Result

Result<T, E> represents an operation that either succeeded with a value (Ok) or failed with a typed error (Err) — no exceptions as control flow.

Creating a Result

use Maybe\Result\Result;

function loadUser(int $id): Result
{
    if ($id <= 0) {
        return Result::err('invalid_id');
    }

    return Result::ok(['id' => $id, 'name' => 'Ana']);
}

Chaining fallible operations

andThen is the workhorse: it chains another operation that can also fail, short-circuiting on the first Err.

$invoice = loadUser(10)
    ->andThen(fn (array $user): Result => checkSubscription($user))
    ->andThen(fn (array $user): Result => createInvoice($user))
    ->map(fn (array $invoice): array => addTotals($invoice));

orElse is the mirror image — recover from an error by producing a new Result:

$user = loadFromCache($id)
    ->orElse(fn (string $error): Result => loadFromDatabase($id));

mapErr transforms the error payload without touching success values:

$result = loadUser($id)->mapErr(fn (string $e): array => ['code' => $e]);

Extracting values

$result->unwrap();                          // value, or throws UnwrapErrException on Err
$result->unwrapErr();                       // error, or throws UnwrapOkException on Ok
$result->unwrapOr($default);                // value or eager fallback
$result->unwrapOrElse(fn ($e) => recover($e)); // value or fallback computed from error
$result->expect('user must exist');         // value, or throws with your message

Prefer match when both branches need handling:

$message = loadUser(10)->match(
    fn (array $user): string => 'User: ' . $user['name'],
    fn (string $error): string => 'Error: ' . $error
);

Converting to Option

$result->okOption();   // Ok(v) -> Some(v), Err(_) -> None
$result->errOption();  // Err(e) -> Some(e), Ok(_) -> None

API summary

Method	Description
Result::ok($v) / Result::err($e)	Constructors
map(fn) / mapErr(fn)	Transform success / error
andThen(fn) / orElse(fn)	Chain fallible operations / recover
match(onOk, onErr)	Exhaustive handling
unwrap() / unwrapErr() / unwrapOr($d) / unwrapOrElse(fn) / expect($msg)	Extraction
okOption() / errOption()	Convert to Option
isOk() / isErr()	Inspection