The lambda calculus doesn't really have assignment, so you can't call a function recursively in the way we're used to in modern programming languages. However,

The lambda calculus doesn't really have assignment, so you can't call a function recursively in the way we're used to in modern programming languages. However, Y allows us to give a recursive function a temporary name which we can then use to apply it recursively. It's trippy, for sure, but pretty much theoretical use only for most people.