Create a MonadThrow typeclass.

[tmciver]

The MonadThrow class might look something like this:

interface MonadThrow extends Monad {
  function throwM($errObj);
}

This will make it easier to make methods generic in their return monads. For example, one could write a database-querying method like so:

class MyRepository {
  function getEntityById(string $id, $monadThrow = new MaybeMonad()) {
    $dbEntity = ... // query the database

    return is_null($dbEntity) ?
      $monadThrow->throwM("Entity with ID: " . $id . " could not be found.") :
      $monadThrow->pure($dbEntity);
  }
}

If the caller desires the return type to be Maybe, they could simply call it like this:

$maybeEntity = $repository->getEntityById("123456");

But if, say, an Attempt is desired, then they could call it like this:

$attemptEntity = $repository->getEntityById("123456", new AttemptMonad());

This presumes, of course, that both MaybeMonad and AttemptMonad have implemented the MonadThrow interface.