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

[GHC]

#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,
> {{{
> <xC(S),1*C1(U)><L,1*C1(U)><L,U>
> }}}
> 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:
> {{{
> <C(S),1*C1(U)>
> }}}
>
> Not 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 :: 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,
 {{{
 <xC(S),1*C1(U)><L,1*C1(U)><L,U>
 }}}
 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:
 {{{
 <C(S),1*C1(U)>
 }}}

 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:2>
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