Expressing foldl in terms of foldr

In chapter 4 of Real World Haskell there is a challenge for the reader: express foldl using foldr. Authors warn that this is not trivial and I must admit that I have not attempted this exercise leaving it for later. Yesterday I read Graham Hutton’s “Tutorial on the universality and expressiveness of fold” and it happens that, among other things, it presents an approach that can be applied to solve this problem. Hutton first presents a specific case in which sum function is defined using foldl expressed with foldr. Then he gives a general formula for expressing foldl with foldr. Despite having a derivation for one specific case and a ready result (without its derivation) it wasn’t straightforward to provide my own derivation of the general solution. This was a mind bending exercise and I’d like to go through the details here.

The solution is based on using the so called universal property of foldr¹. This property states that if we have some function g defined as:

g [] = v
g (x:xs) = f x (g xs)

then

g = foldr f v

Indeed, if we substitute foldr f v into definition of g we get a definition of foldr:

foldr f v [] = v
foldr f v (x:xs) = f x (foldr f v xs)

Moreover, foldr f v is a unique solution to the defining equations of g.

Recall that foldl is defined as:

foldl f v [] = v
foldl f v (x:xs) = foldl f (f v x) xs

The base case of foldr and foldl is identical, but the recursive one is not. Moreover, the recursive case of foldl cannot be rewritten in the form f x (g xs). This means that we need to apply some transformation to definition of foldl so it can be rewritten in that form. Let’s create a function foldl2 defined as:

foldl2 f [] v = v
foldl2 f (x:xs) v = foldl2 f xs (f v x)

Nothing special so far. We just made a function that is the same as foldl, but has the last two parameters swapped. We can rewrite the base case as:

foldl2 f [] v = id v

id is the identity function that accepts one parameter and returns that parameter unchanged. Now we remove the v parameter:

foldl2 f [] = id

Such transformation is known as η-reduction². Let us now concentrate on the recursive case. It can be rewritten as

foldl2 f (x:xs) v = (\w -> foldl2 f xs (f w x)) v

We created an anonymous function with one of the parameters of f factored out. This expression can also be ?-reduced:

foldl2 f (x:xs) = \w -> foldl2 f xs (f w x)

Let’s factor out second parameter of function f in the same manner:

foldl2 f (x:xs) = (\y w -> foldl2 f xs (f w y)) x

And finally let’s factor out foldl2 f xs and just pass it as another parameter to the lambda expression:

foldl2 f (x:xs) = (\y h w -> h (f w y)) x (foldl2 f xs)

We’re almost there. Recall that the universal property requires function of the form³:

g (x:xs) = k x (g xs)

And it so happens that we just converted foldl2 to that form, where k = \y h w -> h (f w y). Comparing last two equation we see that g = foldl2 f, but from the universal property we also know that g = foldr k v, which means that foldl2 f = foldr k v. The notation here might be a bit confusing. From the base case we determined the value of v in the last equality to be equal to id function, which yields foldl2 f = foldr k id. Substituting the value of k we get:

foldl2 f = foldr (\y h w -> h (f w y)) id

Original definition of foldl2 had two more parameters, but they were removed by ?-reductions. Let’s restore these two parameters by adding them to both lhs and rhs:

foldl2 f xs v = foldr (\y h w -> h (f w y)) id xs v

Recall that foldl2 was only a convenience function we used for the derivation. Going back to the original foldl function yields the final result:

foldl f v xs = foldr (\y h w -> h (f w y)) id xs v

OK, formulas don’t lie, but this result is definitely not an intuitive one and deserves some good explanation. You may be surprised that four parameters are passed into foldr, but this should become clear in a moment. We will play with it to get some intuition on how this works.

Let us begin with verifying that this expression is type-correct. Type of foldr is:

ghci> :t foldr
foldr :: (a -> b -> b) -> b -> [a] -> b

So the first parameter to foldr should be of type (a -> b -> b). Lambda that we pass to foldr as the first parameter uses f. This is the function that is passed as first parameter to foldl. Since foldl has type:

ghci> :t foldl
foldl :: (a -> b -> a) -> a -> [b] -> a

We require that f has type a -> b -> a. Let’s define simplest possible function that has that type and then check the type of lambda passed to foldr:

ghci> let f = \a b -> a
ghci> :t (\y h w -> h (f w y))
(\y h w -> h (f w y)) :: t2 -> (t1 -> t) -> t1 -> t

