Empty alternative does not trigger JSON name preservation

[wezm]

Possibly related to #1080, #1145

As per the dhall-json tests nesting{0,1}, the use of a record with fields field, nesting, and contents is supposed to preserve the name of the union alternative alongside the value, either Inline or Nested. However the behaviour does not trigger when the union alternative is empty.

For example, with this input:

let Example = < Left : { foo : Natural } | Middle | Right : { bar : Bool } >

let Nesting = < Inline | Nested : Text >

in  { field    = "name"
    , nesting  = Nesting.Inline
    , contents = Example.Middle
    }

I expect this output:

{ "name": "Middle" }

But instead get:

{ "field": "name", "contents": "Middle", "nesting": "Inline" }

I have added a test case for this in my fork but I'm not familiar enough with Haskell to implement a fix: master...wezm:json-nesting-empty-alternative

[sjakobi]

Thanks for the testcase! That expected output looks very reasonable.

We should also handle empty alternatives with the Nested layout. For

let Example = < Left : { foo : Natural } | Middle | Right : { bar : Bool } >

let Nesting = < Inline | Nested : Text >

in  { field    = "name"
    , nesting  = Nesting.Nested "value"
    , contents = Example.Middle
    }

I'd propose generating { "name": "Middle", "value": {} } by default. With --omitEmpty that would become just { "name": "Middle" }.

I'll submit a PR soon.

[sjakobi]

I'd propose generating { "name": "Middle", "value": {} } by default.

Actually that should probably be { "name": "Middle", "value": null } since Nested already supports non-record alternatives like < Foo : Text >.Foo "bar".

@Gabriel439 Thoughts?

[Gabriella439]

@sjakobi: Yeah, null seems safer. Also, null can be combined with the --omitNull flag so that the value field can be filtered out entirely at the user's discretion