#2899: GADT type inference with existentials
------------------------------------+---------------------------------------
Reporter: jochemb | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 6.10.1 | Severity: normal
Keywords: GADT, existential types | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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The following code works in GHC 6.8.2:
{{{
{-# OPTIONS -fglasgow-exts #-}
module Test where
data Existential = forall a. Ex (GADT a)
data GADT :: * -> * where
GCon :: Int -> GADT ()
-- g :: Existential -> Int
g (Ex (GCon x)) = x
}}}
The compiler correctly infers the type of x in the definition of g, and
correctly infers the type of g.
In GHC 6.10.1, however, this fails with:
{{{
Test.hs:10:12:
GADT pattern match with non-rigid result type `t'
Solution: add a type signature
In the definition of `g': g (Ex (GCon x)) = x
Failed, modules loaded: none.
}}}
The solution, to include the type signature of g (that I have commented
out), works, but why can't GHC figure out the type of g? GHC 6.8.2 could!
An equivalent version with a non-GADT instead, works correctly:
{{{
data GADT a = GCon Int
}}}
Ticket #2206 might be related.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2899>
GHC <http://www.haskell.org/ghc/>
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