No: it goes Just 20 → Just 10 → Just 5 → Nothing, because 5/2 can’t be represented as an integer.

Ah, it's integral division.

I'd expect some kind of expection in that case.

This sounds like a dangerous example of monads (since you don't know if you had a Nothing value up in the chain or a valid value that can't be representated)!

> I'd expect some kind of expection in that case.

Sorry, what exactly do you mean by ‘expectation’ here?

> This sounds like a dangerous example of monads (since you don't know if you had a Nothing value up in the chain or a valid value that can't be representated)!

Well, this particular case is of course oversimplified. There’s plenty of ways of dealing with different kinds of errors. The easiest is to define a sum type of errors:

    data DivisionError = ValueMissing | NonIntegralValue

Then redefine the function to return an ‘Either DivisionError Int’:

    half x
        | even x    = Right (x `div` 2)
        | otherwise = Left NonIntegralValue

Now chaining several calls to ‘half’ will abort at the first error.