Request for JSON specification of Vega functions

cc. @maartenbreddels

We are working on a Python -> Vega-expression transpilation package: https://github.com/QuantStack/py2vega

We provide mocking Python functions for all vega functions. In order to easily keep them up to date with vega it would be super useful to have spec files (JSON files?) for those functions. This spec could be used for automatically generating our code, but it would also be useful for generating the Vega documentation.

It could look like the following:

{
    "string": [
        {"name": "indexof", "args": ["string", "substring"], "doc": "Returns the first index of substring in the input string."},
        [...]
    ],
    [...]
}

[jheer]

I’m interpreting this issue as primarily a request for a standardized JSON specification file that documents all functions in the Vega expression language; that is, all functions exported by the vega-functions package. I’m also flagging this as “help-wanted”, as this is a relatively straightforward (if tedious) task for a community contribution. Folks should join the conversation here if they are interested in working on this, as we will want to first come to agreement on the format of the specification file.

[saulshanabrook]

Few notes from the discussion with @martinRenou and @maartenbreddels:

• They are doing the opposite thing that we are. They want to translate Python expressions into vega code, by parsing the AST. We are doing the opposite, taking Vega expressions and compiling it to Python.
• We could maybe use both tools together for E2E testing, to make sure performing both transforms gave the same result in python.

cc @ian-r-rose