Re: [GHC] #12069: Possible type inference regression: "Expected a type, but 'a' has kind 'k0'"

13 Jun 2016

      #12069: Possible type inference regression: "Expected a type, but 'a' has kind
'k0'"
-------------------------------------+-------------------------------------
        Reporter:  akio              |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"15b9bf4ba4ab47e6809bf2b3b36ec16e502aea72/ghc"
 15b9bf4b/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="15b9bf4ba4ab47e6809bf2b3b36ec16e502aea72"
 Improve typechecking of let-bindings

 This major commit was initially triggered by #11339, but it spiraled
 into a major review of the way in which type signatures for bindings
 are handled, especially partial type signatures.  On the way I fixed a
 number of other bugs, namely
    #12069
    #12033
    #11700
    #11339
    #11670

 The main change is that I completely reorganised the way in which type
 signatures in bindings are handled. The new story is in TcSigs
 Note [Overview of type signatures].  Some specific:

 * Changes in the data types for signatures in TcRnTypes:
   TcIdSigInfo and new TcIdSigInst

 * New module TcSigs deals with typechecking type signatures
   and pragmas. It contains code mostly moved from TcBinds,
   which is already too big

 * HsTypes: I swapped the nesting of HsWildCardBndrs
   and HsImplicitBndsrs, so that the wildcards are on the
   oustide not the insidde in a LHsSigWcType.  This is just
   a matter of convenient, nothing deep.

 There are a host of other changes as knock-on effects, and
 it all took FAR longer than I anticipated :-).  But it is
 a significant improvement, I think.

 Lots of error messages changed slightly, some just variants but
 some modest improvements.

 New tests

 * typecheck/should_compile
     * SigTyVars: a scoped-tyvar test
     * ExPat, ExPatFail: existential pattern bindings
     * T12069
     * T11700
     * T11339

 * partial-sigs/should_compile
     * T12033
     * T11339a
     * T11670

 One thing to check:

 * Small change to output from ghc-api/landmines.
   Need to check with Alan Zimmerman
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12069#comment:1>
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