[Haskell-beginners] How to "cast" FromJSON instance value

Hello, List! I have, for example, function with signature like fn :: FromJSON a => Map Text a -> BlahBlah and I want to treat this `a` as String (or `Text`), to read it with `readMaybe`. Am I right that it's impossible? I get error about "...a is rigid type and could not be matched with Text...". But `Text` is `FromJSON` instance. Is it possible to do it (cast, convert, unwrap, etc)? The problem's source is that early I worked with `Text` values only (getting from HTML form) but now they become any type: int's, bool's, etc). === Best regards, Paul