Typeclasses - Foldable

Posted on December 3, 2018

Let’s think about sets of things that we want to make into one thing.

A classic example might be a list of numbers that we want to add up.

In Javascript we might do something like this:

const added = [1, 2, 3, 4].reduce((total, item) => {
  return total + item;
}, 0);
// added == 10

Or perhaps we could get the maximum of the same list.

const maxNo = [1, 2, 3, 4].reduce((highest, item) => {
  return highest > item ? highest : item;
}, 0);
// maxNo == 4


So in Haskell we have the very similar foldr with the following signature:

Prelude> :i Foldable
class Foldable (t :: * -> *) where
  foldMap :: Monoid m => (a -> m) -> t a -> m
  foldr :: (a -> b -> b) -> b -> t a -> b
  {-# MINIMAL foldMap | foldr #-}

(there is actually loads more but these are the key ones)

foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b
foldMap :: (Foldable t, Monoid m) => (a -> m) -> t a -> m

Here are the above JS functions using foldr.

added :: Int
added = foldr (\a b -> a + b) 0 [1,2,3,4]
-- added = 10
maxNo :: Int
maxNo = foldr (\a b -> if a > b then a else b) 0 [1,2,3,4]
-- maxNo = 4

Not hugely different from the Javascript equivalent really. If you squint you can see the combining function, the initial value, and the data itself.

foldMap works a little differently. Instead of taking a custom combining function and using that to combine the items together, it takes a a -> m function (where the m in question is any Monoid instance). It uses this to turn each item into a Monoid, and then uses the <> and mempty functions for that Monoid to combine the items.

Here’s a newtype I made earlier: MySum. It’s Monoid instance adds numbers together when combined.

newtype MySum a = MySum { getMySum :: a }

-- Semigroup instance
instance (Num a) => Semigroup (MySum a) where
    MySum a <> MySum b = MySum (a + b)

-- Monoid instance
instance (Num a) => Monoid (MySum a) where
    mempty = MySum 0

Now we can use foldMap with the MySum constructor to add up a list of numbers.

addTwo :: MySum Int
addTwo = foldMap MySum [1,2,3,4]
-- addTwo = MySum 10

Great stuff! Now our answer is still wrapped up in a MySum, but it’s easy enough to take it out.

addTwoUnwrapped :: MySum Int
addTwoUnwrapped = getMySum $ foldMap MySum [1,2,3,4]
-- addTwoUnwrapped = 10


This seems laborious, but actually MySum isn’t my invention, I’ve just stolen a thing called Sum that comes with the Haskell Prelude. Therefore we can just do getSum $ foldMap Sum [1,2,3,4,5,6] to Monoidally combine the list items.

It also provides a similar invention for multiplying numbers called Product that works like this:

twentyFour :: Int
twentyFour = getProduct $ foldMap Product [1,2,3,4]
-- twentyFour == 24

Folding can also capture logic, here we are using foldMap to check all of a list is true.

newtype MyAll = MyAll { getMyAll :: Bool }

instance Semigroup MyAll where
    MyAll a <> MyAll b = MyAll (a && b)

instance Monoid MyAll where
    mempty = MyAll True

allOfThem :: Bool
allOfThem = getMyAll $ foldMap MyAll [True,True,True]
-- allOfThem == True

notAll :: Bool
notAll = getMyAll $ foldMap MyAll [False, True, True]
-- notAll == False

(I have also stolen MyAll, it is usually called just All. You can see the pattern here.)

We could also very easily make a MyAny type which uses or (ie, ||) which we could use to return a True whenever a single one of a collection of Bools happens to be True. You might want to have a think about what the mempty value would be for that to work though. That’s up to you.

Anyhow. I’m bored of typing now so I guess this is it for this one.

Make sense? If not, why not get in touch?

Further reading: