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I was playing around with the following code, which tries to map a type family over a type (something I thought would not be allowed):

{-# LANGUAGE
    TypeFamilies
  , KindSignatures
  #-}

type family Id x :: *
type instance Id a = a

type family Map (f :: * -> *) x :: *
type instance Map f [a] = [f a]

x :: Map Id [Bool]
x = []

Both GHC 7.8.3 and the current HEAD (7.9.20140811) accept this program! However, changing the definition of x to [True] gives an error:

    Couldn't match type ‘Id Bool’ with ‘Bool’
    Expected type: Map Id [Bool]
      Actual type: [Bool]
    In the expression: [True]
    In an equation for ‘x’: x = [True]

If I define y :: Id Bool with value True (which works fine) and define x = [y], I even get this error:

    Couldn't match type ‘Bool’ with ‘Id Bool’
    Expected type: Id Bool
      Actual type: Id Bool
    In the expression: y
    In the expression: [y]

Trac metadata

Trac field	Value
Version	7.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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