Functional programming basics part 1: Pure function

ysael pepin on September 13, 2018

What is purity? Purity for a function is defined as one that will always return the same output for the same input and perform no side effects. ... [Read Full]
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I admit that functional programming is one of the fields where I lack any deeper understanding, so excuse my ignorance for a while, but does that mean that any C function declared as void is a pure function as long as its name contains everything it does?


I don't think that the void return type has anything to to with function purity. A function can be pure even if it returns some value. It just shouldn't modify anything outside of it and does not use any variables that are defined outside of its scope. Its return value should be determined solely by its input parameters.


Its return value should be determined solely by its input parameters.

Wouldn't that make any function in strongly typed languages "pure"?


For example:

int a = 5;

int sumOfNumbers(int x, int y) {
  a = 1;
  return x + y;

int sum = sumOfNumbers(12, 33);

This function is not pure as it modifies a.


Yes I would say that Luka is right.

And would add that for me, making a function that returns void is a way of declaring that that particular function is impure and will most likely be made to do side effects stuff.... cause we need to do them sometimes:)


that particular function (...) will most likely be made to do side effects stuff

I thought that this can be solved by changing the signature?


template <typename T>
void processThenOutputConcatenatedInputVariables(T var1, T var2) {
    process(var1, var2); // side effect covered by the signature
    std::cout << var1 << var2;

Unfortunately the signature wont make this function pure.

What you would need in this case is composition which would allow you to compose 2 functions. I will at one point cover composition, in the mean time, you can read that great article by Eric Elliott -->

Unfortunately the signature wont make this function pure.

Why not?


There is nothing to do with the signature of a function.

A function is pure if and only if

  1. The returned value is calculated only with the arguments
  2. It does not change any thing outside of a function (side effect)

So in your example, assuming process(var1, var2) will make some side effect somewhere else in the program, it is not a pure function.

However, if process do not make any changes, then it will still be a pure function.


Any useful function returning void is definitely impure.
Even if it doesn't reference anything besides its parameters, it's still bound to mutate or otherwise bother the parameters.
A pure function:

  1. has no effect on the world except returning something.
  2. the return value depends only on the parameters.

One of the consequences of 2. is that when given the same parameters, it should always produce the same return value.

This is a useful dipping stick that demonstrates that functions that depend on eg time or randomness aren't pure.


Luka is right. For function to be pure it shouldn't change things out of it's scope too which void function can do. It's because whole point of pure functions is to write more predictable code.


Purity simply means:

  • no side-effects
  • same output for input

If a function returns void and has no side-effects, it's pure, but it also doesn't do anything interesting.


I think something that returns VOID is not even a function at all, but a procedure....don't functions need input/ouput by mathematical definitions?


that is a great way to describe it I think!


Currently this is not valid Javascript:

const addYtoX(x, y) => x + y

But this is:
const addYtoX = (x, y) => x + y


oups! I just corrected that, thanks a lot!


Funny that you choose JavaScript for the examples, while it doesn't support pure functions.


There is a good article on the topic that seems to spark some interesting debates-->

I feel we can still benefit from the concept and that indeed JavaScript will remain a language full of surprises ;) Maybe that is why we like it so much in the end?

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