JuniorTheoryCommon

What does the program print?

PHP

<?php

function applyTwice(callable $f, int $x): int
{
    return $f($f($x));
}

$inc = function (int $n): int {
    return $n + 3;
};
echo applyTwice($inc, 1), ' ', applyTwice('abs', -5), "\n";

7 5

Expecting 4, as if $f ran once: $f($f($x)) calls the function on $x, then again on that result, so $inc adds 3 twice.
Expecting a TypeError for 'abs': a string that holds the name of a function is a valid callable, and calling it runs that function.

How would you write applyTwice so that it applies $f a given number of times?
Which values can a callable parameter accept besides an anonymous function?

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JuniorTheoryCommon

What does the program print?

PHP

<?php

function applyTwice(callable $f, int $x): int
{
    return $f($f($x));
}

$inc = function (int $n): int {
    return $n + 3;
};
echo applyTwice($inc, 1), ' ', applyTwice('abs', -5), "\n";

7 5

Expecting 4, as if $f ran once: $f($f($x)) calls the function on $x, then again on that result, so $inc adds 3 twice.
Expecting a TypeError for 'abs': a string that holds the name of a function is a valid callable, and calling it runs that function.

How would you write applyTwice so that it applies $f a given number of times?
Which values can a callable parameter accept besides an anonymous function?

Back to questions Open topic in roadmap →Answer it yourself — Quiz