[GHC] #10555: RULE left-hand side too complicated to desugar

[GHC]

#10555: RULE left-hand side too complicated to desugar
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        Reporter:  yongqli           |                   Owner:
            Type:  bug               |                  Status:  closed
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.2-rc1
      Resolution:  duplicate         |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:  yes
      Blocked By:                    |                Blocking:
 Related Tickets:  #10699            |  Differential Revisions:
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Comment (by simonpj):

 I did look at the "lhs too complicated to desugar" issue.  There's a good
 reason for it, and I'm not sure what to do.

 The issue is this.  Most numeric primops are pure and can't fail, so they
 can be subject to aggressive code motion.  Division, unlike most numeric
 primops, can fail.  So GHC restricts its code motion by `case`-binding it.
 In particular something like
 {{{
   a /## (b /## c)
 }}}
 doesn't obey the "let/app invariant" (see `CoreSyn.hs`) and gets turned
 into
 {{{
   case b /## c of r -> a /## r
 }}}
 But the LHS of a rule is supposed to look like `f e1 .. en`.

 Maybe the let/app invariant is too strong -- after all, we only support
 imprecise exceptions for things like divide-by-zero.  But if you say
 {{{
   if x >## 3## then
      y /## x
   else ...
 }}}
 you jolly well don't expect the `(y /## x)` to be executed until after
 testing `x >## 3`, so we need to restrict code motion.

 Not clear what to do here.   Avoid division primops on the LHS of a rule!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10555#comment:4>
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