[Haskell-beginners] Is it possible to "run" JSON Parser?

Hello List again! :) If I have a `FromJSON` instance, with `parseJSON` which translates `Value` to MyType, how can I run this parser like I "run" state/writer/reader monads? I mean not decoding of a bytestring representation: decode::ByteString -> Maybe MyType but run::Value -> Maybe MyType The reason is that I have already collected Value's , and I have parsers, so what is the problem, it would seem, to run parser on the Value - to convert `Value` into `Maybe MyType`... Is it possible? === Best regards, Paul