As a wild guess, this same construct might not always work with linear types—if we had to use x exactly once, we wouldn't always be able to write `return (x, y)

As a wild guess, this same construct might not always work with linear types—if we had to use x exactly once, we wouldn't always be able to write `return (x, y)`. I haven't really used them, but linear types are supported in Haskell, and you can generalize the Monad class to be able to handle them. This would conceptually correspond to x being some sort of limited resource that we can't copy arbitrarily, which is more than a purely mathematical curio. In mathematics more broadly, the fact that you don't always have the equivalent of the `strength` function is useful because it lets us work with different kinds of products than just tuples. This can even be useful for practical work: we might want to model something like hardware or quantum computations which behave differently than "normal" programs. Requiring all monads in that context to support `strength` would really restrict the sort of things you could useful model as monads.