[GHC] #10460: Allow foreign prim to return Any

[GHC]

#10460: Allow foreign prim to return Any
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        Reporter:  ezyang            |                   Owner:  ezyang
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler (Type    |                 Version:  7.11
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |                Blocking:
      Blocked By:                    |  Differential Revisions:
 Related Tickets:                    |
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Comment (by simonpj):

 Generally ok but

  * The ability to pass `Any` to `foreign import prim` is not documented in
 [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ffi.html
 #ffi-prim 8.1.3 of the user manual], and it jolly well should be.  If you
 want to extend this to arbitrary FFI imports then we should document that
 too.

  * Who converts the argument to `Any`? The caller?   With `unsafeCoerce`?

  * You should never combine a '''safe''' foreign import with `Any`. "Safe"
 means that the call may block and GC may happen, so passing a pointer into
 the heap is a bad idea.  (And this constraint should be documented.)

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10460#comment:2>
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