[GHC] #14542: Renamer / typechecker hang (UndecidableSuperClasses)

[GHC]

#14542: Renamer / typechecker hang (UndecidableSuperClasses)
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
                                     |  UndecidableSuperClasses
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 simonpj):

 Well, every given `(Category p)` constraint will give rise to four
 superclasses
 {{{
 class (Functor p, Dom p ~ Op p, Cod p ~ Nat p (->), Ob (Op p) ~ Ob p) =>
 Category (p :: Cat i) where
 }}}
 That `Functor p` constraint will give rise to two `(Category (Dom p),
 Category (Cod p))` constraints.
 {{{
 class (Category (Dom f), Category (Cod f)) => Functor (f :: i -> j) where
 }}}

 And so on.  So in each round of superclass expansion we get double the
 number of constraints.  Result: exponential blowup.

 So I'd say this is expected bahaviour.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14542#comment:1>
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