# Strict foldl

**Jan Kort
**
kort@science.uva.nl

*Fri, 07 Dec 2001 12:05:20 +0100*

"Ch. A. Herrmann" wrote:
>*
*>* Hi Haskellers,
*>*
*>* which compiler settings do I have to pass to ghc-5.02
*>* in order to achieve that the strictness analyzer
*>* recognizes strictness of (+) in foldl and computes
*>* sum in constant space?
*>*
*>* Prelude> sum [1..10000000]
*>*
*>* had the following effect:
*>*
*>* PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
*>* 23542 herrmann 20 0 250M 130M 97500 R 66.3 52.4 0:21 ghc-5.02
*>*
*>* Of course, one could define a strict foldl oneself:
*>*
*>* > sfoldl f e [] = e
*>* > sfoldl f e (x:xs) = (sfoldl f $! (f e x)) xs
*
There is a foldl' in the Hugs Prelude that does this:
foldl' :: (a -> b -> a) -> a -> [b] -> a
foldl' f a [] = a
foldl' f a (x:xs) = (foldl' f $! f a x) xs
There are some functions in the Hugs Prelude that use
foldl (or foldl1) and some use foldl'. Maybe someone
can explain why certain functions use foldl: reverse,
maximum, minimum and readInt, while others use foldl':
length, sum and product.
I can understand why reverse would use foldl, but
why do maximum, minimum and readInt use foldl ? Maybe
the function foldl1 was based on foldl' at one time ?
Jan