[GHC] #10867: Primop types should not mention ()

[GHC]

#10867: Primop types should not mention ()
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              Reporter:  simonpj     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 I've just noticed that two primpops have types that mention tuples (see
 `compiler/prelude/primops.txt.pp`):
 {{{
 primop  FinalizeWeakOp "finalizeWeak#" GenPrimOp
    Weak# a -> State# RealWorld -> (# State# RealWorld, Int#,
               (State# RealWorld -> (# State# RealWorld, () #)) #)


 primop  Check "check#" GenPrimOp
       (State# RealWorld -> (# State# RealWorld, a #) )
    -> (State# RealWorld -> (# State# RealWorld, () #) )
 }}}
 But they really really shouldn't.  The unit tuple is a type defined in
 Haskell, and it makes an awkward cycle if the supposedly-truly-primitive
 primpops refer to it.  (Discovered during the saga of [Phab:D757].)

 Most primpops do not do this; eg
 {{{
 primop  WriteMutVarOp "writeMutVar#"  GenPrimOp
    MutVar# s a -> a -> State# s -> State# s
 }}}

 Now `check#` and `finalizeWeak#` are pretty obscure primops and I don't
 think anyone will mind if I fix their type signatures.

 But in addition to doing that, I'll have to look at their implementations.
 '''But where ''are'' those implementations??'''  I can't find anything in
 the RTS.

 While thinking about it
  *  Both come with literally zero Haddock documentation which is terrible.
 The type of both is pretty obscure.
  * `check#` is a very short name for very specialised STM operation.
 Surely we should call it `addSTMInvariant` or something like that?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10867>
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