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

#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: -------------------------------------+------------------------------------- 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: {{{ }}} Not the lack of an `x`. I believe this may be what causes #13916. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13977 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler