seq# is intentionally lazy in its argument, to allow explicit ordering in an IO context
Hmnm. Can you give an example? Nothing in `seq#`'s documentation says
#15226: GHC doesn't know that seq# produces something in WHNF -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Runtime | Test Case: performance bug | perf/should_run/T15226, 15226a Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4796 Wiki Page: | -------------------------------------+------------------------------------- Comment (by dfeuer): Replying to [comment:16 simonpj]: that. It jolly well should! Considering `seq#` strict can be rather bad, I believe. If we turn `.... seq# x s` into `case x of x' {DEFAULT__ -> .... seq# x' s}` then we'll see that `x'` is evaluated and erase the `seq#`. That sort of thing is the very sort of trouble `seq#` was intended to avoid. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15226#comment:19 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler