# foldl traverses with State, foldr traverses with anything

## Whether to foldl or foldr?

Avi Press gave an excellent talk at Scale By the Bay 2023 about difficulties using Haskell at a startup. He mentions that even experienced Haskellers don’t always know how to use fundamental parts of the language. In particular,

even experienced Haskell engineers aren’t always going to know whether to foldl or foldr.

In this article I’ll deduce a firm rule that allows you to make the correct choice. I will stick to the versions of these functions that operate on lists; their generalization to Foldable warrants a separate article. In summary, the answer is

But why? Let’s see, by examining what these functions do.

## The definitions

We’ll work with the traditional definitions of foldl and foldr, given below. The implementations in base are more complicated, for performance reasons, but the reasoning in this article applies to them too.

foldl :: (b -> a -> b) -> b -> [a] -> b
foldl f z [] = z
foldl f z (a : as) = foldl f (f z a) as

foldr :: (a -> b -> b) -> b -> [a] -> b
foldr f z [] = z
foldr f z (a : as) = f a (foldr f z as)

### What’s the difference?

Ostensibly, the difference between foldl and foldr is the that former “associates a binary operation to the left” and the latter “associates a binary operation to the right”, as follows.

foldl (**) z [x1, x2, ..., xn] ==
(...((z ** x1) ** x2) **...) ** xn

foldr (**) z [x1, x2, ..., xn] ==
x1 ** (x2 ** ... (xn ** z)...)

Those are indeed descriptions of the calculated results, but that distinction is not particularly important. If we only care about the result then it’s easy enough to convert between foldl and foldr, as shown below. The important difference is how they calculate the result; the conversion below does not preserve behaviour. For example, foldl (-) 0 calculates a sequence of subtractions in constant space1; foldr (flip (-)) 0 uses O(n) space.

foldl f z == foldr (flip f) z . reverse
foldr f z == foldl (flip f) z . reverse

We will apply our analysis of foldl to its strict counterpart, foldl', too.

## foldl traverses with State

So what is the precise difference between how the two folds calculate their results? Consider this: suppose we didn’t have foldl, we only had traverse for State. Nonetheless, we could recover foldl! (In fact we only need traverse_, a weaker form of traverse, and in these examples we’ll use for_ = flip traverse_ for syntactic convenience.) The example below shows how. It converts a function that performs for_ (restricted to State) into a function that performs foldl.

foldlFromForState ::
(forall a b. [b] -> (b -> State a ()) -> State a ()) ->
forall a b.
(a -> b -> a) ->
a ->
[b] ->
a
foldlFromForState for_ f z bs = flip evalState z $do for_ bs$ \b -> do
a <- get
put (f a b)
get

### foldl and for_ (restricted to State) are equivalent

And not only can we get foldl from for_ (restricted to State), we can get for_ (restricted to State) from foldl. They are equivalent! Importantly, they are equivalent in both result and performance characteristics.

forStateFromFoldl ::
(forall a b. (a -> b -> a) -> a -> [b] -> a) ->
forall a b.
[b] ->
(b -> State a ()) ->
State a ()
forStateFromFoldl foldl bs f = do
z <- get
put (foldl g z bs)
where
g a b = execState (f b) a

That is to say, having foldl is equivalent to being able to traverse_ in State. If you have a foldl in your program you may as well have used traverse_ or for_ with State (or vice versa).

