I was wrong. See https://ghc.haskell.org/trac/ghc/ticket/5129#comment:17 which I have just added. Simon | -----Original Message----- | From: Simon Peyton Jones | Sent: 08 January 2015 15:05 | To: 'Roman Cheplyaka'; Edward Z. Yang; David Feuer | Cc: ghc-devs | Subject: RE: seq#: do we actually need it as a primitive? | | No (2) would not suffer from #5129. Think of | | type IO a = State# -> (State#, a) | return x = \s -> (s, x) | (>>=) m k s = case m s of (s, r) -> k r s | | (it's a newtype actually, but this will do here). | | (2) says | | = \x -> (return $! x) >>= return | = \x. \s. case return $! x s of (s1, r) -> return r s1 | = \x\s. x `seq` case (s,x) of (s1, r) -> return r s1 | = \x\s. x `seq` (s,x) | | which is fine.