[GHC] #10595: BuiltinRules override other rules in some cases.

[GHC]

#10595: BuiltinRules override other rules in some cases.
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        Reporter:  gjsimms           |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  high              |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by simonpj):

 GHC has a crude-but-effective way to control the order of application of
 rules, called "phases".  See the reference to phase control in
 [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/rewrite-
 rules.html 7.23.1 in the manual].

 Phases count down; currently 2, 1, 0.

 Currently, though, a class-method has a built-in rule (selection from
 dictionary) which is always active.  There is no way for the user to
 override this, to make it active in (say) phase 1 and later. If you could,
 that would solve your problem.

 The only straightforward solution I can see is to make the built-in rule
 for class methods inactive in phase 2, so that user-written rules take
 precedence.   The trouble with this is that it will delay the moment at
 which the per-instance functions (which may have rules of their own)
 become visible.

 Rather than attempt a change with global consequences, I suggest that you
 simply make a new intermediate function, just as you have done with
 `ida'`.  Instead of NOINLINE you can say `INLINE [1]` which will inline it
 in phase 1.  Now write your rules for `ida'`.

 This has the effect of delaying the built-in class-method rule for `ida`
 without affecting any other functions.

 I'll add a paragraph to the user manual about this. And I'll close this as
 wont-fix, because there is a good workaround and no obviously better
 design.


 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10595#comment:4>
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