[Haskell-cafe] Space and time leaks
Dan Weston
westondan at imageworks.com
Thu Oct 4 14:59:36 EDT 2007
Ronald Guida wrote:
> I need some help with space and time leaks.
>
> I know of two types of space leak. The first type of leak occurs when
> a function uses unnecessary stack or heap space.
>
> GHCi> sum [1..10^6]
> *** Exception: stack overflow
>
> Apparently, the default definition for "sum" has a space leak.
> I can define my own sum in terms of strict foldl ...
>
> > sum' xs = foldl' (+) 0 xs
>
> ... and it doesn't overflow the stack.
>
> GHCi> sum' [1..10^6]
> 500000500000
> (0.27 secs, 112403416 bytes)
But it fills the heap? My intuition is that
foldr (+) 0 [1..10^6]
is the fusion of a good producer and consumer so no heap is wasted
constructing [1..10^6] but the stack is instead filled with
(1+),(2+),...,(999999+) unary functions, whereas
foldl' (+) 0 [1..10^6]
does not fill the stack with anything but does fill the heap with
[1..10^6] because foldl' is no longer a good consumer?
If so, it seems that the stack is smaller than the heap (or else the
size of (1+) is much larger than that the element of an [Int].
Anyone know the truth of any of this?
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