b

instance C a => C (Maybe a) where
  go f a b = go f (f a b)

The current head complains:

    Could not deduce (F a b ~ F a b) from the context (C a)
    NB: `F' is a type function, and may not be injective
    Expected type: b -> F a b
      Actual type: F (Maybe a) b
  1. 12.3 accepts it.
Trac metadata
Trac field Value
Version 7.1
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler (Type checker)
Test case
Differential revisions
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