[GHC] #12670: Representation polymorphism validity check is too strict

[GHC]

#12670: Representation polymorphism validity check is too strict
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  typeable
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by bgamari):

 The patch that Simon was referring to in comment:4 was
 eefe86d96d40697707c3ddfb9973a30a1897241f. Unfortunately this doesn't fix
 the issue which described by this ticket; the typechecker still rejects
 the program given in the ticket description.

 Richard and I discussed this a few weeks ago and came to a solution which
 looks something like this,
 {{{#!patch
 diff --git a/compiler/typecheck/TcPatSyn.hs
 b/compiler/typecheck/TcPatSyn.hs
 index 5c62121..62724c8 100644
 --- a/compiler/typecheck/TcPatSyn.hs
 +++ b/compiler/typecheck/TcPatSyn.hs
 @@ -13,6 +13,7 @@ module TcPatSyn ( tcInferPatSynDecl, tcCheckPatSynDecl
    ) where

  import HsSyn
 +import TcHsSyn( checkForRepresentationPolymorphism )
  import TcPat
  import Type( mkTyVarBinders, mkEmptyTCvSubst
             , tidyTyVarBinders, tidyTypes, tidyType )
 @@ -312,6 +313,9 @@ tc_patsyn_finish lname dir is_infix lpat'
               arg_tys    = tidyTypes env2 arg_tys'
               pat_ty     = tidyType  env2 pat_ty'

 +       -- Check that the arguments aren't representationally polymorphic
 +       ; mapM_ (checkForRepresentationPolymorphism empty) arg_tys
 +
         ; traceTc "tc_patsyn_finish {" $
             ppr (unLoc lname) $$ ppr (unLoc lpat') $$
             ppr (univ_tvs, req_theta, req_ev_binds, req_dicts) $$
 diff --git a/compiler/typecheck/TcTyClsDecls.hs
 b/compiler/typecheck/TcTyClsDecls.hs
 index c009bc9..9335676 100644
 --- a/compiler/typecheck/TcTyClsDecls.hs
 +++ b/compiler/typecheck/TcTyClsDecls.hs
 @@ -2296,6 +2296,9 @@ checkValidDataCon dflags existential_ok tc con
            -- Check all argument types for validity
          ; checkValidType ctxt (dataConUserType con)

 +          -- Check for representationally polymorphic fields
 +        ; mapM_ (checkForRepresentationPolymorphism empty)
 (dataConOrigArgTys con)
 +
            -- Extra checks for newtype data constructors
          ; when (isNewTyCon tc) (checkNewDataCon con)

 diff --git a/compiler/typecheck/TcValidity.hs
 b/compiler/typecheck/TcValidity.hs
 index 6cc40a5..c9f3bda 100644
 --- a/compiler/typecheck/TcValidity.hs
 +++ b/compiler/typecheck/TcValidity.hs
 @@ -487,8 +487,6 @@ check_type _ _ _ (TyVarTy _) = return ()

  check_type env ctxt rank (FunTy arg_ty res_ty)
    = do  { check_type env ctxt arg_rank arg_ty
 -        ; when (representationPolymorphismForbidden ctxt) $
 -          checkForRepresentationPolymorphism empty arg_ty
          ; check_type env ctxt res_rank res_ty }
    where
      (arg_rank, res_rank) = funArgResRank rank
 }}}

 Unfortunately this still doesn't fix the Core Lint issues that inevitably
 pop up when you try to use such a type.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12670#comment:7>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler