5th January 2010, 03:08 pm

There is a lot of confusion about the meaning of “functional” and “declarative” as descriptions of programming languages and paradigms. For instance, Haskell is sometimes advertised as a “purely functional programming language” and other times as “the world’s finest imperative programming language”. I added some playful confusion (and clarity, I hope) with my post The C language is purely functional.

I still regularly hear people ask and disagree about whether Haskell’s monadic I/O is “functional”. I would say yes in one (technical) sense of “functional”. But not the same sense in which we say “functional programming has simple, precise semantics” or “functional programming has good compositional properties” or “functional programming is good for reasoning about”. Monadic I/O is a clever trick for encapsulating sequential, imperative computation, so that it can “do no evil” to the part that really does have precise semantics and good compositional properties.

It’s because of this confusion that I’ve started using the more specific term “denotational programming” in my blog subtitle and elsewhere, as an alternative to what I used to call “functional programming”. While there are other notions of “functional”, applicable even to monadic IO, I think “denotational” captures the fundamental and far-reaching benefits that we called “good for reasoning about” and “powerfully compositional”.

When I bash monadic I/O, my real complaint isn’t with the technical invention–which I like. Rather, I’m cranky about confusion and misleading communication, and about distraction from what we originally meant by “functional programming”–from what I call “denotational programming”.

I don’t mind that we haven’t yet been liberated from the von Neumann model. As long as remember we haven’t.

As long as we keep on searching.

There is a lot of confusion about the meaning of “functional” and “declarative” as descriptions of programming languages and paradigms. For instance, Haskell is sometimes advertised as a “purely functional...