poster' :: [MediaType] -> Maybe (Either String String)
poster' (x:xs) = case x of
    (Poster a) -> Just a
    _ -> poster' xs
poster' :: [MediaType] -> Maybe (Either String String)
poster' (x:xs) = case x of
    (Poster a) -> Just a
    _ -> poster' xs
poster' (x:xs) = case x of
    (Poster a) -> Just a
    _ -> poster' xs