#15226: GHC doesn't know that seq# produces something in WHNF -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: merge Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4796 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Good point. A cast would complicate. I suppose that in {{{ case seq# x s |> g of (# s', x' #) -> ... }}} we'd have to have {{{ g :: (# State# t1, t2 #) ~ (# s1, s2 #) }}} where `x :: t2` and `x' :: s2`. So you'd want to transform to {{{ case seq# x s |> g of (# s', x' #) -> let x = x' |> sym (Nth 2 g) }}} because `Nth 2 g :: t2 ~ s2`. (I might not have this exactly right.) This all seems a bit ad-hoc for `seq#` but I don't really see an alternative, sadly. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15226#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler