[GHC] #10744: Allow oneShot to work with unboxed types

[GHC]

#10744: Allow oneShot to work with unboxed types
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        Reporter:  akio              |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by simonpj):

 I think it'd be fine to allow `oneShot` to accept unboxed types; just a
 matter of using `openAlphaTyVar` etc in `MkId.oneShot`.  Give it a try and
 submit a patch.

 Moreover, the worst that can happen if you mis-use it, by applying it to a
 function that is called many times, is that computation may be repeated.
 For example
 {{{
 f xs y = let x = expensive_fn y
          in map (oneShot (\v. x+v)) xs
 }}}
 The `oneShot` claims (falsely unless `xs` has length at most one) that
 `(\v. x+v)` is called at most once. So GHC will move the call to
 `expensive_fn` inside the lambda thus:
 {{{
 f xs y = map (oneShot (\v. expensive_fn y + v)) xs
 }}}
 That will work just fine, but it'll call `expensive_fn` once per element
 of `xs`.

 So, no seg-faults at least!

 It would be good if someone felt like extending the Haddock comments for
 `GHC.Magic.oneShot` to explain this.

 Thanks

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10744#comment:8>
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