[GHC] #15490: Can Template Haskell and RULES be combined?

[GHC]

#15490: Can Template Haskell and RULES be combined?
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        Reporter:  ChaiTRex          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      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):

 Template Haskell is the wrong tool for this, since it deals with Haskell
 (the surface language), but rules actually apply to Core.

 But I suppose you can write a GHC plugin that installs a `BuiltInRule`
 (see
 https://downloads.haskell.org/~ghc/latest/docs/html/libraries/ghc-8.4.3/CoreSyn.html#t:CoreRule)
 which can have arbitrary functions to decide whether the rule fires, and
 what to replace it with.

 Or write a GHC Plugin with a Core2Core pass that simply rewrites the code
 as desired.

 In all cases, the plugin will have to be loaded explicitly, i.e. these
 kind of rules are not exported.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15490#comment:1>
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