[GHC] #7398: RULES don't apply to a newtype constructor

[GHC]

#7398: RULES don't apply to a newtype constructor
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        Reporter:  shachaf           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  7.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #6082, #10418,    |  Differential Rev(s):
  #13290                             |
       Wiki Page:                    |
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Comment (by simonpj):

 Currently, wen you have
 {{{
 {-# RULES "foo" forall x. f (g x) = h x #-}
 }}}
 we have to delay inlining `g` until the rule has had a decent
 chance to fire.  We currently do this manually, usually thus
 {{{
 {-# NOINLINE[1] f,g #-}
 }}}
 The obvious thing for newtypes (and indeed other data constructors, if
 they have wrappers that unpack their arguments) is to delay inlining them.
 Something like
 {{{
 newtype T a = MkT (Maybe a)
 {-# NOINLINE[1] MkT #-}
 }}}
 We don't support that right now, but we could.

 The alternative is to try to make the rule work ''after'' inlining `MkT`,
 by being clever about casts.  That might be possible.

 But it doesn't work for data constructors like
 {{{
 data S = MkS {-# UNPACK #-} !Int
 }}}
 where the wrapper evaluates and unboxes the argument.  After inlining, the
 original `(f (MkS x))` turns into `f (case x ov I# y -> MkS y)`, which is
 a lot harder to match.

 So I think my suggestion, for now, is that we might want to allow users to
 put a NOINLINE pragma on data constructors.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7398#comment:15>
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