It depends. A C function is actually a procedure: a list of statements that have some effect on the computer, and then (maybe) shove some return value through t

It depends. A C function is actually a procedure: a list of statements that have some effect on the computer, and then (maybe) shove some return value through the stack at the end. A function in languages like Clean or Haskell or Clojure or Agda or Coq can only return values, and have no effect on the machine (like mutating memory). Of course, this construct is used only as a way to express ideas. The notion of a function that has no other effect on the machine is actually a lie. The time it takes to evaluate the function is a real effect. So is the memory it uses. It's not a perfect abstraction (none are) but for many types of problems, pure functions can work as a tool to help a programmer. Sorry, typed this on a phone, hard to edit.