#3592: Eta-contraction gives a rather bogus type error message
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Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.10.3
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Os: MacOS X
Architecture: x86 |
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Changes (by simonpj):
* difficulty: => Unknown
* summary: ghc: panic! on overloaded type synonym => Eta-contraction
gives a rather bogus type error message
Comment:
With the HEAD (and I think 6.12) we get
{{{
T3592.hs:7:4:
Could not deduce (Show (T a)) from the context ()
arising from a use of `show' at T3592.hs:7:4-7
Possible fix:
add (Show (T a)) to the context of the type signature for `f'
or add an instance declaration for (Show (T a))
In the expression: show
In the definition of `f': f = show
}}}
which at least is not a crash. So that has already been fixed.
Your type signature means this
{{{
f :: forall a. (Show a => a) -> String
}}}
If we eta-expand the definition of f to
{{{
f x = show x
}}}
then we get the totally reasonable error message:
{{{
T3592.hs:7:11:
Could not deduce (Show a) from the context ()
arising from a use of `x' at T3592.hs:7:11
Possible fix:
add (Show a) to the context of the type signature for `f'
In the first argument of `show', namely `x'
In the expression: show x
In the definition of `f': f x = show x
}}}
I think we should get the same error from the eta-contracted version, so
I'm going to re-title this bug report, and put it in my list for the type
inference overhaul.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3592#comment:1>
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