The same analysis works for the strict left fold, foldl', in place of lazy left fold, foldl. To obtain foldl' from for_ we would have to change foldlFromForState to use put $! f a b in place of put (f a b). forStateFromFoldl' would be a version of for_ (restricted to State) that forces its state after every iteration. ## foldr traverses with anything How does the behaviour of foldr differ? Suppose we didn’t have foldr, we only had for_ (the general version). Nonetheless, we could recover foldr. The example below shows how; it converts an Applicative-polymorphic for_ into foldr. foldrFromFor :: (forall b f. Applicative f => [b] -> (b -> f ()) -> f ()) -> forall a b. (b -> a -> a) -> a -> [b] -> a foldrFromFor for_ f z bs = runEndoApplicative z$ for_ bs $\b -> mkEndoApplicative (f b) I’ve used the following convenient type definition and functions: type EndoApplicative a = Const (Endo a) mkEndoApplicative :: (a -> a) -> EndoApplicative a () mkEndoApplicative = Const . Endo runEndoApplicative :: a -> EndoApplicative a () -> a runEndoApplicative a (Const (Endo f)) = f a ### foldr and for_ (the general version) are equivalent And not only can we get foldr from for_, we can get for_ from foldr. They are equivalent. Again, the equivalence is one not only of result but also of performance characteristics. forFromFoldr :: (forall a b. (b -> a -> a) -> a -> [b] -> a) -> forall b f. Applicative f => [b] -> (b -> f ()) -> f () forFromFoldr foldr bs f = foldr (\b rest -> f b *> rest) (pure ()) bs That is to say, having foldr is equivalent to being able to traverse_. If you have a foldr in your program you may as well just have used traverse_ or for_ with an appropriate choice of Applicative (or vice versa). ## Example of the difference What can we do with this new knowledge? Let’s look at the example of printing all even elements of a list. We can do so using foldr but the equivalent in terms of for_ (choosing the Applicative to be IO) is clearer. The “equivalent” in terms of foldl is wrong; it uses O(n) space (and completely fails on infinite lists). printEvensFoldr :: [Int] -> IO () printEvensFoldr = foldr (\i rest -> when (even i) (print i) *> rest) (pure ()) printEvensFor :: [Int] -> IO () printEvensFor is = for_ is$ \i -> when (even i) (print i)

printEvensFoldl :: [Int] -> IO ()
printEvensFoldl =
foldl
(\rest i -> when (even i) (print i) *> rest)
(pure ())
. reverse

## Solving an old riddle

An old riddle challenges us to write foldl in terms of foldr. Personally I find the riddle impossible to solve directly and even when I know the answer I can hardly understand it. With the code above, though, we can solve the riddle with no further thought. We know how to turn foldr into for_ and for_ into foldl, so we simply compose.

foldlFromFoldr ::
(forall a b. (b -> a -> a) -> a -> [b] -> a) ->
forall a b.
(a -> b -> a) ->
a ->
[b] ->
a
foldlFromFoldr foldr =
foldlFromForState (forFromFoldr foldr)

After purely mechanical simplification (see appendix below) this becomes

foldr (\b rest a -> rest (f a b)) id bs z

We didn’t need to use any brainpower to solve the riddle! The riddle can also be solved for foldl'. It ends up the same except with a strict application:

foldr (\b rest a -> rest $! f a b) id bs z ## Should I use foldl', foldl or foldr? Now we’re ready to resolve the original dilemma. We’ve seen that the only difference in functionality between foldl and foldr is that the latter can be used in a wider range of situations. In consequence we can specify when one should be used in preference to another. ### Never use foldl Firstly, regarding the choice between the two left folds, always use the strict version foldl', not the lazy version foldl. The latter can cause space leaks when strictness analysis is off; the former always avoids those space leaks. I’ve never seen a reason to use foldl.2 This advice is common knowledge in the Haskell community and not directly the subject of this article but it seems appropriate to reiterate it. ### Use foldl' in preference to foldr Regarding the choice between left and right fold, our first thought might have been been to take into account the ostensible distinction that the former “associates to the left” and the latter “to the right”. But we are no longer distracted by this mirage. We saw above that foldl' is equivalent to for_ restricted to (a strict use of) State, and foldr is equivalent to general for_, so foldl' is a special case of foldr. According to the Principle of Least Power you should use foldl' in preference to foldr when you can, this is, when the operation you want to perform is to traverse the list with a state parameter. ### Maybe just use for_ foldr does not do a different job to foldl': it does a more general version of the same job. This was not immediately clear, however; it required careful analysis. The lack of clarity around the behavior of folds might be an argument for avoiding them and instead using for_ with an appropriate choice of Applicative. Personally, I find for_ much clearer than foldr in many cases. The base implementation of (!?), below, is a case in point. It is a foldr that simulates a for_ in a composition of StateT and Either by handwriting the bind ((>>=)). (!?) :: [a] -> Int -> Maybe a xs !? n | n < 0 = Nothing | otherwise = foldr (\x r k -> case k of 0 -> Just x _ -> r (k-1)) (const Nothing) xs n I find it much clearer written as a literal for_, as below (but it can’t be, because StateT isn’t in base). The two implementations should have equal performance when compiled, assuming sufficient inlining, because for_ for lists in base is implemented in terms of foldr. xs !? n = | n < 0 = Nothing | otherwise = fromEither$ do
flip evalStateT n $do for_ xs$ \x -> do
get >>= \case
0 -> lift (Left (Just x))
k -> put (k - 1)
Left Nothing

fromEither :: Either a a -> a
fromEither = either id id

### Isn’t for_ imperative?

Yes. Although I don’t know how to precisely define “imperative style” I am confident in saying that the style of (!?) defined with foldr is functional and the style of (!?) defined with for_ is imperative. Yet they calculate exactly the same thing in exactly the same way. In fact, I think the implementation with for_ is both imperative and clearer. How can that be? Don’t functional programmers eschew imperative style? Actually, no: I also think that Haskell is the world’s finest imperative programming language!

