[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
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 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.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13977>
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