Fun

The Fun component provides functional programming utilities for composing, decorating, and controlling function execution. These small combinators let you build data pipelines, inject side effects without disrupting flow, and defer expensive computations until they are actually needed.

Usage

Piping Values Through Functions

pipe() chains closures left-to-right. Each function receives the output of the previous one:

use Psl\Fun;
use Psl\Str;

$transform = Fun\pipe(
    static fn(string $s): string => Str\trim($s),
    static fn(string $s): string => Str\lowercase($s),
    static fn(string $s): string => Str\replace($s, ' ', '-'),
);

$transform('  Hello World  '); // 'hello-world'

Composing Two Functions

after() connects two functions where the output type of the first can differ from the input type of the second:

use Psl\Fun;
use Psl\Str;

$strlen = Fun\after(static fn(string $s): string => Str\trim($s), static fn(string $s): int => Str\length($s));

$strlen('  hi  '); // 2

Side Effects with tap()

tap() runs a callback for its side effect and returns the original value unchanged -- useful for logging or debugging inside a pipeline:

use Psl\Fun;
use Psl\IO;
use Psl\Str;

$process = Fun\pipe(
    Fun\tap(static fn(string $v) => IO\write_error_line('input: %s', $v)),
    static fn(string $s): string => Str\uppercase($s),
    Fun\tap(static fn(string $v) => IO\write_error_line('output: %s', $v)),
);

$process('hello'); // 'HELLO' (logs are emitted as a side effect)

Lazy Evaluation

lazy() wraps an initializer so the value is computed once on first access, then cached:

use Psl\Fun;
use Psl\Vec;

$config = Fun\lazy(static fn(): array => Vec\fill(1000, 'value'));

$config(); // computes the array
$config(); // returns cached result

Conditional Logic

when() returns one of two results based on a predicate:

use Psl\Fun;

$classify = Fun\when(
    static fn(int $n): bool => $n >= 18,
    static fn(int $_n): string => 'adult',
    static fn(int $_n): string => 'minor',
);

$classify(21); // 'adult'
$classify(12); // 'minor'

Utility Functions

identity() returns a closure that passes its argument through unchanged -- handy as a default transformer or no-op callback:

use Psl\Fun;

$fn = Fun\identity();
$fn(42); // 42

rethrow() returns a closure that rethrows any exception it receives -- useful as an error callback:

use Psl\Fun;

$handler = Fun\rethrow();

try {
    $handler(new \RuntimeException('boom'));
} catch (\RuntimeException $e) {
    echo $e->getMessage(); // 'boom'
}

See src/Psl/Fun/ for the full API.