In a language with Haskell's guarantees, you can mechanically get from the standard "bind" function to prove that "m (m a)" can be converted to "m a" [1]. That

In a language with Haskell's guarantees, you can mechanically get from the standard "bind" function to prove that "m (m a)" can be converted to "m a" [1]. That function is called "join", and is a lesser-well known way to write monad implementations which is equivalent to the more famous bind function. (IMHO, join is a better way to understand the typeclass intuitively. The standard bind is better to program with in general use, but much harder to grok.) So, in Haskell, it's more than just "the APIs would tend to encourage not nesting the types"... having a valid monad implementation means they really are equivalent. [1]: https://stackoverflow.com/questions/3382210/monad-join-function https://stackoverflow.com/questions/3382210/monad-join-funct...