Well there is the "official" definition on Wikipedia.[0]
Informally, my definition of a functional programming language is one that
supports functional style as the default. (Hooray for using a recursive
definition. :p) The language should strive for referential transparency and the
language and compiler should be optimized for functional idioms.
Let's define the sum of a list functionally[1]:
# Python
sum = lambda x: reduce(int.__add__, x, 0)
# Haskell
sum = foldl' (+) 0
You could write this same function in C using function pointers and arrays.
However it's not very convenient to do so. You would need to write a reduce
function, an add function, anonymous functions don't exist, and syntactically it
would be very messy. On top of all this, the compiler is not optimized for
reduce functions vs an imperative for loop.[2]
More generally, functional style typically refers a style of programming where
most functions are pure and side effects are contained specifically. This allows
for easier reasoning of a program's correctness.
John Carmack has a post about functional programming in C++:
[1]: This ignores the fact that sum() is already provided in most functional
languages.
[2]: Python straddles my definition of functional programming and that's why
it's awesome and frustrating at the same time. It supports many staple
functional idioms but is not optimized for it. Function calls are very
expensive, and thus recursive performance is also hurt tremendously.
