[GHC] #13614: Rewrite rules not applied exhaustively when simplifying from plugin

[GHC]

#13614: Rewrite rules not applied exhaustively when simplifying from plugin
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        Reporter:  nomeata           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHC API           |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nomeata):

 Replying to [comment:1 simonpj]:
 > It's not just that they aren't getting applied; somehow `foo 1` is
 getting rewritten to `foo 2` which is deeply strange.

 That is not strange, one of the rules is
 {{{
 "foo1" [1] foo 1 = foo 2
 }}}
 after all. But I wonder why it does not then apply the second rule.

 > I'm not sure why you are simplifying twice.

 Just in case it takes multiple simplifier iterations to apply rules
 exhaustively (although it does not help).

 My hypothesis is as follows:

 Rewrite rules are attached (via the `IdInfo` field `ruleInfo`)to the `Id`
 of the outermost function on the LHS of the rule; in this case `foo` (and
 not looked up in some mapping, as one might naively expect). As far as I
 can tell, the rule `foo1` and `foo2` are attached to all occurrences of
 `foo` in the module – excluding the ones on the RHS of the rules
 themselves!

 This would explain this behaviour. It would rewrite

     foo[ruleInfo=foo1,foo2] 1 `fun` foo[ruleInfo=foo1,foo2] 2

 to

     foo[ruleInfo=] 2 `fun` foo[ruleInfo=] 3

 and then not apply any more rules (because `ruleInfo`) is empty.

 This hypothesis would explain the behavior; what puzzles me is that it
 works as expected if I compile the module as normal. And, pragmaticall,
 what I wave to do so that my plugin acts as expected.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13614#comment:2>
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