Re: [GHC] #1311: newtypes of unboxed types disallowed - documentation bug and/or feature request

22 Dec 2015

      #1311: newtypes of unboxed types disallowed - documentation bug and/or feature
request
-------------------------------------+-------------------------------------
        Reporter:  Isaac Dupree      |                Owner:  osa1
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:  ⊥
       Component:  Compiler          |              Version:  7.7
      Resolution:                    |             Keywords:
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 osa1):

 So I think this turned out to be more complicated than we first thought
 (as usual). Richard, could you tell me if I'm going in the right
 direction? Here's what I did so far:

 {{{#!diff
 --- a/compiler/typecheck/TcTyClsDecls.hs
 +++ b/compiler/typecheck/TcTyClsDecls.hs
 @@ -1490,7 +1490,7 @@ tcGadtSigType doc name ty@(HsIB { hsib_vars = vars
 })
                tcHsTyVarBndrs gtvs $ \ _ ->
                do { ctxt <- tcHsContext cxt
                   ; btys <- tcConArgs DataType hs_details
 -                 ; ty' <- tcHsLiftedType res_ty
 +                 ; ty' <- tcHsOpenType res_ty
                   ; field_lbls <- lookupConstructorFields name
                   ; let (arg_tys, stricts) = unzip btys
                         bound_vars = allBoundVariabless ctxt `unionVarSet`
 }}}

 Second change:

 {{{#!diff
 --- a/compiler/typecheck/TcHsType.hs
 +++ b/compiler/typecheck/TcHsType.hs
 @@ -1862,8 +1862,7 @@ tcDataKindSig :: Kind -> TcM [TyVar]
  -- We use it also to make up argument type variables for for data
 instances.
  -- Never emits constraints.
  tcDataKindSig kind
 -  = do  { checkTc (isLiftedTypeKind res_kind) (badKindSig kind)
 -        ; span <- getSrcSpanM
 +  = do  { span <- getSrcSpanM
          ; us   <- newUniqueSupply
          ; rdr_env <- getLocalRdrEnv
          ; let uniqs = uniqsFromSupply us
 @@ -1880,11 +1879,6 @@ tcDataKindSig kind
      mk_tv loc uniq occ kind
        = mkTyVar (mkInternalName uniq occ loc) kind

 -badKindSig :: Kind -> SDoc
 -badKindSig kind
 - = hang (ptext (sLit "Kind signature on data type declaration has non-*
 return kind"))
 -        2 (ppr kind)
 -
  {-
  Note [Avoid name clashes for associated data types]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  }}}

 Those are all for accepting this program:

 {{{#!haskell
 {-# LANGUAGE MagicHash, KindSignatures, GADTs #-}

 module Main where

 import GHC.Types
 import GHC.Prim
 import GHC.Exts

 newtype Blah :: TYPE 'Unlifted where
   Blah :: Int# -> Blah

 main = return ()
 }}}

 For now I'm not trying to infer unlifted types, I'm trying to make cases
 with explicit kind signatures working.

 With these changes, GHC is failing with this panic:

 {{{
 ghc-stage1: panic! (the 'impossible' happened)
   (GHC version 7.11.20151222 for x86_64-unknown-linux):
         ASSERT failed! file compiler/typecheck/TcTyClsDecls.hs, line 1630
 }}}

 This is because we have some code that assume result types of ... I think
 GADTs? ... are lifted. (I think newtypes defined this way are type checked
 using GADT type checker?)

 Am I doing this right? How should I proceed after this point? I feel like
 I shouldn't lift these restrictions and instead I need some special cases
 for newtypes. Am I right?

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/1311#comment:12
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