[GHC] #11032: Missing result type handling for State# s in foreign import prim.

28 Oct 2015

      #11032: Missing result type handling for State# s in foreign import prim.
-------------------------------------+-------------------------------------
           Reporter:  ekmett         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
  (FFI)                              |
           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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 I've been using `foreign import prim` for a while now to craft custom
 primops on an as-needed basis.

 Attempting to write

 {{{#!hs
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE UnboxedTuples #-}
 {-# LANGUAGE UnliftedFFITypes #-}
 {-# LANGUAGE GHCForeignImportPrim #-}
 {-# LANGUAGE ForeignFunctionInterface #-}

 foreign import prim "pinThreadzh"   pinThread#   :: State# s -> (# State#
 s, Int#, Int# #)
 foreign import prim "unpinThreadzh" unpinThread# :: State# s -> State# s
 }}}

 threw me for a loop when it didn't work.

 The former works fine. The latter complains that `State# s` isn't a valid
 result type!

 {{{
 src/Concurrent/Internal/Thread.hs:23:1:
     Unacceptable result type in foreign declaration:
       ‘State# s’ cannot be marshalled in a foreign call
     When checking declaration:
       foreign import prim safe "static unpinThreadzh" unpinThread#
         :: State# s -> State# s
 }}}

 Builtin primitives work this way all the time, so I'm guessing we just
 missed it in a case statement somewhere inside the compiler when dealing
 with `foreign import prim` support.

 I can likely hack around it by giving back something like `(# State# s #)`
 for now but that is a rather unsatisfying solution.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11032
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