How does Foldable.foldl work on Num a = a

Question

In LYAH, there is a piece of code that looks like this.

data Tree a = Empty | Node a (Tree a) (Tree a) deriving (Show, Read, Eq)  

instance F.Foldable Tree where  
    foldMap f Empty = mempty  
    foldMap f (Node x l r) = F.foldMap f l `mappend`  
                             f x           `mappend`  
                             F.foldMap f r  

ghci> F.foldl (+) 0 testTree  
42  
ghci> F.foldl (*) 1 testTree  
64800

As far as I know, foldMap is of type foldMap :: (Monoid m, Foldable t) => (a -> m) -> t a -> m, but Num a => a itself is not of type Monoid, so I am wondering how does Foldable.foldl actually work here? And since foldMap is called internally by Foldable.foldl, what is the type of the Monoid?

How does Foldable.foldl work on Num a => a

1 Answers