I recently learned that a book which has a very similar approach to teaching category theory as kittenlab was written in 1988, using standard ML: http://www.cs.man.ac.uk/~david/categories/book/book.pdf. I’ve only skimmed through it so far, but it seems like it does a lot of similar things to Kittenlab/Catlab, like working through many constructions via algorithms in the category of finite sets. If you are programming-minded and want to learn about category theory, I’d suggest taking a look!

Also, this is a great quote from the introduction:

And all this in 1988! Feels like there was a brief dark age and we’re in a renaissance now.