caryoscelus:~#

As long as you understand immutability, it isn’t hard to come to conclusion that types are “kinda like” sets. This is blatantly obvious for numeric types, but if you heard of cartesian product and discriminated union, it also becomes clear that data definitions in pure fp work in a similar fashion.

Then it is simply a matter of contemplating idea that sets can be defined by membership rules, i.e. by giving a predicate (well-defined in a sense, so as to avoid paradoxes) that should hold for all of its members (and vice verse, everything for which it is true is member of set in question).

Now proposition and predicates are not exactly the same, of course. But they are so similar that finding a (perhaps partial) correspondence between them is a really easy thing to do. Predicates can be made to propositions using quantifiers (i.e. saying “for all x P is true”, or “there is x such that P is true for it”); propositions can be made into predicates by kinda removing quantifiers, but although that might get a bit tricky, it is quite intuitive that it can be done at least for many of propositions.

That’s really all there is to it. But then of course, there’s a number of obstacles to that path.

And another note i should perhaps stress is that this correlation is no more than intuition (and quite incomplete at that); if you want to be mathematical about it, you’d need to give rigorous definitions for things you work with. But in order to get interested in such definitions in the first place, it could be useful to get the sense of it.