[GHC] #14387: listToMaybe doesn't participate in foldr/build fusion

[GHC]

#14387: listToMaybe doesn't participate in foldr/build fusion
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        Reporter:  duog              |                Owner:  duog
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  8.2.1
      Resolution:                    |             Keywords:
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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Changes (by duog):

 * owner:  (none) => duog


Comment:

 Replying to [comment:2 simonpj]:

 Thank you very much for your explanations.

 > Note the join point.  But `findIndex4` and `findIndex` do not.  Maybe
 that was a typo.
 Yes it was, sorry about that.

 >
 > > Having a join point means constant stack space, not having a join
 point means linear stack space.
 >
 > This definitely isn't true.  Here's `findIndex` (the one that does not
 get a join point):
 > In the `False` branch inside `go`, there's a tail-call to `go` (no stack
 growth). In the `True` branch, `go` returns a cons-cell, and stops.  Then
 the `case go x o# of ...` scrutinises that cons cell and returns a `Just`.
 All done. No stack growth in either case.
 Ah I see, I didn't understand that tail-calls to let bindings worked like
 that. I will examine the Notes on join points to try to understand the
 difference between calling a join point and tail-calling a let binding; I
 guess that the tail-call is a bit more expensive because the let binding
 has additional code for the case when it is not tail-called.

 >
 > > it seems that changing the definition of listToMaybe to use `foldr`
 would be a win
 >
 > Yes I agree.  It's a bit like making `map` use `foldr` rather than being
 written directly.  But you'd need an INLINE pragma on it.
 >
 > I can't see a downside.  If it doesn't fuse you get the original
 `listToMaybe` back instead.   If someone makes that change (in `base`) do
 add a Note to explain.

 I will prepare a patch.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14387#comment:3>
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