#1608: Newtype deriving error messages coming out too late
--------------------------------------+-------------------------------------
Reporter: simonpj | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone: 6.10
Component: Compiler (Type checker) | Version: 6.6.1
Severity: normal | Keywords:
Difficulty: Unknown | Os: Unknown
Testcase: | Architecture: Unknown
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Consider this
{{{
newtype Ego a = Ego a deriving (Ord)
f :: Ord a => Ego a -> Ego a -> Bool
f e1 e2 = e1 < e2
}}}
GHC 6.7 reports an error like this:
{{{
Could not deduce (Eq (Ego a)) from the context (Ord a)
arising from a use of `<' at Foo10.hs:6:10-16
In the expression: e1 < e2
In the definition of `f': f e1 e2 = e1 < e2
}}}
This error should have come when you gave the instance decl. The reason
is that GHC's newtype deriving is generating an instance like
{{{
instance Eq (Ego a) => Ord (Ego a)
}}}
It would make more sense to report the missing instance for Eq right
there, rather than abstracting over it.
Thanks to Dougal Stanton for pointing this out:
http://www.haskell.org/pipermail/haskell-cafe/2007-August/030224.html
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1608>
GHC <http://www.haskell.org/ghc/>
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