[GHC] #10505: more specific types in the generated *_stub.h files

[GHC]

#10505: more specific types in the generated *_stub.h files
-------------------------------------+-------------------------------------
              Reporter:  imz         |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:
              Priority:  normal      |           Version:  7.6.1
             Component:  Compiler    |  Operating System:  Linux
  (FFI)                              |   Type of failure:  Other
              Keywords:              |        Blocked By:
          Architecture:              |   Related Tickets:  #2979, #8222
  Unknown/Multiple                   |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 Having a declaration like:

 {{{#!hs
 foreign export ccall "respond" respond :: CWString -> IO CWString
 }}}

 all the types in the corresponding `_stub.h` are `HsPtr`.

 Wouldn't it be possible to write more specific types reflecting my
 expectations, i.e., `wchar_t *`?

 It would be nice in order to signal automatically to the C code which uses
 the Haskell functions that the types of the Haskell functions changed. In
 simple cases, one could simply include the stub.h, and get automatically
 an error if the C code doesn't match the new types.

 I've tried using a CTYPE pragma (cf. #2979 , #8222 ) on a newtype to get
 such results:

 {{{#!hs
 newtype {-# CTYPE "wchar_t *" #-} CWString' = CWString' CWString
 foreign export ccall "respond" respond :: CWString'  -> IO CWString
 }}}

 but that still produces `HsPtr`. (Well, I see, the CTYPE pragma was mostly
 intended for imports rather than exports.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10505>
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