Functional Programming

Pure Function

Functions that only operate on their input parameters.

Categories:

1. Impure Functions
2. Pure Functions:
   1. Not Useful Pure Functions
   2. Useful Pure Functions

Impure Pure Function

Functions that read and/or write to an out-of-scope variable.

// Example #1

$var = 10;

function add($a, $b) {
    $var = $a + $b;

    return $var;
}

print $var;    // Print 10

print add(1, 2);    // Print 13

// Examples #2
send($request);
update($query);
write($fileName);

Pure Function

Functions that only operate on their input parameters.

// Example #1

function addTen() {
    return 10;
}

print addTen();    // Print 10

// Better use a constant
define("TEN", 10);

print TEN;    // Print 10

Not-Useful Pure Function

Pure Function

Functions that only operate on their input parameters and return a value.

// Example #1

function add($a, $b) {
    return $a + $b;
}

print add(1, 2);    // Print 3

Useful Pure Function

Pure Function vs. Impure Function

Impure Function may change a variable somewhere deep in the code base.

Makes debugging harder.

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