#3440: Improve error message for GADT failures
--------------------------------------+-------------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.12.1
Component: Compiler (Type checker) | Version: 6.10.4
Severity: normal | Keywords:
Difficulty: Unknown | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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If you write
{{{
type family Fam a :: *
data GADT :: * -> * where
GADT :: a -> Fam a -> GADT (Fam a)
unwrap :: GADT (Fam a) -> (a, Fam a)
unwrap (GADT x y) = (x, y)
}}}
then typechecking `unwrap` should certainly fail. And it does, but with a
horrible message:
{{{
Main.hs:9:21:
Couldn't match expected type `a' against inferred type `a1'
`a' is a rigid type variable bound by
the type signature for `unwrap' at Main.hs:8:20
`a1' is a rigid type variable bound by
the constructor `GADT' at Main.hs:9:8
In the expression: x
In the expression: (x, y)
In the definition of `unwrap': unwrap (GADT x y) = (x, y)
}}}
It would be better to say something more like:
{{{
Cannot deduce (a ~ a1) from (Fam a ~ Fam a1)
}}}
See the thread at
[http://thread.gmane.org/gmane.comp.lang.haskell.cafe/62322]
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3440>
GHC <http://www.haskell.org/ghc/>
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