funny type inference error with ghc7.6rc1
carter.schonwald at gmail.com
Fri Aug 17 20:00:32 CEST 2012
When playing with the current hackage versions of Epic and Idris to make
them play nice with ghc7.6rc1
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.
heres some gists with links to more info
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?
Just to be clear, when I add the infererred type ascriptions without the
type class constraint, everything type checks in those modules. So my
confusion is why the inference adding those unused class constraint
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