I think category theory is a language you need to learn at some point if you are serious about math or computer science. I've refused to do so for quite a long

I think category theory is a language you need to learn at some point if you are serious about math or computer science. I've refused to do so for quite a long time, but there is just too much interesting material out there that uses this language. The biggest problem is not falling asleep while learning it :-) The most interesting part of category theory is topos theory, in my opinion. This is a generalisation of set theory, and can thus claim to be indeed useful for foundations.