The C language is purely functional

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. “C programmers” really program not in C, but in the purely functional language cpp (the “C Preprocessor”). Now along the way, some mistakes were made that tarnished the theoretical beauty of cpp+C:

Fortunately for purists, the relatively obscure programming language Haskell restored the original untarnished beauty of K&R’s vision by fixing these defects.

Source: conal.net