#13253: Exponential compilation time with RWST & ReaderT stack with `-02` -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler