#5642: Deriving Generic of a big type takes a long time and lots of space -------------------------------------+------------------------------------- Reporter: basvandijk | Owner: bgamari Type: bug | Status: merge Priority: normal | Milestone: Component: Compiler | Version: 7.3 Resolution: | Keywords: deriving- | perf, Generics Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: T5642 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D2304 Wiki Page: | -------------------------------------+------------------------------------- Comment (by rrnewton): One more small experiment. Before diving into the compiler internals, I thought it might be possible to do standard test case minimization. I.e. can we create some stub definitions for the relevant bits of `GHC.Generics`, while preserving the bad behavior? [https://gist.github.com/rrnewton/df17230b748df4ce1f06b49831348000 The file in this gist] attempts to do so, using the following stub definitions: {{{ class Generic a where from :: a -> M1 a data M1 a where M1 :: b -> M1 a data U1 p = U1 data (:+:) (f :: * -> *) (g :: * -> *) p = L1 (f p) | R1 (g p) }}} This is a lower bound -- it fails to produce the really slow behavior. It is simplified enough that the time for compiling just the "from" case drops from 5.7s all the way down to 0.85s. Getting back to the bad behavior will require putting more of the complexity of "M1" back into play... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5642#comment:46 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler