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  • Glasgow Haskell CompilerGlasgow Haskell Compiler
  • GHCGHC
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  • #15740
Closed
Open
Issue created Oct 11, 2018 by Richard Eisenberg@raeDeveloper

Type family with higher-rank result is too accepting

GHC accepts this garbage:

type family F2 :: forall k. k -> Type
data SBool :: Bool -> Type
data Nat
data SNat :: Nat -> Type
type instance F2 = SBool
type instance F2 = SNat

The family F2 should have an arity of 0, meaning that only one instance is possible -- and the RHS of that instance must have kind forall k. k -> Type. In other words, even accepting only one of the instances above is hogwash.

This is from ticket:11719#comment:161416, but you don't have to read that to understand this.

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Version 8.6.1
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler
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