New sum codec does not report accurate errors

[wclr]

Brand new sum codecs made by @m-bock make writing sum type codecs a breeze. But there is a problem with decoding error reports that in most cases happen to be are irrelevant.

For example, for this type

data Sample
  = Foo
  | Bar Int
  | Baz Boolean String Int

When parsing (wrong value type):

        $ Str.joinWith "\n"
            [ "{"
            , "  \"tag\": \"Bar\","
            , "  \"values\": \"42\""
            , "}"
            ]

the error reported is: (Named "Sample" (TypeMismatch "Expecting tag Baz, got Bar")). However, one would expect to see something like this: (Named "Sample" (Named "case Bar" (TypeMismatch "Int")))

This is because it currently propagates decoding errors for all cases (without respect to the actual tag value) and reports the error for the last case.

To solve this I refactored the code a bit and added tests for error reporting, going to make a PR.

[m-bock]

@wclr Sorry for the late reply!
Thanks for reporting, this totally makes sense. I'll have a closer look into your PR!