Re: [GHC] #13680: Can't use TypeApplications with empty list expression

10 May 2017

      #13680: Can't use TypeApplications with empty list expression
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:                    |  TypeApplications
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 RyanGlScott):

 A have a (very slight) hunch what might be happening here. `[]` is wired
 in to GHC
 [http://git.haskell.org/ghc.git/blob/4d9167b087abd2f4dad4ccfaba7bbde177fd2797...
 here]:

 {{{#!hs
 nilDataCon :: DataCon
 nilDataCon  = pcDataCon nilDataConName alpha_tyvar [] listTyCon
 }}}

 And if you follow the definition of `pcDataCon` deep enough, you'll
 discover it eventually calls
 [http://git.haskell.org/ghc.git/blob/4d9167b087abd2f4dad4ccfaba7bbde177fd2797...
 this code]:

 {{{#!hs
 pcDataConWithFixity' :: Bool -> Name -> Unique -> RuntimeRepInfo
                      -> [TyVar] -> [TyVar]
                      -> [Type] -> TyCon -> DataCon
 -- The Name should be in the DataName name space; it's the name
 -- of the DataCon itself.

 pcDataConWithFixity' declared_infix dc_name wrk_key rri tyvars ex_tyvars
 arg_tys tycon
   = data_con
   where
     data_con = mkDataCon dc_name declared_infix prom_info
                 (map (const no_bang) arg_tys)
                 []      -- No labelled fields
                 (mkTyVarBinders Specified tyvars)
                 (mkTyVarBinders Specified ex_tyvars)
                 []      -- No equality spec
                 []      -- No theta
                 arg_tys (mkTyConApp tycon (mkTyVarTys tyvars))
                 rri
                 tycon
                 []      -- No stupid theta
                 (mkDataConWorkId wrk_name data_con)
                 NoDataConRep    -- Wired-in types are too simple to need
 wrappers

     no_bang = HsSrcBang NoSourceText NoSrcUnpack NoSrcStrict

     wrk_name = mkDataConWorkerName data_con wrk_key

     prom_info = mkPrelTyConRepName dc_name
 }}}

 The use of `Specified` is a tad suspicious. I tried changing it to
 `Required`, but unfortunately, the bug still persists after this. So there
 must be something else that explains this behavior, but it eludes me for
 now...

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13680#comment:4
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