Output of my live coding impro at Full Stack Europe 2022

fseu.php

<?php declare(strict_types=1);
// @mathiasverraes
// parser is a function that takes some input and returns (either (structured value AND remainder) OR error)
// parser is a function that takes some input and returns  a Result

$a = function ($input) {
    $parsed = substr($input, 0, 1);
    $remainder = substr($input, 1);
    if ($parsed === 'a') return succeed($parsed, $remainder); else return fail("expected a");
};
$char = function (string $char) {
    return function ($input) use ($char) {
        $parsed = substr($input, 0, 1);
        $remainder = substr($input, 1);
        if ($parsed === $char) return succeed($parsed, $remainder); else return fail("expected $char");
    };
};


$seq = function ($p1, $p2) {
    return function ($input) use ($p1, $p2) {
        $r1 = $p1($input);
        if ($r1->isSuccess()) {
            $r2 = $p2($r1->remainder());
            return $r2;
        } else {
            return $r1;
        }
    };
};


$append = function ($p1, $p2) {
    return function ($input) use ($p1, $p2) {
        $r1 = $p1($input);
        if ($r1->isSuccess()) {
            $r2 = $p2($r1->remainder());
            if ($r2->isSuccess()) {
                return succeed($r1->parsed() . $r2->parsed(), $r2->remainder());
            }
            return $r2;
        } else {
            return $r1;
        }
    };
};

$either = function ($p1, $p2) {
    return function ($input) use ($p1, $p2) {
        $r1 = $p1($input);
        if ($r1->isSuccess()) {
            return $r1;
        } else {
            return $p2($input);

        }
    };
};


$map = function ($parser, $map) {
    return function ($input) use ($map, $parser) {
        $r = $parser($input);
        if ($r->isSuccess()) {
            return succeed($map($r->parsed()), $r->remainder());
        } else {
            return $r;
        }
    };
};


final class Result
{
    private $parsed;
    private $remainder;
    private $error;
    private bool $success = false;

    private function __construct()
    {
    }

    public function parsed()
    {
        return $this->parsed;
    }

    public function remainder()
    {
        return $this->remainder;
    }

    public function isSuccess(): bool
    {
        return $this->success;
    }

    public function isFail(): bool
    {
        return !$this->success;
    }

    public function error()
    {
        return $this->error;
    }

    public static function succeed($parsed, $remainder): Result
    {
        $r = (new Result());
        $r->success = true;
        $r->parsed = $parsed;
        $r->remainder = $remainder;
        return $r;
    }

    public static function fail(string $error): Result
    {
        $r = (new Result());
        $r->success = false;
        $r->error = $error;
        return $r;
    }
}

function fail(string $error)
{
    return Result::fail($error);
}

function succeed($parsed, $remainder)
{
    return Result::succeed($parsed, $remainder);
}