Moving away from Monads to handle errors

example.php

<?php

function generateFunctionsAndClass(int $i)
{
    $i1 = $i - 1;
    return <<<TEMPLATE
enum E{$i} { case Error; }
class R{$i} {}
function f{$i}(R{$i1} \$input): R{$i}|E{$i} { return new R{$i}; }
function e{$i}(R{$i1} \$input): R{$i}|E{$i} { return E{$i}::Error; }
TEMPLATE;
}

$code = join("\n", [
    'class R0 {}',
    ...array_map(
        generateFunctionsAndClass(...), range(1, 5)
    )
]);
eval($code);

function isOfSingleType($value, ReflectionNamedType $type): bool
{
    $type_name = $type->getName();
    if ($type->isBuiltin()) {
        return (get_debug_type($value) === $type_name);
    } else {
        return ($value instanceof $type_name);
    }
}

function isOfType($value, ReflectionType $type): bool
{
    return match (true) {
        $type instanceof ReflectionNamedType => isOfSingleType($value, $type),
        $type instanceof ReflectionUnionType => array_reduce(
            $type->getTypes(),
            fn (bool $carry, ReflectionType $type) => $carry || isOfType($value, $type),
            initial: false,
        ),
        $type instanceof ReflectionIntersectionType => array_reduce(
            $type->getTypes(),
            fn (bool $carry, ReflectionType $type) => $carry && isOfType($value, $type),
            initial: true,
        ),
    };
}


/**
 * @template T
 * @param T $x
 * @return T
 */
function identity(mixed $x): mixed {
    return $x;
}

/**
 * @template T
 * @template S
 * @template R
 * @param Closure(T): S $f
 * @param Closure(S): R $g
 * @return Closure(T): R
 */
function apply(Closure $f, Closure $g): Closure {
    return static fn($v) => $g($f($v));
}

function applyAll(Closure ...$closures): Closure
{
    return array_reduce(
        $closures,
        callback: apply(...),
        initial: identity(...)
    );
}

function applyIfValid(Closure $f, Closure $g): Closure {
    $input_type = (new ReflectionFunction($g))->getParameters()[0]->getType();
    return static function($v) use ($f, $g, $input_type) {
        $inter = $f($v);
        if (isOfType($inter, $input_type)) {
            return $g($inter);
        } else {
            return $inter;
        }
    };
}

function applyAllIfValid(Closure ...$closures): Closure
{
    return array_reduce(
        $closures,
        callback: applyIfValid(...),
        initial: identity(...)
    );
}

$g = applyAllIfValid(f1(...), f2(...), e3(...), f4(...), f5(...));
var_dump($g(new R0));