#5910: Holes with other constraints
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Reporter: xnyhps | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.5
Keywords: holes | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: Incorrect warning at
compile-time
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by dreixel):
You might have to treat ambiguity errors in a special way. They can still
be deferred with `-fdefer-type-errors`, and the error message adequately
says there are two possible instances:
{{{
No instance for (Show (Foo a0)) arising from a use of `show'
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
instance Show (Foo Bar1) -- Defined at Holes.hs:7:11
instance Show (Foo Bar2) -- Defined at Holes.hs:8:11
Possible fix: add an instance declaration for (Show (Foo a0))
In the expression: show (Foo undefined)
In an equation for `test': test = show (Foo undefined)
}}}
So maybe you could use this in the feedback for holes with an ambiguous
type.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5910#comment:10>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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