Support configuring which keys are used for tag & values

[LiamGoodacre]

Depends on #11
Not tied to the names, feel free to suggest better ones.

[thomashoneyman]

@LiamGoodacre Would you mind explaining a little about the motivation behind this change?

I understand this allows for choosing what keys to use for tag / values, but I'm not sure what new flexibility that opens up.

[thomashoneyman]

I inadvertently merged this as I thought I was looking at the simple dependency update. It's been merged and then reverted in master. I'd like to continue this conversation briefly before merging officially, if you don't mind. Sorry about that!

[thomashoneyman]

This can be cherry-picked into master when ready. Thanks!

[LiamGoodacre]

No problem! My motivation for this is to interoperate with an existing (fixed) JSON structure that uses different keys than tag and values. I didn't want to reimplement all of this just with different keys.

[thomashoneyman]

What do you think of supporting both options -- continue allowing a default that uses tag and values, in a type class member decodeRep, which relies on the other type class member, decodeRepWith? That would allow current users to continue using their code as-is in a new version without having to update everything with new explicit encodings.

class DecodeRep r where
  decodeRep :: Json -> Either String r
  decodeRepWith :: Encoding -> Json -> Either String r

decodeRep = decodeRepWith defaultEncoding

[LiamGoodacre]

Happy to also have decodeRep - but I'm wondering if we should export it separately without it being a type class member?

[thomashoneyman]

Thanks for re-opening the PR. I was thinking the same thing.