#8147: Exponential behavior in instance resolution on fixpoint-of-sum -------------------------------------+------------------------------------- Reporter: jkoppel | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: performance, Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by dfeuer): Although there's still a bug, it has changed somewhat. We now get a context reduction stack overflow: {{{ Test.hs:43:8: error: • Reduction stack overflow; size = 201 When simplifying the following type: Functor (X201 :+: (X202 :+: (X203 :+: (X204 :+: (X205 ... Use -freduction-depth=0 to disable this check (any upper bound you could choose might fail unpredictably with minor updates to GHC, so disabling the check is recommended if you're sure that type checking should terminate) • In the expression: cata lift' In an equation for ‘lift’: lift = cata lift' | 43 | lift = cata lift' | ^^^^^^^^^^ }}} Removing the reduction depth limit brings my machine to its knees. I can compile the program with `n=120` and a reduction depth limit of 300. One other thing of note: removing the redundant `Functor` constraint on the `Lift f g` instance (which for some reason is not detected as redundant in 8.2.1, though it was in 8.0.2) improves matters considerably, but they're still pretty bad even in that case. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8147#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler