# efficiency question

Sat, 9 Feb 2002 00:34:07 +0000

```On Friday 08 February 2002 23:52, Hal Daume III wrote:
> I've tried using a strict fold:
>
> foldl' f a []     = a
> foldl' f a (x:xs) = (foldl' f \$! f a x) xs
>
> but that has no effect (or minimal effect).

That wouldn't work even if if laziness is the problem because that would only
cause the elements of the list to be evaluated to head normal form, the
elements of the pair would not be evaluated so you'd have a 'suspension of
(minus and plus) operations'.

> (\x (a,b) -> (x+a,x-b))
try
> (\x (a,b) -> (((,) \$! x-a)\$! x-b) )

I just noticed that you were the one who sent me the DeepSeq module.
This is the kind of place where I want to use it.

And Konst Sushenko wrote:
>>My guess is that it is due to the laziness of the addition/subtraction
>>in (,)

Seems to me like lazyness is not the right guess because both functions Hall
first posted were lazy. So I think it's just the overhead of applying (,)
besides (+) and (-) in each step. Do I make sense or am I missing something?

J.A.

```