#10555: RULE left-hand side too complicated to desugar -------------------------------------+------------------------------------- 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: -------------------------------------+------------------------------------- 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler