On 11-04-14 01:57 PM, Andrew Coppin wrote:
3. How is pseq different from seq?
An example to show that there are non-unique evaluation orders to fulfill the mere strictness promise of seq: import Data.List(foldl') () & () = () main = print (foldl' (&) () (replicate 2500000 ())) with ghc with -O0 : fast, no stack overflow with ghc with -O or -O2: slow, stack overflow ghc versions 6.12.3, 7.0.2, 7.0.3 Look at the core code (-ddump-simpl) to verify evaluation orders. Repeat the experiment and observations with pseq: import GHC.Conc(pseq) () & () = () foldl' :: (a -> b -> a) -> a -> [b] -> a foldl' op z xs = go z xs where go z [] = z go z (x:xs) = let z' = op z x in z' `pseq` go z' xs main = print (foldl' (&) () (replicate 2500000 ()))