Discussion on: Learning Functional Programming

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Replies for: Not really - having each function do a precise task is a general good practice, not a quality of pure functions. A setter, for example, is an impur...

Valentin Silvestre • Dec 15 '17

Ok, but you sometimes NEED to do impure function no ?
How could you not change the type of some variables ?

Ex :
You want to get array data to convert into object, you need to do an impure function no ?

Idan Arye • Dec 15 '17

Not really:

class Obj:
    def __init__(self, array):
        # ...


array = [1, 2, 3]
obj = Obj(array)

I converted the array to an object, but the function is still pure(or, more precisely, can be pure - I didn't write Obj.__init__'s body and Python does not guarantee function purity). We still have the original array - we did not change existing values, only returned new ones.

A better example will adding an element to an array:

def foo(array, value):
    array.append(value)


def bar(array, value):
    return array + [value]


array = [1, 2, 3]

foo(array, 4)
assert array == [1, 2, 3, 4]

array2 = bar(array, 5)
assert array == [1, 2, 3, 4]
assert array2 == [1, 2, 3, 4, 5]

foo is impure, because it changes an existing value. bar is pure, because it does not change an existing value - it creates a new one.

Valentin Silvestre • Dec 15 '17

Now I got it !

Last question, why do we have to do this ?

Malcolm Kee • Dec 15 '17

FP is not about eliminating side effect at all, because a software that does not has any side effect has no use at all. Instead, FP is about making as much area of your code to be pure function so that they are predictable and consistent, while allow only specific module to be impure, i.e. having side effect.

Valentin Silvestre • Dec 15 '17

.. So, pure function could also be a priority for OOP no ?

Kasey Speakman • Dec 15 '17 • Edited

I would think so, yes. You find that most Smalltalk-inspired OOP advocates tend to view sharing of data as a bad thing. Instead, data is private to an object, and you just tell the object what you want it to do (by sending it a message or invoking a method). The object then induces local mutations to itself. It's still not technically a pure function, but this flavor of OOP follows the spirit of not mutating a shared external state. Because that's what makes programs hard to change over time.

A pure function has additional benefits like thread safety.

Valentin Silvestre • Dec 18 '17

Ahah thread.. I'm doing PHP :(