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  • Glasgow Haskell Compiler
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  • #3638
Closed
Open
Created Nov 04, 2009 by rl@cse.unsw.edu.au@trac-rl

Redundant signature required with RULES and GADTs

Here is a small program:

data T a where TInt :: T Int

foo :: T Int -> Int
foo TInt = 0

{-# RULES "foo"  forall x. foo x = case x of { TInt -> 0 } #-}

GHC complains:

    GADT pattern match with non-rigid result type `t'
      Solution: add a type signature for the entire case expression
    In a case alternative: TInt -> 0
    In the expression: case x of { TInt -> 0 }
    When checking the transformation rule "foo"

And indeed, changing the rule to

{-# RULES "foo"  forall x. foo x = case x of { TInt -> 0 } :: Int #-}

works. The signature is redundant, though, because the type of the case is determined by the type of the rule head.

Trac metadata
Trac field Value
Version 6.13
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler (Type checker)
Test case
Differential revisions
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