Comments exist to provide information beyond primitive, domain-agnostic types (String, Int, etc.), but without the overhead of more elaborate modelling into domain-specific types (ChequingAccounts, Widgets, and so forth).

I may want to communicate further information about the inhabitants or values of a particular type, without introducing extraneous or superfluous types:

    -- given a path to a PEM encoded PKCS#8 formatted RSA private key and a
    -- JWT Claims Set (RFC 7519) represented as a strict ByteString,
    -- return a strict ByteString representing a base64 encoded RSA256-signed JWS.
    generateJWT :: FilePath -> B.ByteString -> IO (Maybe B.ByteString)
    generateJWT fp claims = (fmap unJwt <$>) . (maybe (return Nothing) (fmap eitherToMaybe . encodeClaims) =<<)
                                             $ fromPKCS8 fp
      where eitherToMaybe = preview _Right
            encodeClaims  = (flip $ rsaEncode RS256) claims