What's the difference between a keyword or a statement, and a function call?

Question

I was thinking about this recently since Python 3 is changing print from a statement to a function.

However, Ruby and CoffeeScript take the opposite approach, since you often leave out parentheses from functions, thereby blurring the distinction between keywords/statements and functions. (A function call without parentheses looks a lot like a keyword.)

Generally, what's the difference between a keyword and a function? It seems to me that some keywords are really just functions. For example, return 3 could equally be thought of as return(3) where the return function is implemented natively in the language. Or in JavaScript, typeof is a keyword, but it seems very much like a function, and can be called with parentheses.

Thoughts?

Answer 1

A function is executed within a stack frame, whereas a keyword statement isn't necessarily. A good example is the return statement: If it were a function and would execute in its own stack, there would be no way it could control the execution flow in the way it does.

Answer 2

Keywords and functions are ambiguous. Whether or not parentheses are necessary is completely dependent upon the design of the language syntax.

Consider an integer declaration, for instance:

int my_integer = 4;

vs

my_integer = int(4)

Both of these examples are logically equivalent, but vary by the language syntax.

Programming languages use keywords to reserve their finite number of basic functions. When you write a function, you are extending a language.

Answer 3

Keywords are lower-level building blocks than functions, and can do things that functions can't.

You cite return in your question, which is a good example: In all the languages you mention, there's no way to use a function to provide the same behavior as return x.