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Foldr Foldl Foldl'

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To foldr, foldl or foldl' that's the question! This article demonstrates the differences between these different folds by a simple example.

If you want you can copy/paste this article into your favorite editor and run it.

We are going to define our own folds so we hide the ones from the Prelude:

> import Prelude hiding (foldr, foldl)

Say we want to calculate the sum of a very big list:

> veryBigList = [1..1000000]

Lets start with the following:

> foldr f z []     = z
> foldr f z (x:xs) = x `f` foldr f z xs
 
> sum1 = foldr (+) 0
 
> try1 = sum1 veryBigList

If we evaluate try1 we get:

*** Exception: stack overflow

Too bad... So what happened:

try1 -->
foldr (+) 0 [1..1000000] -->
1 + (foldr (+) 0 [2..1000000]) -->
1 + (2 + (foldr (+) 0 [3..1000000])) -->
1 + (2 + (3 + (foldr (+) 0 [4..1000000]))) -->
1 + (2 + (3 + (4 + (foldr (+) 0 [5..1000000])))) -->
-- ...
-- ...  My stack overflows when there's a chain of around 500000 (+)'s !!!
-- ...  But the following would happen if you got a large enough stack:
-- ...
1 + (2 + (3 + (4 + (... + (999999 + (foldr (+) 0 [1000000]))...)))) -->
1 + (2 + (3 + (4 + (... + (999999 + (1000000 + ((foldr (+) 0 []))))...)))) -->
 
1 + (2 + (3 + (4 + (... + (999999 + (1000000 + 0))...)))) -->
1 + (2 + (3 + (4 + (... + (999999 + 1000000)...)))) -->
1 + (2 + (3 + (4 + (... + 1999999 ...)))) -->
 
1 + (2 + (3 + (4 + 500000499990))) -->
1 + (2 + (3 + 500000499994)) -->
1 + (2 + 500000499997) -->
1 + 500000499999 -->
500000500000

The problem, as you can see, is that a large chain of (+)'s is created which eventually won't fit in your stack anymore. This will then trigger a stack overflow exception.

For a nice interactive animation of the above behaviour see: http://foldr.com

Lets think about how to solve it...

One problem with the chain of (+)'s is that we can't make it smaller (reduce it) until at the very last moment when it's already too late.

The reason we can't reduce it, is that the chain doesn't contain an expression which can be reduced (a so called "redex" for reducible expression.) If it did we could reduce that expression before going to the next element.

Well, we can introduce a redex by forming the chain in another way. If instead of the chain 1 + (2 + (3 + (...))) we could form the chain (((0 + 1) + 2) + 3) + ... then there would always be a redex.

We can form the latter chain by using a function called foldl:

> foldl f z []     = z
> foldl f z (x:xs) = foldl f (z `f` x) xs
 
> sum2 = foldl (+) 0
 
> try2 = sum2 veryBigList

Lets evaluate try2:

*** Exception: stack overflow

Good Lord! Again a stack overflow! Lets see what happens:

try2 -->
foldl (+) 0 [1..1000000] -->
foldl (+) (0 + 1) [2..1000000] -->
foldl (+) ((0 + 1) + 2) [3..1000000] -->
foldl (+) (((0 + 1) + 2) + 3) [4..1000000] -->
foldl (+) ((((0 + 1) + 2) + 3) + 4) [5..1000000] -->
-- ...
-- ... My stack overflows when there's a chain of around 500000 (+)'s !!!
-- ... But the following would happen if you got a large enough stack:
-- ...
foldl (+) ((((((0 + 1) + 2) + 3) + 4) + ...) + 999999) [1000000] -->
foldl (+) (((((((0 + 1) + 2) + 3) + 4) + ...) + 999999) + 1000000) [] -->
 
((((((0 + 1) + 2) + 3) + 4) + ...) + 999999) + 1000000 -->
(((((1 + 2) + 3) + 4) + ...) + 999999) + 1000000 -->
((((3 + 3) + 4) + ...) + 999999) + 1000000 -->
(((6 + 4) + ...) + 999999) + 1000000 -->
((10 + ...) + 999999) + 1000000 -->
 
(499998500001 + 999999) + 1000000 -->
499999500000 + 1000000
500000500000 -->

For a nice interactive animation of the above behaviour see: http://foldl.com (actually this animation is not quite the same :-( )

Well, you clearly see that the redexen 0 + 1, (0 + 1) + 2, etc. are created. So why doesn't the chain reduce sooner than before?

The answer is that GHC uses a lazy reduction strategy. This means that GHC only reduces an expression when its value is actually needed.

The reduction strategy works by reducing the outer-left-most redex first. In this case it are the outer foldl (+) ... [1..10000] redexen which are repeatedly reduced. So the inner (((0 + 1) + 2) + 3) + 4 redexen only get reduced when the foldl is completely gone.

We somehow have to tell the system that the inner redex should be reduced before the outer. Fortunately this is possible with the seq function:

seq :: a -> b -> b

seq is a primitive system function that when applied to x and y will first reduce x, then reduce y and return the result of the latter. The idea is that y references x so that when y is reduced x will not be a big unreduced chain anymore.

Now lets fill in the pieces:

> foldl' f z []     = z
> foldl' f z (x:xs) = let z' = z `f` x 
>                     in seq z' $ foldl' f z' xs
 
> sum3 = foldl' (+) 0
 
> try3 = sum3 veryBigList

If we now evaluate try3 we get the correct answer and we get it very quickly:

500000500000

Lets see what happens:

try3 -->
foldl' (+) 0 [1..1000000] -->
foldl' (+) 1 [2..1000000] -->
foldl' (+) 3 [3..1000000] -->
foldl' (+) 6 [4..1000000] -->
foldl' (+) 10 [5..1000000] -->
-- ...
-- ... You see that the stack doesn't overflow
-- ...
foldl' (+) 499999500000 [1000000] -->
foldl' (+) 500000500000 [] -->
500000500000

You can clearly see that the inner redex is repeatedly reduced first.


Usually the choice is between
foldr
and
foldl'
, since
foldl
and
foldl'
are the same except for their strictness properties, so if both return a result, it must be the same.
foldl'
is the more efficient way to arrive at that result because it doesn't build a huge thunk. However, if the combining function is lazy in its first argument,
foldl
may happily return a result where
foldl'
hits an exception:
> (?) :: Int -> Int -> Int
> _ ? 0 = 0
> x ? y = x*y
>
> list :: [Int]
> list = [2,3,undefined,5,0]
> 
> okey = foldl (?) 1 list
>
> boom = foldl' (?) 1 list

Let's see what happens:

okey -->
foldl (?) 1 [2,3,undefined,5,0] -->
foldl (?) (1 ? 2) [3,undefined,5,0] -->
foldl (?) ((1 ? 2) ? 3) [undefined,5,0] -->
foldl (?) (((1 ? 2) ? 3) ? undefined) [5,0] -->
foldl (?) ((((1 ? 2) ? 3) ? undefined) ? 5) [0] -->
foldl (?) (((((1 ? 2) ? 3) ? undefined) ? 5) ? 0) [] -->
((((1 ? 2) ? 3) ? undefined) ? 5) ? 0 -->
0
 
boom -->
foldl' (?) 1 [2,3,undefined,5,0] -->
    1 ? 2 --> 2
foldl' (?) 2 [3,undefined,5,0] -->
    2 ? 3 --> 6
foldl' (?) 6 [undefined,5,0] -->
    6 ? undefined -->
<nowiki>*** Exception: Prelude.undefined</nowiki>

For another explanation about folds see the Fold article.