#14542: Renamer / typechecker hang (UndecidableSuperClasses) -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler