-- Port of part of Control.Monad.Error to use ATs (and then to phrac)

data Either a b = Left a | Right b;

class Error e where {
    noMsg :: e;
    strMsg :: List Char -> e;
}

instance Error (List Char) where {
    noMsg = Nil;
    strMsg s = s;
}

class Monad m where {
    return :: a -> m a;
    (>>=) :: m a -> (a -> m b) -> m b;
}

(>>) p q = p >>= \_ -> q;

class Monad m => MonadError m where {
    type MError m;
    throwError :: MError m -> m a;
    catchError :: m a -> (MError m -> m a) -> m a;
}
    
instance (Error e) => Monad (Either e) where {
    return = Right;
    (>>=) p q = case p of
                Left e -> Left e;
                Right x -> q x;;          
}

instance (Error e) => MonadError (Either e) where {
    type MError (Either e) = e;
    throwError = Left;
    catchError p h = case p of
                     Left e -> h e;
                     x -> x;;
}

main :: Either (List Char) Unit;
main = return ();