[tel]

Could you expand on effects which preserve extensionality some more? I'm fairly sure I understand it, but I've never quite follows what Harper's "benign effects" meant in detail and I'd like to see how it all connects.

[jonsterling]

The most benign effect of them all is laziness. Haskell uses this in lieu of all other effects, basically.

So memoizing data structures, or a higher order function for memoizing, utilizes a "benign effect".

[tel]

So extensionally equal simply means that there is a model with and without the effect such that extensionally, according to some model of observation, we cannot discern the existence of the effect?

So, laziness is benign in total fragments.

[jonsterling]

I wasn't thinking about models, but perhaps you could? I just mean that you get the same results for equal inputs; laziness is fine in this respect (but not in the presence of some other effects).