[koomi]

Aeson's «fromJSON» returns a «Result» which is basically «Either String». Most other functions operate on «Parser», which also includes an error message if things go wrong.

The only functions returning «Maybe» are «decode» and friends, and those have «Either String» variants.

You can use «modifyFailure» and «typeMismatch» (and the «with» wrappers) to get more descriptive messages, without a bit of help they are quite useless.