[lmm]

I think the main thing I'm objecting to is the possibility of multiple paths through the function. Doing it this way there's only one possible flow: a series of calls that pass blocks to validation objects (that then may or may not execute them).

Compare how smalltalk didn't have a conditional control flow statement. Rather, the boolean type has a polymorphic method that takes a block and then executes it or not.

The other nice thing about this approach is it makes the language simpler and more regular because it's implemented using standard language constructs rather than a special statement. E.g. you can write a "gather" function that takes a collection of monads and returns a monad of the collection, and this is generic in the monad (so the same function works to turn a List of Futures into a Future of a List, a List of Validations into a Validation of a List, etc.)