Is it necessary that a monad "builds up a computation and then runs"? In fact it's very hard for a monad to do that because the type of bind is

    (>>=) :: m a -> (a -> m b) -> m b

so you can really only make progress if you first build a bit (`m a`), then run it (to get `a`) then build the next bit (applying `a` to `a -> m b`), then run that. So "building" and "running" must necessarily be interleaved. It's an odd myth that "Haskell's IO purely builds an impure computation to run".

Are you saying "monad" is a synonym of "interface"?

Not a synonym, but `Monad` is one of the commonly used interfaces in Haskell (not the only one).

OK I think I understand now, thank you. My takeaways:

1. Yes, Zig is doing basically the same thing as Haskell

2. No, it's not a monad in Zig because it's an imperative language.

It still is a monad. It's just Zig doesn't have language support for monads, so it's less ergonomic.

Just as modular addition over ints in Zig forms a group, even if Zig has no notion of groups. It's just a property of the construct.

Laziness has nothing to do with it.

What that means practically for Zig, I'm unsure.