[GHC] #393: functions without implementations

[GHC]

#393: functions without implementations
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        Reporter:  c_maeder          |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Compiler (Type    |              Version:  None
  checker)                           |
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Probably not worth it, but fake definitions are generated for
 Commentary/PrimOps

 {{{#!hs
 newArray# :: Int# -> a -> State# s -> (# State# s,MutableArray# s a #)
 newArray# = let x = x in x

 sameMutableArray# :: MutableArray# s a -> MutableArray# s a -> Int#
 sameMutableArray# = let x = x in x

 nullAddr# :: Addr#
 nullAddr# = let x = x in x
 }}}

 If this feature is enabled the bodies could be omitted and if we make
 holes representation-polymorphic (#12531) it's not such a reach to allow
 top-level unlifted bindings as well when body is omitted:

 {{{#!hs
 newArray# :: Int# -> a -> State# s -> (# State# s,MutableArray# s a #)

 sameMutableArray# :: MutableArray# s a -> MutableArray# s a -> Int#

 nullAddr# :: NullAddr#
 }}}

 This might avoid hacky tests mentioned in `[Compiling GHC.Prim]`.

 ----

 === Note `[Placeholder declarations]` ===
 > We are generating fake declarations for things in
 [https://hackage.haskell.org/package/ghc-prim-0.4.0.0/candidate/docs/src
 /GHC-Prim.html GHC.Prim], just to keep GHC's renamer and typechecker happy
 enough for what Haddock needs.  Our main plan is to say
 > {{{#!hs
 > foo :: <type>
 > foo = foo
 > }}}
 > We have to silence GHC's complaints about unboxed-top-level declarations
 with an ad-hoc fix in `TcBinds`: see Note `[Compiling GHC.Prim]` in
 `TcBinds`.
 >
 >
 > That works for all the primitive functions except `tagToEnum#`. If we
 generate the binding
 > {{{#!hs
 > tagToEnum# = tagToEnum#
 > }}}
 > GHC will complain about "`tagToEnum#` must appear applied to one
 argument". We could hack GHC to silence this complaint when compiling
 `GHC.Prim`, but it seems easier to generate
 > {{{#!hs
 > tagToEnum# = let x = x in x
 > }}}
 > We don't do this for *all* bindings because for ones with an unboxed RHS
 we would get other complaints (e.g.can't unify "`*`" with "`#`").

 === Note `[Compiling GHC.Prim]` ===
 > Module `GHC.Prim` has no source code: it is the host module for
 primitive, built-in functions and types.  However, for Haddock-ing
 purposes we generate (via `utils/genprimopcode`) a fake source file
 `GHC/Prim.hs`, and give it to Haddock, so that it can generate
 documentation.  It contains definitions like
 > {{{#!hs
 > nullAddr# :: NullAddr#
 > }}}
 > which would normally be rejected as a top-level unlifted binding. But we
 don't want to complain, because we are only "compiling" this fake mdule
 [sic] for documentation purposes.  Hence this hacky test for `gHC_PRIM` in
 `checkStrictBinds`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/393#comment:35>
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