What does the algorithm look like in other imperative programming languages? Below are two implementations in Python. Each shows the weakness of Python’s support for imperative programming compared to Haskell. In the first example the scopes of k, ret and x are limited to no less than the rest of the function, and the break is implicitly scoped to the closest enclosing for – you don’t get a choice about that. By contrast, the corresponding Haskell variables are scoped to their precise range of use, and the break equivalent is scoped precisely to its enclosing handler, fromEither. The scope is maintained even across function call boundaries so you can refactor and abstract. The second Python example is no better; it just trades a too-large scope of ret for an abstraction-resistant scope of early return.

I find imperative style programming in Haskell clear to understand and easy to reason about exactly because Haskell’s type system and expression-based nature allows fine-grained effect tracking and precise control of the scopes of values and effects.

Python example avoiding early return:

def lookup(xs, n):
if n < 0: return None
k = n
ret = None
for x in xs:
if k == 0:
ret = x
break
k -= 1

return ret

Python example with early return:

def lookup(xs, n):
if n < 0: return None
k = n
for x in xs:
if k == 0:
return x
k -= 1

return None

## Conclusion

By carefully analysing the behaviour of three Haskell folds on lists we were able to determine when we should use each. We even discovered that “imperative style” programming in Haskell can be clearer than “functional style”. In summary, never use foldl, prefer foldl' to foldr where you can, or maybe just forget them all and use for_ instead.

## References and commentary

### Monoids

This article is the culmination of an idea I wrote about years ago: What is foldr made of?. Brent Yorgey wrote a follow-up, foldr is made of monoids. One lesson of the present article is that foldr being “made of monoids” is equivalent to it being “made of applicatives whose result type we ignore”, because when you have an applicative whose result type you ignore, that’s equivalent to having a monoid.

There is also a specific Applicative that corresponds directly to any given Monoid m, that is, Const m, and my earlier blog post showed that there is a specific Moniod that foldr is “made of”: Endo a; that’s why we ended up using Const and Endo to define foldrFromFor. Instead of “foldr traverses with anything” we could have said “foldr traverses with Const (Endo _)” (but that’s much less catchy). However, it is worth observing that the following characterizations are also valid:

• foldl foldMaps with Dual (Endo _)
• foldr foldMaps with any Monoid
• foldl' foldMaps with StrictEndo _
• foldr foldMaps with Endo _

(where StrictEndo is a strict version of Endo, which doesn’t seem to exist anywhere in the Haskell ecosystem). The lens library takes advantage of these correspondences in its definitions of foldlOf', foldlOf and foldrOf (although it uses Endo (Endo _) instead of StrictEndo _).

In any case, the slogan “foldl traverses with State, foldr traverses with anything” seems the most catchy and the easiest to use as a guide to practice.

### Other references

Alexis King has a post from 2019 explaining the difference between foldl and foldr. She agrees that one should never use foldl. Regarding the distinction between foldl' and foldr she writes

When the accumulation function is strict, use foldl’ to consume the list in constant space, since the whole list is going to have to be traversed, anyway. When the accumulation function is lazy in its second argument, use foldr.

Those rules of thumb are hard to apply because they presuppose that you already have an (“accumulation”) function (which is also a misnomer: in the case of foldr it doesn’t accumulate). That is, it’s a rule that you can use if you start with a function, to determine which fold to use with that function. By contrast, this article presents a rule that you can use if you start with a problem, to determine which fold can solve your problem. If your problem is “traverse with (only) a state”-shaped then the answer is foldl'; if the problem is “traverse with anything else”-shaped then the answer is foldr(or in either case you could just use for_).

Yao Li et al. address the choice of folds in Reasoning about the Garden of Forking Paths from the point of view of laziness and demand analysis.

