Re: [GHC] #13977: ExnStr doesn't propagate "outwards"

18 Jul 2017

      #13977: ExnStr doesn't propagate "outwards"
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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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Description changed by bgamari:

Old description:
...
While looking into #8091 (see ticket:8091#comment:9) and #13916, I
noticed something that I found rather surprising. Consider this program,
{{{#!hs
hello :: STR Char -> STM Char
hello a = a `orElse` pure 'a'
}}}
Recall that `catchRetry#` (which `orElse` is defined in terms of) has the
following demand signature,
{{{

}}}
Note the `x` here: this means it is `ExnStr`, since it doesn't bottom
even if the first argument does (despite the fact that it's strict).
So, the question is this: What demand should `hello` place on its first
argument? I would have thought that it should be precisely the same as
the demand that `catchRetry#` places on **its** first argument. However,
the demand analyser seems to conclude this:
{{{

}}}
Note the lack of an `x`. I believe this may be what causes #13916.
New description:

 While looking into #8091 (see ticket:8091#comment:9) and #13916, I noticed
 something that I found rather surprising. Consider this program,

 {{{#!hs
 hello :: STM Char -> STM Char
 hello a = a `orElse` pure 'a'
 }}}

 Recall that `catchRetry#` (which `orElse` is defined in terms of) has the
 following demand signature,
 {{{

 }}}
 Note the `x` here: this means it is `ExnStr`, since it doesn't bottom even
 if the first argument does (despite the fact that it's strict).

 So, the question is this: What demand should `hello` place on its first
 argument? I would have thought that it should be precisely the same as the
 demand that `catchRetry#` places on **its** first argument. However, the
 demand analyser seems to conclude this:
 {{{

 }}}

 Note the lack of an `x`. I believe this may be what causes #13916.

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13977#comment:3
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