On Tue, 2008-01-22 at 11:55 -0500, Michael Speer wrote:
rexn ns pps = let ( ~( xs , rps ) , ~( ~( nxs ) , ~( rxs , rrps ) ) ) = ( exn nxs pps ,
Not one of the lazy marks was required in the current version.
Pattern bindings via let are always irrefutable; the ~s here are all redundant. This took me a little while to figure out; I was browsing Control.Monad.State.Lazy and trying to figure out how it was different than Control.Monad.State.Strict; they look almost exactly the same. But then I noticed code that looked like the following: (strict) m >>= f = State $ \s -> case runState m s of (a, s') -> runState (f a) s' (lazy) m >>= f = State $ \s -> let (a, s') = runState m s in runState (f a) s' The strict code deconstructs the pair in a case expression, so "runState m s" will always get evaluated at least enough to know that it is going to return a pair and not _|_. The lazy code immediately calls (f a) and m may not get evaluated at all if f is lazy and doesn't access the state. -- ryan