#8095: TypeFamilies painfully slow -------------------------------------+------------------------------------- Reporter: MikeIzbicki | Owner: goldfire Type: bug | Status: new Priority: high | Milestone: 8.4.1 Component: Compiler (Type | Version: 7.6.3 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: #5321, #11598, | Differential Rev(s): #12506 | Wiki Page: | -------------------------------------+------------------------------------- Comment (by mbieleck): Here's another idea, based on comment:15: Replace every coercion resulting from flattening type family applications with `UnivCo`. I think that could fix the problem Simon mentioned, because the resulting `UnivCo` should have the same free variables as the original chain of coercions. But I know approximately nothing about GHC internals, so this may be terribly wrong. I am experimenting with an implementation of this idea. The code is {{{ --- a/compiler/typecheck/TcFlatten.hs +++ b/compiler/typecheck/TcFlatten.hs @@ -1139,16 +1139,18 @@ flatten_fam_app tc tys -- Can be over-saturated -- in which case the remaining arguments should -- be dealt with by AppTys do { let (tys1, tys_rest) = splitAt (tyConArity tc) tys - ; (xi1, co1) <- flatten_exact_fam_app tc tys1 + ; (xi1, _co1) <- flatten_exact_fam_app tc tys1 -- co1 :: xi1 ~ F tys1 -- all Nominal roles b/c the tycon is oversaturated - ; (xis_rest, cos_rest) <- flatten_many (repeat Nominal) tys_rest + ; (xis_rest, _cos_rest) <- flatten_many (repeat Nominal) tys_rest -- cos_res :: xis_rest ~ tys_rest + ; zonkedTy <- liftTcS $ zonkTcType $ mkTyConApp tc tys + ; role <- getRole ; return ( mkAppTys xi1 xis_rest -- NB mkAppTys: rhs_xi might not be a type variable -- cf Trac #5655 - , mkAppCos co1 cos_rest + , mkUnsafeCo role (mkAppTys xi1 xis_rest) zonkedTy -- (rhs_xi :: F xis) ; (F cos :: F xis ~ F tys) ) } }}} Here I'm trying to generate an `UnsafeCo` of the same type as the original. This passes all tests but T13822, which fails with a Core Lint error, and I have no idea why. Can someone help me debug this? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8095#comment:35 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler