#12234: 'deriving Eq' on recursive datatype makes ghc eat a lot of CPU and RAM -------------------------------------+------------------------------------- Reporter: slyfox | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: deriving-perf 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: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * cc: RyanGlScott (added) Comment: Yikes. Interestingly, the way the `Eq (Fix f)` instance is constructed plays a large role in the slowdown. Since it's a newtype, GHC defaults to deriving that instance via `GeneralizedNewtypeDeriving`, i.e., {{{#!hs instance Eq (f (Fix f)) => Eq (Fix f) where (==) = coerce ((==) :: f (Fix f) -> f (Fix f) -> Bool) :: Fix f -> Fix f -> Bool }}} But if you implement it the "stock" way, i.e., {{{#!hs instance Eq (f (Fix f)) => Eq (Fix f) where In x == In y = x == y }}} Then it compiles instantly. So perhaps the coercion solver is to blame here? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12234#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler