[GHC] #9495: Do What I Mean RULES for foldr2 look shady

[GHC]

#9495: Do What I Mean RULES for foldr2 look shady
-------------------------------------+-------------------------------------
       Reporter:  dfeuer             |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  libraries/base     |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:  Runtime
     Blocked By:                     |  crash
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 There's a comment in the source:

 {{{
 The foldr2/right rule isn't exactly right, because it changes
 the strictness of foldr2 (and thereby zip)

 E.g. main = print (null (zip nonobviousNil (build undefined)))
           where   nonobviousNil = f 3
                   f n = if n == 0 then [] else f (n-1)

 I'm going to leave it though.
 }}}

 This rule is intended to allow `foldr2` to fuse with ''either'' argument
 list. There are thus two problems, one already documented and the other
 not:

 1. The rule can turn working code into non-working code, although this
 seems to be ''relatively'' unlikely. (The problem the above example is
 showing is that if the left list ends at the same time the right list
 bottoms out, the world goes boom. So `foldr2 f [1,2,3,4]
 (1:2:3:4:undefined)` appears to be a problem. You could argue this is not
 a big deal, I suppose.

 2. The `foldr2/left` and `foldr2/right` rules are not confluent. They are
 both active in all phases, but if both list arguments are good producers,
 they will each want to rewrite the expression differently. So if you
 actually care about which one fuses, you need to explicitly block fusion
 with one argument using `NOINLINE`, which of course could easily muck up
 some other optimization.

 My uninformed opinion: nix the `foldr2/right` rule, and change the
 documentation to indicate that `foldr2` fuses with its ''left'' argument.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9495>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler