Given the Fibonacci infinite list
fibo a b = a : fibo b (a+b)
And given the two following calls:
foldl (+) 1 (take 1000000 $ fibo 1 1)
foldr (+) 1 (take 1000000 $ fibo 1 1)
I expected the first one (foldl) to allocate a huge amount of memory because of thunks and in fact that's what happens.
However, I didn't expect the same for the second one. Because of how the foldr is defined I thought a usual stack like evaluation would have been performed for the right argument of (+) (because of its strictness).
Actually even in this case I have a huge amount of memory that is allocated.
What is happening?
foldr fz xs
is inefficient when f
is strict on its second argument (like (+)
), since it evaluates to
f1 x1 (f x2 (f x3 ...
and this can not start to evaluate until the very end of the list. If f
were not strict on the second argument, evaluation could instead start sooner.
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