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It would be nice if type parameters of associated types could be more specific than those in the instance header. This is currently rejected with the message "Type indexes must match class instance head", but could be accepted:

{-# LANGUAGE TypeFamilies #-}

class C a where
  type T a

instance C [a] where
  type T [Bool] = Int
  type T [Int]  = Int

More typically, this is useful where we want to use an equality constraint to make type inference easier, but need to match on the actual type in an associated type:

{-# LANGUAGE TypeFamilies, FlexibleInstances #-}

class C a where
  type T a

instance a ~ [b] => C a where
  type T [b] = Int

This showed up in the implementation of OverloadedRecordFields. Of course, one can always work around it using a normal (non-associated) type family.

Note that we already allow type families to specialise variables that do not occur in the instance header:

{-# LANGUAGE TypeFamilies #-}

class C a where
  type T a b

instance C [a] where
  type T [a] [Bool] = Bool
  type T [a] [Int]  = Int

Trac metadata

Trac field	Value
Version	7.7
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
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