The C language is purely functional

There has been a scurry of reaction on twitter and reddit to Robert Fischer’s post saying that Scala is Not a Functional Programming Language. James Iry responded by saying that analogous reasoning leads to the conclusion that Erlang is Not Functional

My first inclination was to suggest that Haskell, as commonly practiced (with monadic IO), is not a functional language either. Instead, I’m going to explain how it is that the C language is purely functional.

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Early inspirations and new directions in functional reactive programming

In 1989, I was a grad student nearing completion at Carnegie Mellon. Kavi Arya gave a talk on “functional animation”, using lazy lists. I was awe-struck with the elegance and power of that simple idea, and I’ve been hooked on functional animation ever since.

At the end of Kavi’s talk, John Reynolds offered a remark, roughly as follows:

You can think of sequences as functions from the natural numbers. Have you thought about functions from the reals instead? Doing so might help with the awkwardness with interpolation.

I knew at once I’d heard a wonderful idea, so I went back to my office, wrote it down, and promised myself that I wouldn’t think about Kavi’s work and John’s insight until my dissertation was done. Otherwise, I might never have finished. A year or so later, at Sun Microsystems, I started working on functional animation, which then grew into functional reactive programming (FRP).

In the dozens of variations on FRP I’ve played with over the last 15 years, John’s refinement of Kavi’s idea has always been the heart of the matter for me.

Lately, I’ve been rethinking FRP yet again, and I’m very excited about where it’s leading me.

The semantic model of FRP has been based on behaviors of infinite duration and, mostly on absolute time. Recently I realized that some problems of non-modular interaction could be elegantly addressed by switching to finite duration and relative time, and by adopting a co-monadic approach.

Upcoming post will explore these ideas.