Recall that -> is right-associative, which means that above type is equivalent to t2 -> (t1 -> t) -> (t1 -> t). Parentheses at the end can be added and the meaning is the same. This corresponds to our expected type of (a -> b -> b). Here, the value of b is assumed to be t1 -> t. If we substitute (t1 -> t) for b in the type signature of foldr we get

(a -> (t1 -> t) -> (t1 -> t)) -> (t1 -> t) -> [a] -> (t1 -> t)

Note that last parentheses can be dropped, which result in function that has four parameters:

(a -> (t1 -> t) -> (t1 -> t)) -> (t1 -> t) -> [a] -> t1 -> t

We already verified that lambda passed to foldr is of type (a -> (t1 -> t) -> (t1 -> t)). The second parameter, id function, is of type (a -> a), which corresponds to (t1 -> t) in the type signature. Therefore usage of id imposes additional restriction that t1 ~ t ((~ notation means that two types are the same.)), which means that type signature can be rewritten as:

(a -> (t -> t) -> (t -> t)) -> (t -> t) -> [a] -> t -> t

[a] corresponds to parameter xs, the list that we are folding. t corresponds to initial value of accumulator and the last t is the return type.

Now that we have verified type-correctness of the solution, let’s see how it works in practice. Let’s say we want to fold a list of three elements using (+) as folding function and 0 as initial value of the accumulator. In other words, we want to calculate the sum of elements in a list. If we use foldr, the evaluation process will look like this:

foldr (+) 0 [1,2,3] =
(+) 1 (foldr (+) 0 [2,3]) =
(+) 1 ((+) 2 (foldr (+) 0 [3])) =
(+) 1 ((+) 2 ((+) 3 (foldr (+) 0 []))) =
(+) 1 ((+) 2 ((+) 3 0 )) =
(+) 1 ((+) 2 3) =
(+) 1 5 =
6

If we instead use foldl, evaluation will look like this:

foldl (+) 0 [1,2,3] =
foldl ((+) 0 1) [2,3] =
foldl ((+) ((+) 0 1) 2) [3] =
foldl ((+) ((+) ((+) 0 1) 2) 3) [] =
((+) ((+) ((+) 0 1) 2) 3) =
((+) ((+) 1 2) 3) =
((+) 3 3) =
6

Both folds produce the same result, which is a direct consequence of first duality theorem for folds⁴. Now let’s see how evaluation will proceed if we use foldl expressed using foldr:

foldl (+) 0 [1,2,3] =
foldr (\y h w -> h ((+) w y)) id [1,2,3] 0 =
(\h w -> h ((+) w 1)) (foldr (\y h w -> h ((+) w y)) id [2,3]) 0 =
(\h w -> h ((+) w 1)) (\h w -> h ((+) w 2)) (foldr (\y h w -> h ((+) w y)) id [3]) 0 =
(\h w -> h ((+) w 1)) (\h w -> h ((+) w 2)) (\h w -> h ( (+) w 3) ) (foldr (\y h w -> h ( (+) w y) ) id []) 0 =
(\h w -> h ((+) w 1)) (\h w -> h ((+) w 2)) (\h w -> h ((+) w 3)) id 0 =
(\h w -> h ((+) w 1)) (\h w -> h ((+) w 2)) (w -> id ((+) w 3)) 0 =
(\h w -> h ((+) w 1)) (w -> id ((+) ((+) w 2) 3)) 0 =
(w -> id ((+) ((+) ((+) w 1) 2) 3)) 0 =
id ((+) ((+) ((+) 0 1) 2) 3))

We’ve reached expression that is the same as the one we reached when evaluating foldl. Well, in fact that is what we expected. After all this is also foldl! So the whole trick is based on using foldr to generate a function that accepts initial value of the accumulator and produces the same expression we would get when using foldl (plus the identity function).

I hope that this post made it clear how to express foldl using foldr. This is of course by no means an exhaustive treatment of the subject of folds. There’s a lot more. I think that Hutton’s paper is a good starting point. Bird’s and Wadler’s “Introduction to Functional Programming” also seems to be a very valuable resource, though I’ve read only chapter about folds⁵. There’s still some more stuff to figure out about folds, like the difference in behaviour of foldr and foldl for infinite lists or expressing foldr using foldl (for finite lists only).

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