What is the name of a function whose result depends only on its parameters?

Question

I'm writing a toy compiler thingy which can optimise function calls if the result depends only on the values of the arguments. So functions like xor and concatenate depend only on their inputs, calling them with the same input always gives the same output. But functions like time and rand depend on "hidden" program state, and calling them with the same input may give different output. I'm just trying to figure out what the adjective is that distinguishes these two types of function, like "isomorphic" or "re-entrant" or something. Can someone tell me the word I'm looking for?

Answer 1

The term you are looking for is Pure

• http://en.wikipedia.org/wiki/Pure_function

Answer 2

I think it's called Pure Function:

In computer programming, a function may be described as pure if both these statements about the function hold:

• The function always evaluates the same result value given the same argument value(s). The function result value cannot depend on any hidden information or state that may change as program execution proceeds or between different executions of the program, nor can it depend on any external input from I/O devices.
• Evaluation of the result does not cause any semantically observable side effect or output, such as mutation of mutable objects or output to I/O devices.

The result value need not depend on all (or any) of the argument values. However, it must depend on nothing other than the argument values.

Answer 3

I guess you could say the adjective is "pure" if you go by "pure function".

I always learnt that a function whose output is always the same when the arguments are always the same is called "deterministic". Personally, I feel that that is a more descriptive term. I guess a "pure function" is by definition deterministic, and it seems a pure function is also required to not have any side-effects. I assume that that need not be the case for all deterministic functions (as long as the return value is always the same for the same arguments).

Wikipedia link: http://en.wikipedia.org/wiki/Deterministic_algorithm

Quote:

Given a particular input, it will always produce the same output, and the underlying machine will always pass through the same sequence of states.