I am an avid functional programmer (it’s my default when working on solo projects) and I teach a PL class that covers the lambda calculus, LISP, etc. But the bulk of the course uses F#. I’ve been using F# regularly for the last 13 years.

It’s pretty easy to stick to pure F# if what you want is the pure functional programming experience. But what I like about it is its pragmatism, and this is a big reason why it’s the language I chose for the course. It is by-value, eagerly evaluated by default, and has an easy-to-learn idiomatic syntax. It has a large and well-behaved standard library, and you can use C#’s excellent standard library if you need additional things (e.g., mutable data structures). I have used F# in many performance-sensitive applications, and the fact that I can say “you know, inside this function, I’m going to use mutability, raw pointers, and iteration” has been a lifesaver in some places. But because it is a functional language, I can also abstract all that away and pretend that it is functional.

I understand why other FP folks dislike this approach. But the insistence on purity makes many problems artificially difficult. Debugging a lazily evaluated program is a nightmare. There are lots of times I want a well-behaved language but I am not willing to do CS research just to solve common algorithmic problems. The generally pragmatic streak from the SML family makes them easy to love.