funny type inference error with ghc7.6rc1

Simon Peyton-Jones simonpj at
Mon Aug 20 10:44:06 CEST 2012

That does seem odd.  Can you give instructions for how to reproduce this?  The fewer dependencies the better :-)


From: glasgow-haskell-users-bounces at [mailto:glasgow-haskell-users-bounces at] On Behalf Of Carter Schonwald
Sent: 17 August 2012 19:01
To: GHC Users List
Subject: funny type inference error with ghc7.6rc1

Hey All,

When playing with the current hackage versions of Epic and Idris to make them play nice with ghc7.6rc1
and    (current version on github now builds on ghc 7.6,

I ran into some funny type inference problems. Namely, using the idris-  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 variables!

thanks all,
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