[GHC] #13253: Exponential compilation time with RWST & ReaderT stack with `-02`

[GHC]

#13253: Exponential compilation time with RWST & ReaderT stack with `-02`
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        Reporter:  phadej            |                Owner:  bgamari, osa1
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15630            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by tdammers):

 On further investigation, the `MonadIO` constraint is not the one that
 makes the specializer blow up; removing the `MonadIO` constraints and
 changing `Handler` to `type Handler = IO` retains the blowup. Which leaves
 us with the `Semigroup`, `Monoid`, `Functor`, `Applicative` and `Monad`
 instances for `FormResult`.

 So I implemented non-polymorphic `formMap` and `formAp` to replace `fmap`
 and `<*>`, and used `FormSuccess` directly instead of `pure`, allowing me
 to delete the `Functor` and `Applicative` instances for `FormResult`.
 Things still blow up in the `SpecConst` step, until I add a `NOINLINE
 formMap` pragma - and suddenly everything is "fine".

 Interestingly, changing the `Monoid` and `Semigroup` instances such that
 all methods are implemented in a pathologically trivial way (`mappend x y
 = FormMissing`, etc.) doesn't make things faster, so it's not that
 specializing on either of those is the problem.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13253#comment:38>
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