On Saturday 04 July 2009, Uwe Hollerbach wrote: > Good evening, all, following up on my question regarding space leaks, > I seem to have stumbled across something very promising. I said I was > using this tiny function "lastOrNil" to get the last value in a list, > or the empty (scheme) list if the haskell list was empty. The uses of > it were all of the form > > lastOrNil (mapM <something> <some list>) > > so I wrote a different function mapML to do this directly: > > mapML fn lst = mapMLA (List []) fn lst > > where mapMLA r _ [] = return r > > mapMLA ro fn (x:xs) = > > do rn <- fn x > > mapMLA rn fn xs > > This isn't an accumulator, it's a replacer (or, if you like, the > "accumulation" is "drop the old one on the floor"), it starts out with > the scheme empty list that I want as the default, and it never even > builds the list which it'll just dump an instant later. Shazam! Memory > usage dropped by roughly an order of magnitude in my little Collatz > benchmark, and incidentally runtime improved by 25% or so as well. The > horror! :-)
Hi,
IMHO expressing mapML using StateT would be a bit cleaner ;)
mapML :: (Monad m) => (a -> m List) -> [a] -> m List
mapML fn lst = execStateT mapMLAs (List [])
where
mapMLAs = sequence_ $ map mapMLA lst
mapMLA x = (lift $ fn x) >>= put
--
Marcin Kosiba
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