php functional-php library less_than函数

Question

I saw in source code of php-functional library such function :

function less_than($b)
{
    return function ($a ) use ($b) {
        return $a < $b;
    };
}

I can't get there $a comes from. How is this going to work?

We pass only $b to function less than and $a is undefined.

This is the link to source code where I took it from.

Answer 1

The idea of functional programming is that functions are first-class objects. So you can pass functions as parameters; and you can return functions.

So here you can construct a function:

$func_less = less_than(3);

So we basically construct a function like:

$func_less = function ($a) {
    return $a < 3;
};

and now $func_less is a function that takes a parameter $a and checks if that $a is less than three. For example:

php > $func_less_three = less_than(3);
php > var_dump($func_less_three(2));
bool(true)
php > var_dump($func_less_three(14));
bool(false)

So first we construct a function $func_less_three, then we call that function with two numbers 2 and 14. Since 2 is less than 3, it returns bool(true) on the first call, and since 14 is not less than 3, it return bool(false) on the second call.

In most functional programming languages like Haskell, all functions take exactly one parameter. So instead of adding two numbers together, one first constructs a function that adds a (in that function) constant value to a parameter.