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After a stumbled upon this question I thought about how I would define a pure function. I have two rules to accept a function as pure which somewhat differ from the definition cited in the top answer. I'm interested to know if my view is correct or not. My "2 rules" are:

  • The function will not change the state of the program.
  • The input is constant for the same arguments or the same local public program state.

To clarify the second point, by local public state I mostly mean public class members. Local means that the state is strongly associated with the function and is exclusive to it (ie. not some global variables). My reasoning for using that definition is that it is just a more expanded view (perhaps too specific to CS) on the term arguments, as I see local public state being only a different interface for passing function arguments. Is this definition acceptable or is the inclusion of a class context destroys the "prunes" of the function?

Edit: As an example for what I mean consider the following class (C++). 

class Foo
{
public: 
    int Number;

    int foo(int v) { return Number + v; }
}

Considering that an instance function call is actually the following call:

foo(&this, 123)

How is this different from passing the public data (Number) via a struct? 

struct Foo
{
    int Number;
} 

foo(Foo { 1 }, 123);
Matan Shahar
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  • Definitely no. I think at max would allow it to use a constant defined in the class like public static double PI = 3.14; So " function always evaluates the same result value given the same argument value" – tgkprog Nov 01 '17 at 07:15
  • This is covered in the [Wikipedia page on Pure function](https://en.wikipedia.org/wiki/Pure_function) where it mentions "free variables". A constant, defined outside the pure function, won't break the purity but a free variable will. Note that a constant really must be a true constant, not just whatever the flavor of the month is right now. – Lasse V. Karlsen Nov 01 '17 at 07:18

1 Answers1

4

No, that "or the same local public program state" is not in the pure method definition everyone else agrees with.

A pure function computes a value based on the input parameters and the input parameters alone, and does not, beyond returning the computed value, produce any observable side effects.

I advice you to take a look at the Wikipedia page on Pure function as it shows some examples.

--

Also note that "no observable side effects" does not mean "no side effects whatsoever", but the gray area overlapping those two generally only include runtime-specific side effects that you have no control over and generally do not observe.

This also includes state such as "there is enough memory to allocate the memory needed".

Lasse V. Karlsen
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  • Thanks for the answer. I've edited the question to show why I feel that a class is a different interface for passing arguments. If you could address that point which I'm in not so sure about this will answer my question fully – Matan Shahar Nov 01 '17 at 07:32
  • No, that is a free variable and makes the function unpure. – Lasse V. Karlsen Nov 01 '17 at 08:13
  • No external state really means no external state **to that function**. A field member parallel to the method is external. Period. You may wish to create your own concept but it isn't the "pure function" concept everyone else has agreed upon. – Lasse V. Karlsen Nov 01 '17 at 08:14