improve the action space

[ppasupat]

(: init ...) formulas should modify the current context

[sidaw]

Proposal

Value can be used to construct other Value, or be set by itself

• for example, a channel cannot be constructed without a type
• so we should be able to put InitChannelDef into $.encoding
• How can we use some of the checks in init for set actions?

Ideally, the interface should provide a clean way for us to add new actions deemed necessary perhaps we need $Path $Action, where action is performed on the path, and comes with its own checks?

Currently

• $Action <- $Path $Value
• $Value is constructed from grounded arguments
  • and possibly some floating arguments when needed
  • we can already new SemanticFn for new types of values, which can be checked via schema validation
  • but unclear how value-specific checks should work (maybe in JoinFn, but not ideal).

Do you think you can merge some of the parts used in init into set formulas and design a good interface for doing more customized checks, in addition to just schema validation?

[ppasupat]

@sidaw Could you give some examples of path actions ($Action) and the "value-specific checks"?

Also, shouldn't the check be path-specific instead of value-specific? What do you mean by value-specific?

[ppasupat]

Here's one proposal for generic value setting and constraint checking:

• value = primitiveValue | compositeValue
• primitiveValue is either grounded or floated
  • For floated primitiveValue, we can use a special token "ANY" that expands into a random compatible value when needed
• compositeValue = template + value + value + ...
  • template is like FIELDDEF = {"field": ___, "type": ___}
  • Define one template for each BNF category
  • Put template-specific checks here!
• action = path + value
  • e.g., path = "$.encoding.color"; value = {"field": "year", "type": "quantitative"}
  • Put path-specific checks here!
• ROOT = action

[sidaw]

@ppasupat Sounds good. We do not need to handle many of the categories, but we should handle important ones (from looking at vega-lite examples, encoding.blah seems the most important by far, until we start to consider compositions of whole specs) and make it conceptually clean to add more.