re: Functional programming basics part 1: Pure function VIEW POST


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.


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.


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.


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.


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.

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