Why does bind flatten the List?

Question

I wonder, you say that flatMap is monad's bind method. The bind method takes a function that maps monad's contained item to another monad. This is how

option.flatMap(item => another option (f(item)))

gives me another monad with f(item) inside. But, what does it mean applied to List? By strightforward extension, if you map every List item to a (list) monad, you get the whole list of lists. Why does bind flatten the outcome when applied to the list?

"*to another monad*" should read "…maps the contained item to a container of the same monad" — Bergi, Jun 18 '16 at 17:27

score 1 · Accepted Answer · answered Jun 18 '16 at 18:07

Like the Option monad's bind does not return an option of an option, the List monad's bind does not return a list of lists. This joining is the whole point of monads - they are more than mere functors:

// Functor m
fmap :: (a -> b) -> m a -> m b
// Monad m
bind :: (a -> m b) -> m a -> m b // not m (m b)!

In the case of lists, this is done by flattening them.

Why does bind flatten the List?

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