sum, perfomance?

[John van Groningen]

Igor Artelnykh wrote:
>In StdList one has
>
>sum:: !.[a] -> a |  + , zero  a
>sum xs = accsum zero xs
>where
>        accsum n [x:xs] = accsum (n + x) xs
>        accsum n []     = n
>
>
>why not
>
>sum:: !.[a] -> a |  + , zero  a
>sum [] = zero
>sum [x:xs] = x + sum xs

The second definition uses O(n) stack space, the first one does not because it is tail recursive.

>or even
>
>sum = foldl (+) 0
>
>Is it a perfomance issue?

Yes, when StdList was written the Clean compiler could not transform 'foldl (+) 0' into an efficient version. The current compiler can, so it could now be defined as:

sum l = foldl (+) zero l

Regards,

John van Groningen