What is Justin syntax?

[dckc]

In yesterdays' Office Hours, we were using agoric follow to look at some stuff similar to...

agoric follow :published.agoricNames.vbankAsset
...
  [
    "ibc/toyatom",
    {
      brand: slot(14,"Alleged: IbcATOM brand"),
      denom: "ibc/toyatom",
      displayInfo: {
        assetKind: "nat",
        decimalPlaces: 6,
      },
      issuer: slot(15,"Alleged: IbcATOM issuer"),
      issuerName: "IbcATOM",
      proposedName: "ATOM",
    },
  ],
]

Note that unlike JSON, property names are not quoted. And there are function call expressions such as slot(15,"Alleged: IbcATOM issuer").

I tried to get agoric follow to produce JSON so I could slice-and-dice it with jq. Meanwhile, @tgrecojs noticed that the default format (above) is "justin" and asked what that is:

$ agoric follow --help
...
Options:
  -o, --output <format>                 value output format (default: "justin")

We didn't get back to his question yesterday, so I'm writing it up here...

[dckc]

Justin is one of the jessica grammars; it's a "pure expression grammar", between JSON and Jessie:

// Justin is the safe JavaScript expression language, a potentially
// pure terminating superset of JSON and subset of Jessie, that relieves
// many of the pain points of using JSON as a data format:
//   * unquoted indentifier property names.
//   * comments.
//   * multi-line strings (via template literals).
//   * undefined.
//   * includes all floating point values: NaN, Infinity, -Infinity
//   * will include BigInt once available.

agoric follow uses decodeToJustin from @endo/marshal to decode marshalled data to Justin.

test-marshal-justin.js is another form of specification / documentation. There we see that unlike JSON, Justin can represent undefined, bigints, etc.