[GHC] #14806: Officially sanction certain unsafeCoerce applications with unboxed unary tuples

[GHC]

#14806: Officially sanction certain unsafeCoerce applications with unboxed unary
tuples
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Documentation     |              Version:  8.2.2
      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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Comment (by dfeuer):

 Consider

 {{{#!hs
 adjust, adjust' :: (a -> a) -> Seq a -> Seq a
 }}}

 The lazy version can leak memory, but it's been around for years so we
 need to support it. The study version is efficient. Both versions can be
 defined using a more general operation:

 {{{#!hs
 adjust# :: (a -> (# a #)) -> Int -> Seq a -> Seq a
 adjust f = adjust# (\x -> (# f x #))
 adjust' f = adjust# (\x -> let !x' = f x in (# x' #))
 }}}

 `adjust#` allows the given function to perform as much work as it wants,
 then suspend as much further work as it wants. The definitions of `adjust`
 and `adjust'` above add a closure allocation and a bit of indirection over
 the equivalent direct definitions, unless we inline and duplicate code.
 For `adjust`, which already leaks memory, that's tolerable. For `adjust'`,
 it's not. What can we do? Define

 {{{#!hs
 adjust' f = adjust# (unsafeCoerce f)
 }}}

 That should just work. Since the result of applying `f` is interested by
 `adjust#` as an unboxed unary tuple, it gets forced.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14806#comment:4>
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