[GHC] #10418: Incorrect RULE warning on constructor, and inablilty to {-# INLINE [0] #-} constrcutor

14 May 2015

      #10418: Incorrect RULE warning on constructor, and inablilty to {-# INLINE [0] #-}
constrcutor
-------------------------------------+-------------------------------------
              Reporter:  duncan      |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 This is mentioned in passing in ticket #10417, but pulling it out here to
 track it.

 Given a type
 {{{#!hs
 data Decoder s s' where
   Done     :: Decoder s s
   Sequence :: Decoder a b -> Decoder b c -> Decoder a c
 }}}

 and rules:

 {{{#!hs
 {-# RULES "Sequence/Done"
     forall sa. Sequence Done sa = sa
   #-}

 {-# RULES "Sequence/Sequence"
     forall a b c. Sequence (Sequence a b) c
                 = Sequence a (Sequence b c)
   #-}
 }}}
 GHC gives us warnings like:
 {{{

     Rule "Sequence/Done" may never fire
       because ‘RulesExample.Sequence’ might inline first
     Probable fix: add an INLINE[n] or NOINLINE[n] pragma on
 ‘RulesExample.Sequence’
 }}}
 but it's currently syntactically invalid to write such a pragma:
 {{{#!hs
 {-# INLINE [0] Done #-}
 {-# INLINE [0] Sequence #-}
 }}}
 Now there's two problems here:

  1. This warning is wrong most of the time
  2. The warning is actually right sometimes, but there's nothing you can
 do about it.

 For 1, most of the time constructors remain as themselves in core, with no
 wrappers to get in the way of rule matching. In these cases the warning is
 just wrong.

 For 2, for some constructors ghc does rewrite them. In this case the
 rewrite rule really
 will fail to match. But we cannot tell GHC to delay the rewriting of them
 because we cannot write INLINE pragmas for them. So for this case it would
 be useful to allow us to write INLINE pragmas.

 In the above example, GHC will rewrite `Done` during optimisation, e.g. an
 excerpt from ticket #10417:
 {{{
 $WDone = \ @ s_anC -> Done @~ <s_anC>_N

 working_example2 =
   \ @ b_aw2 -> AlterStack (working_example3) ($WDone)
 }}}
 This can and will interfere with rule matching. In this case it would be
 desirable to be able to write
 {{{
 {-# INLINE [0] Done #-}
 }}}
 Currently that's a syntax error.

 For the case of nullary constructors, a workaround is to consistently use
 a smart constructor with an `INLINE [0]` pragma, and do all the rule
 matching on that.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10418>
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