### foldM

There is corresponding characterization of foldM: “foldM traverses with StateT”, as demonstrated by the following, which is a minor adjustment to the foldl equivalents.

foldMFromForStateT ::
( forall a b m.
[b] ->
(b -> StateT a m ()) ->
StateT a m ()
) ->
forall a b m.
(a -> b -> m a) ->
a ->
[b] ->
m a
foldMFromForStateT for_ f z bs = flip evalStateT z $do for_ bs$ \b -> do
a <- get
put =<< lift (f a b)
get

forStateTFromFoldM ::
( forall a b m.
(a -> b -> m a) ->
a ->
[b] ->
m a
) ->
forall a b m.
[b] ->
(b -> StateT a m ()) ->
StateT a m ()
forStateTFromFoldM foldM bs f = do
z <- get
put =<< lift (foldM g z bs)
where
g a b = execStateT (f b) a

### Why is (!?) written that way?

It would be even clearer to write (!?) as below. Why not just do that instead, instead of considering foldr and for_? Because when written in terms of foldr GHC can apply short cut fusion, a rewrite rule that leads to an optimization.

0 !? (x:_) = Just x
_ !? [] = Nothing
n !? (_:xs) = (n-1) !? xs

Thanks to tobz619 for raising this question and suggesting the alternative implementation.

### “It’s always traverse”

There’s a running joke in the Haskell world that “it’s always traverse”, that is, many complicated transformations can be boiled down to a use of traverse. This article sheds more light on that phenomenon; foldl, foldl', foldr and foldM are just different flavours of traverse_.

### Foldable

This article only discusses the relationship between foldl and foldr as functions on lists, not as functions in the Foldable class; that can of worms deserves an article of its own. However, the slogan “foldl traverses with State, foldr traverses with anything” can be seen as specifying a putative law for how the Foldable versions of these functions should behave. Determining whether that law holds in practice, and role of the mysterious foldr', requires further analysis.

## Appendix: Calculations for solving the riddle

He we do the calculation for foldl. The calculation for foldl' is almost identical. Starting from the original definition of foldlFromFoldr, after inlining foldlFromForState and forFromFoldr, we get

foldlFromFoldr ::
(forall a b. (b -> a -> a) -> a -> [b] -> a) ->
forall a b.
(a -> b -> a) ->
a ->
[b] ->
a
foldlFromFoldr foldr f' z bs' =
flip evalState z $do for_ bs'$ \b -> do
a <- get
put (f' a b)
get
where
for_ bs f = foldr (\b rest -> f b *> rest) (pure ()) bs

Then we can extract the body of for_ as a variable g

flip evalState z $do for_ bs' g get where for_ bs f = foldr (\b rest -> f b *> rest) (pure ()) bs g b = do a <- get put (f' a b) Then inline for_ flip evalState z$ do
foldr (\b rest -> g b *> rest) (pure ()) bs
get
where
g b = do
a <- get
put (f a b)

Write g in terms of modify

flip evalState z $do foldr (\b rest -> g b *> rest) (pure ()) bs get where g b = modify (flip f b) and then inline g flip evalState z$ do
foldr (\b rest -> modify (flip f b) *> rest) (pure ()) bs
get

and use execState rather than evalState with get

flip execState z $do foldr (\b rest -> modify (flip f b) *> rest) (pure ()) bs Now we can take advantage of an interesting property of foldr, that when h . g == id we have the equality foldr (\a -> f a) z == h . foldr (\a -> g . f a . h) (g z) (I don’t know if this is a free theorem or whether you have to prove it by induction.) Observing that execState and modify are inverses we get flip execState z$ do
modify $foldr (\b rest -> execState$ modify (flip f b) *> modify rest)
(execState (pure ()))
bs

We can combine the two modifys to get

flip execState z $do modify$
foldr
(\b rest -> execState $modify (rest . flip f b)) (execState (pure ())) bs and use that execState . modify == id to get foldr (\b rest -> rest . flip f b) (execState (pure ())) bs z which is foldr (\b rest a -> rest (f a b)) id bs z The same calculation for foldl' yields foldr (\b rest a -> rest$! f a b) id bs z

1. assuming strictness analysis is on, otherwise it has space leak – foldl' always calculates it in constant space↩︎

2. If you’ve miraculously discovered a case where foldl is better then please tell me because I would be astonished to learn about it.↩︎