[Haskell-cafe] Re: Space and time leaks
haskell at list.mightyreason.com
Fri Oct 5 05:05:36 EDT 2007
Dan Weston wrote:
> 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]
>> (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?
Nope, it runs in small space. The [1..10^6] is evaluated lazily, it is never
larger than the thunk that represent the rest of the list. If foldl' is a "good
consumer" then GHC can avoid making the list's cons-cells (:). If foldl' is not
a "good consumer" then then each cons cell is created and then quickly garbage
> 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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