Conal Elliott » 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.
First, let me make that claim more precise. What I really mean is that almost everyone who “writes C programs” is really programming in a purely functional language. Modulo some minor historical accidents.
“C programmers” really program not in C, but in the purely functional language cpp (the “C Preprocessor”). As with any purely functional language, cpp consists only of (nestable) expressions, not statements. Now here’s the clever part: when evaluated (not executed), a cpp expression yields a (pure) value of type C, which is an ADT (abstract data type) that represents imperative programs. That value of type C is then executed (not evaluated) by the cpp language’s RTS (run-time system). As a clever optimization, cpp’s RTS actually includes a code generator, considerably improving performance over more naïve implementations of the C abstract data type.
(So now you can see that, “C programmer” is almost always a misnomer when applied to a person. Instead people are cpp programmers, and cpp programs are the real C programmers (producers of C). I don’t think I can buck this misnomer, so I’ll stay with the herd and use “C programming” to mean cpp programming.)
