(re-joining the list -- I forgot to "reply all") Vincent Kraeutler wrote:

Roberto Zunino wrote:

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Vincent Kraeutler wrote:

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i see that the definition of fix (from Control.Monad.Fix) could not be any simpler:

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fix f = let x = f x in x

I actually consider

fix f = f (fix f)

to be simpler. Alas, it breaks sharing...

;-) sharing? v.

The two functions fix1 f = let x = f x in x fix2 f = f (fix2 f) have the same semantics. However I believe many implementations run them with different performance. Consider y = fix1 (2:) This would be expanded to y = let x = 2:x in x A typical implementation would then represent the infinite list using (roughly) a circular pointer-list, i.e. x = cons(2, pointer-to-x) . So, the tail of the list is shared with the list itself, causing a constant space to be allocated for the list, even if a large portion of the list is demanded as in (take 1000000 y). On the contrary, y = fix2 (2:) would be y = 2 : fix2 (2:) and, unless some optimization magic kicks in, the representation for y is not a circular list. Each time a new list element is demanded, a new cons cell will be allocated. Running (take 1000 y) would then "waste" space for 1000 copies of 2. This is because the tail is not shared. Zun.