#14610: newtype wrapping of a monadic stack kills performance -------------------------------------+------------------------------------- Reporter: mrkkrp | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Resolution: | Keywords: JoinPoints Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: #14620 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nomeata): Here is an idea. Consider a recursive function `foo` with type {{{ foo :: forall t, a -> r t foo @t a = E[foo @t2 x `cast` co)] -- E[_] is tail call context, and the cast prevents join-point-hood }}} But we can transform this into a form that is allowed by now, just by introducing casts: {{{ foo :: forall t, a -> r t foo @t a = go @t a refl where go :: forall t', a -> (r t ~ r t') -> r t go @t' a = E[go @t'2 co] }}} Note that now the return type does no longer depend on the arguments of `go`, so it is a join point. This transformation smells like a worker-wrapper transformation, but I am not sure. Anyways, I am not arguing that we should do this transformation in GHC, but do it in the proofs so that we can simply happily remove the restriction in `isValidJoinPointType`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14610#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler