[GHC] #13380: raiseIO# result looks wrong

[GHC]

#13380: raiseIO# result looks wrong
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           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 `primops.txt.pp` includes the following note:

 > `raiseIO#` needs to be a primop, because exceptions in the `IO` monad
 must be ''precise'' - we don't want the strictness analyser turning one
 kind of bottom into another, as it is allowed to do in pure code.
 >
 > But we ''do'' want to know that it returns bottom after being applied to
 two arguments, so that this function is strict in `y`
 > {{{#!hs
 > f x y | x>0       = raiseIO blah
 >       | y>0       = return 1
 >       | otherwise = return 2
 > }}}

 I believe the "But we do" portion is, unfortunately, entirely wrong. From
 a user perspective, it is extremely strange to replace a precise exception
 with an imprecise one. That is, I strongly believe the above code should
 be considered ''lazy'' in `y`. The function `f` calculates an `IO` action;
 the value of `y` is only necessary to calculate that action if `x` is non-
 positive.

 We can, conservatively, say for certain that the second component of the
 result of `raiseIO#` (i.e., the "return value") is bottom. We can also
 say, given

 {{{#!hs
 case raiseIO# e s of
   (# s', r #) -> q
 }}}

 that `q` is dead (we could replace it by `(# s', undefined #)`).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13380>
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