#3592: Eta-contraction gives a rather bogus type error message
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Reporter: guest | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone: 7.4.1
Component: Compiler (Type checker) | Version: 6.10.3
Keywords: | Testcase:
Blockedby: | Difficulty: Unknown
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by simonpj):
For this module
{{{
{-# LANGUAGE RankNTypes #-}
module T3592 where
type T a = Show a => a
f :: T a -> String
f = show
g :: T a -> String
g x = show x
}}}
We now get
{{{
T3592.hs:7:5: -- Eta contracted version
No instance for (Show (T a))
arising from a use of `show'
Possible fix: add an instance declaration for (Show (T a))
In the expression: show
In an equation for `f': f = show
T3592.hs:10:12: -- Not eta contracted
No instance for (Show a)
arising from a use of `x'
In the first argument of `show', namely `x'
In the expression: show x
In an equation for `g': g x = show x
}}}
So the error messages are slightly different, because of the implicit
instantiation of 'x' when it is explicit, but I think both are reasonable.
I think I'll declare this one done.
I'll add a test.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3592#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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