So let me ask again: in what sense do people "do mathematics" in Haskell?

They don't really "do" category theory, for example; simply noting that something forms a category is mathematics only in the most trivial sense.

> All that stuff about types and categories and monads and so on is maths too.

I think the question he's asking is: What papers have been published in math journals where the work was done in Haskell? Which mathematicians use Haskell to do mathematics work?

Not much afaict. For mathematics, it's usually Coq, maybe Lean or Agda. For scientific computing it's Python or Julia.

To be fair, Agda is a fusion of Martin-Löf type theory and Haskell if my memory serves me.

You can use HaskellR (https://tweag.github.io/HaskellR/) for everything scientific that is being written in Python and Julia.

Everything in computers is math. Floating point numbers are math. Java generics with inheritance are math. It's not a point of distinction.