example.hs

findById :: User.Id -> forall s. Tx Postgres s (Maybe User)
findById userId = do
    result <- maybeEx [stmt|
        SELECT id, email, created_at, updated_at
        FROM users
        WHERE id = $userId
    |]

    return case result of
        Just row -> toUser row
        Nothing  -> Nothing

-- versus:

findById :: User.Id -> forall s. Tx Postgres s (Maybe User)
findById userId = fmap toUser <$> maybeEx q where q = [stmt|
        SELECT id, email, created_at, updated_at
        FROM users
        WHERE id = $userId
    |]
    
-- maybe even

findById :: User.Id -> forall s. Tx Postgres s (Maybe User)
findById userId = do
    result <- maybeEx [stmt|
        SELECT id, email, created_at, updated_at
        FROM users
        WHERE id = $userId
    |]

    user <- return $ do
        row <- result
        return (row & toUser)

    return user