Vega-expressions undefined functions

Some documented vega functions seem to not work inside of vega-expressions. Functions like toDate and timeParse are throwing: Error: Unrecognized function: timeParse.

I see that, unlike other functions, they are not tested here: https://github.com/vega/vega/blob/master/packages/vega-expression/test/codegen-test.js.

Functions that do not seem to be tested are:

• Type Checking functions
• Type Coercion functions
• Formatting functions
• Color functions
• Scale functions

[martinRenou]

Thanks a lot for your answers! 😃 I will look at vega-parser and vega-runtime more closely.

[jheer]

That looks like it is close to what we do in Vega. However, instead of passing the external function implementations as a parameter, we bind it to the this context of each parsed expression. This happens in the vega-runtime package (which is invoked by the vega-view code I shared earlier). The vega-parser runs the expression parser to generate the JavaScript code; generated expression code is included within the fully parsed representation of an input Vega JSON specification, which the parser outputs. The vega-runtime library takes the code generated by the expression parser and uses the Function constructor to generate a callable function for each parsed expression.

Here’s the code we use for that: https://github.com/vega/vega/blob/master/packages/vega-runtime/src/expression.js

Note that different parsed expressions may take different input arguments based on the context in which they are used in Vega (e.g., within data transforms, signal updates, encodings, etc). Common arguments include a datum object and the values of signals that the expression depends on (denoted as the _ object). I do not know if these more detailed concerns affect you or not…