Re: [GHC] #15619: List comprehension seems to prevent some rewrite rules to fire

11 Sep 2018

      #15619: List comprehension seems to prevent some rewrite rules to fire
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        Reporter:  nobrakal          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I have not looked in detail, but this looks delicate.
 {{{
     transpose (star 0 <list>)

 ==> { inline star }
     transpose (case <list> of
                  [] -> vertex 0
                  _  -> vertex 0)
 }}}
 At this point the `transpose/vertex` rule can't fire.

 If we discover that `<list>` is non-empty (which is the case here), the it
 will fire.

 But is `[1..2]` non empty?  It expands to `enumFromTo 1 2` or something
 like that.  It's hard for GHC to tell that's non-empty.
 You may say that it should expand to `[1,2]`, but if it was `[1..10000]`
 would you want it to expand?  And what about `[n..m]`?

 I'm not saying we couldn't do better here, but at the moment I don't see a
 simple, robust way to do so.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15619#comment:1
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