Do What I Mean RULES for foldr2 look shady
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:
- 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.
foldr2/rightrules 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.