[lmm]

> if you can’t observe any difference from safe code, then despite having different intensional/operational definitions, they are extensionally equivalent, and that’s mostly what I care about.

That's true if the only things you care about are the things that are visible from safe code. But if you care about performance characteristics (which is presumably the whole point of this kind of monad) then code that gives the same result but has different performance characteristics is not equivalent for your purposes.

Ultimately, if x and y are marked as equivalent then a future maintainer will expect to be able to blindly replace x with y - and by convention that's true of <hnmarkupisbad> and ap. So you shouldn't define <hnmarkupisbad> and ap in such a way that replacing one with the other will change important characteristics of the code - you should only make things look equivalent in your code if they are equivalent for your purposes.