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  • #7082

Closed
Open
Created Jul 17, 2012 by guest@trac-guest

Default type family instances

The following code does not compile in GHC 7.4, unless line (2) is uncommented. I thought line (1) would remove need for (2). If it does not, probably declaration (1) should cause an error.

{-# LANGUAGE TypeFamilies #-}
class R m where
  type D m a :: *
  type D m a = ()                -- (1)
  f :: m a -> D m a -> ()

instance R Maybe where
  -- type D Maybe a = ()         -- (2)
  f = undefined

x = f (Nothing :: Maybe Int) ()
Trac metadata
Trac field Value
Version 7.4.1
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler (Type checker)
Test case
Differential revisions
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