# Expressions And Operators: Function Call

A function is called using the () operator, which optionally contains a comma-separated list of expressions called arguments. Consider the following function definition and call:

newtype Point = (float, float);

function create_point(float $x, float$y): Point {
return tuple($x,$y);
}

function distance(Point $p1, Point$p2): float {
$dx =$p1[0] - $p2[0];$dy = $p1[1] -$p2[1];
return \sqrt($dx *$dx + $dy *$dy);
}

<<__EntryPoint>>
function main(): void {
echo "Distance is ".distance(tuple(3.5, -6.2), tuple(-2.4, 3.6))."\n";
}

Output
Distance is 11.438968484964


Here, the function distance is called from main using the called-function's name followed by the function-call operator containing a list of two arguments. Each argument corresponds by position to a parameter in the called function's definition. An argument can have any type that is compatible with the corresponding parameter. In a function call, the arguments are evaluated in the order left-to-right. This is important to understand if any of the argument values contains side-effects; for example:

distance(tuple($v++, -6.2), tuple(-$v, 3.6))


Ordinarily, the function-call operator is preceded by the name of the function to be called; however, what that operator really needs is an expression that designates a function, and alternative approaches are available beside using that function's name. Consider the following example, which indexes into an array of anonymous-function objects, and calls the resulting unnamed function:

<<__EntryPoint>>
function main(): void {
$table = vec[ (int$p) ==> $p * 2, // doubler (int$p) ==> $p * 3, // tripler (int$p) ==> $p * 4, // quadrupler ];$seq = new Sequence();

$v =$table[$seq->next()]($seq->next()); // eval is L-to-R
echo "\$v =$v\n";

$v =$table[$seq->next()]($seq->next()); // eval is L-to-R
echo "\$v =$v\n";
}

class Sequence {
private int $last = -1; public function next(): int {$this->last++;
return $this->last; } }  Output $v = 2
$v = 12  In the function call, $table[$seq->next()]($seq->next())


the expression designating the function is evaluated before the expressions in the argument list, so given that $seq->next() has not been called before, the call is $table[0](1)


which calls the doubler function, which returns 1x2=2.

In the second call,

$table[$seq->next()]($seq->next())  given that $i has been called twice, the call is

$table[2](3)  which calls the quadrupler function and results in 3x4=12. When a function is called, the value of each argument passed to it is assigned to the corresponding parameter in that function's definition, if such a parameter exists. Any parameter having a default value, but no corresponding argument, takes on that default value. For example: function f3(int$p1 = -1, float $p2 = 99.99, string$p3 = '??'): void { ... }
f3();                   // $p1 is -1,$p2 is 99.99, $p3 is ?? f3(123); //$p1 is 123, $p2 is 99.99,$p3 is ??
f3(123, 3.14);          // $p1 is 123,$p2 is 3.14, $p3 is ?? f3(123, 3.14, 'Hello'); //$p1 is 123, $p2 is 3.14,$p3 is Hello


If the called function is variadic, the function call can have any number of arguments, provided the function call has at least an argument for each parameter not having a default value. When an argument corresponds to the ellipsis in the called function's definition, the argument can have any type.

Direct and indirect recursive function calls are permitted. For example:

function factorial(int $i): int { if ($i > 1) return $i * factorial($i - 1);
else if ($i === 1) return$i;
else return 0;
}


When an instance method or constructor is called, the instance used becomes the value of $this in the invoked method or constructor. However, if no instance was used (for example, in the call C::instance_method()) the invoked instance has no $this defined.

By default, arguments are passed by value; however, if a parameter contains the inout modifier, the corresponding argument must also contain that modifier. See the swap function in defining a function for an example.