README

A small pile of PHP monads (and friends) that makes functional-style code not terrible.

Why this exists (a tiny tale of regret)

I made a mistake.

I decided to make a parser, and I’d gotten very used to what the Sprache library does for .NET. Underneath it is a bunch of LINQ magic - which PHP does not have. So: a couple of years of poking at ANTLR, reading about packrat parsers, trying to remember the difference between LL and LR (no need to explain it to me), and getting really, really peeved.

Then I ran across OCaml, and all these friendly tutorials showing how easy and pleasant combinator parsers are. Great! Still not PHP. sigh

The “obvious solution” was to write a parser in C and bind it into PHP. No. Wait. Bad idea.

So I looked at everything I’d picked up over the years… we hates it, we does; it burns, precious!

The need didn’t go away: I still want a combinator parser. My first drafts were drowning in edge cases and brittle failure paths.

Start over. Give the parser a foundation. This library is that foundation. Tada!

It’s licensed Apache-2.0 - please do whatever you want with it. Send PRs; I know I’m not covering everything (yet) in functions or tests. I’m calling this “good enough for now.” Enjoy!

Features

The usual suspects:
- Maybe – optional values without null sadness
- Either – success (Right) or error (Left) with typed error channels
- IO – wrap side effects (delay, mapError, handleErrorWith, bracket)
- Reader – thread read-only context (config, services) through pure code
- State – state threading: S -> (S, A)
- Writer – accumulate logs alongside values
- ListM – list/sequence monad (cartesian/branching)
Clear exceptions (0.1.0+): domain-specific error types instead of generic runtime errors
Helpful docs under docs/
PHPUnit + Infection setup (mutation testing)

Coming from other FP ecosystems?
See docs/aliases.md for a Rosetta Stone of names (Haskell, Cats, Arrow, Fantasy Land, Elm, Rust, Swift, F#).

Install

Requires PHP 8.4+

composer require penguinpark/monad

Autoloads under PenguinPark\Monad\.

Quick taste

Maybe

use PenguinPark\Monad\Maybe\Maybe;

$answer = Maybe::of(41)
    ->map(fn (int $x) => $x + 1)
    ->getOrElse(0); // 42

Either

use PenguinPark\Monad\Either\Either;
use PenguinPark\Monad\Either\Left;
use PenguinPark\Monad\Either\Right;

$parseInt = function(string $s): Either {
    return ctype_digit($s) ? new Right((int)$s) : new Left('bad int');
};

$result = $parseInt('123')
    ->map(fn (int $n) => $n + 1)                 // Right(124)
    ->getOrElse(0);                              // 124

$error  = $parseInt('nope')->getOrElse(0);       // 0

IO

use PenguinPark\Monad\IO\IO;
use InvalidArgumentException;

// Lazy effect
$io = IO::delay(fn () => file_get_contents('/etc/hosts'))
    ->map('strlen')
    ->mapError(fn (\Throwable $e) => new InvalidArgumentException('read failed', previous: $e));

$len = $io->unsafeRun(); // int

Resource-safety:

use PenguinPark\Monad\IO\IO;

$readFirstLine = IO::bracket(
    acquire: fn () => fopen('/etc/hosts', 'r'),
    use:     fn ($fh)  => IO::delay(fn () => fgets($fh)),
    release: fn ($fh)  => IO::delay(fn () => fclose($fh))
);

$line = $readFirstLine->unsafeRun();

Writer

use PenguinPark\Monad\Writer\Writer;

[$log, $val] = Writer::tell('start')
    ->map(fn () => 'value')
    ->tell('done')
    ->run();        // $log = ['start','done'], $val = 'value'

Error model (custom exceptions)

Common failure modes throw specific exceptions under PenguinPark\Monad\Exception:

UnwrapNothing, UnwrapLeft
ContractViolation (shape/type contracts)
InvalidBindReturnType, ApCallableExpected, BracketReturnTypeExpected

This lets tests assert precise failures instead of string messages.

Validation (Applicative) - optional, if enabled

If you’ve pulled in the 0.1.1 changes:

Validation, Valid, Invalid accumulate all errors via ap.
Designed for data validation; there is no flatMap (applicative focus).

use PenguinPark\Monad\Validation\Validation;

$build = Validation::of(fn ($e) => fn ($a) => fn ($s) => compact('e','a','s'));

$res = $build
  ->ap(vEmail('bad@ex'))   // -> invalid('email: bad format')
  ->ap(vAge(15))           // -> invalid('age: must be ≥ 18')
  ->ap(vSalary(-10));      // -> invalid('salary: must be positive')

$out = $res->fold(fn (array $errs) => $errs, fn ($ok) => $ok);

Project status

Current tag: 0.1.1 (custom exceptions, solid core)
Next up:
- Docs: keep expanding usage examples and the aliases guide
- Nice-to-haves: Either::sequence/traverse, IO::sequence/traverse

Development

composer test       # phpunit
composer mutate     # infection --min-msi=100

CI runs on branch canon. Docs-only changes skip CI by default.

Contributing

Issues and PRs welcome. Please include tests. If you’re fixing edge cases in IO/Writer, mutation tests are your friend.

License

Apache-2.0 - see LICENSE.

penguinpark / monad

Maintainers

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