#7173: Unnecessary constraints in inferred type
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Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.2
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Carter Schonwald reports: when playing with the current hackage versions
of Epic and Idris to make them play nice with ghc7.6rc1
* http://hackage.haskell.org/package/idris-0.9.2.1
*http://hackage.haskell.org/package/epic-0.9.3 (current version on github
now builds on ghc 7.6, https://github.com/edwinb/EpiVM)
I ran into some funny type inference problems. Namely, using the
idris-0.9.2.1 source and iteratively seeing how ghc complains,
I repeated found that ghc would infer extraneous class constraints with
variables that don't appear in the function type!
eg `(Num a, Ord a) => PArg -> Doc`, when the correct type to infer would
be `PArg -> Doc`.
Here are some gists with links to more info
https://gist.github.com/3365312
https://gist.github.com/3365073
https://gist.github.com/3364775
Anyways, I'm not sure what to make of this, is this a reasonable artifact
of type inference getting confused on functions with a large number of
case analyses when various typeclass extensions are enabled? Or Is this a
bug in terms of what inference should be able to handle?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7173>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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