Implement TOML difference

[chshersh]

Currently if you implement Codec for file that contains more fields that you need, tomland will use only those fields that you specified. However, sometimes it's desired to tell about redundant or unused fields. In JSON it is completely okay to ignore some fields, but in TOML static configuration having some fields in the config may lead to frustration in the future. It would be nice to have some way of telling which fields were not used.

[willbasky]

Should it be cli command or stderr message after decoding?

[chshersh]

@willbasky Under one issue I proposed the following data type as a result of decoding TOML:

data Result a
    = Failure String
    | Warning [DecodeWarning] a
    | Success a

I believe this should be a separate function(s) in some experimental Toml.Diff module. And design for this feature is a subject for discussion 🤔

[willbasky]

Ok. Then for this issue it will be within result message | Warning [DecodeWarning] a

And for other issue it will be separate function. And maybe we will find some other useful functions to be invented.

[willbasky]

Now we have DecodeException.
decode returns Either DecodeException a
Should it returns Result a? where

data Result a
    = DecodeException
    | Warning [DecodeWarning] a
    | Success a

[chshersh]

@willbasky In future we might want to change decode to return Result. But only after this experimental approach will be battle-tested.

[willbasky]

So, now, should it be in separate module. Some decode-like function that returns Result a?

[chshersh]

@willbasky Exactly

[chshersh]

The function is starting to become more useful. So here is the roadmap for implementing the feature:

• Implement the tomlDiff :: TOML -> TOML -> TOML function. This is probably the most difficult part of this issue.
• Add one more constructor to TomlDecodeError, something like NotExactDecode TOML.
• Implement the exactDecode function with the following logic:
  1. Run decode to get the value.
  2. Convert the resulting value back to TOML.
  3. Find diff between the original TOML and the resulting TOML.
  4. If the diff is not empty, fail with NotExactDecode TOML with diff inside error message
• Write some tests 🙂