FoldrW/buildW issues

[Joachim Breitner]

Hi,


Am Mittwoch, den 24.09.2014, 16:37 -0400 schrieb David Feuer:
> On Sep 12, 2014 2:35 PM, "Joachim Breitner" <mail at joachim-breitner.de>
> wrote:
> > I once experimented with a magic "oneShot :: (a -> b) -> (a -> b)"
> > function, semantically the identity, but tell the compiler not to
> share
> > the result of the computation. Using that in the definition of
> > foldl-as-foldr, one can get the same effect as Call Arity, but a bit
> > more reliable. I need to investigate if that solves the
> sumConcatInits
> > problem.
> 
> One nice thing about this idea (which sounds like it must be related
> to the "state hack", but is more explicit) is that it presumably
> applies also to similar situations in the State and ST monads, when a
> state transformer is only used once. Could you explain, perhaps, what
> compiler transformation this enables, and how you implemented it?

I guess it is used in various places, and I woudn’t know all of them.
The one that I was aiming for was is this one

  case x of
    True -> z
    False -> \s(one-shot). e

to

  \s(one-shot) . case x of
    True -> z s
    False -> e

explained in Note [Combining case branches] at
https://github.com/ghc/ghc/blob/master/compiler/coreSyn/CoreArity.lhs#L671

The implementation is attached to my previous mail.


>  It would be nice if the compiler could figure this out for itself,

Well, that’s what I thought, and CallArity is what I came up with :-)

Greetings,
Joachim

-- 
Joachim “nomeata” Breitner
  mail at joachim-breitner.de • http://www.joachim-breitner.de/
  Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0xF0FBF51F
  Debian Developer: nomeata at debian.org

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