Support for Let Expressions

[david-waltermire]

Committer Notes

Adding support for let expressions in constraints.

Also cleaned up the Metaschema XML schema, moving all constraint-related constructs to a section of the schema.

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[aj-stein-nist]

Minor wording request and want to clarify and make sure I understand the scoping (context/sub-context) spec as written.

[wendellpiez]

My questions are less about the design than about process.

What are we promising if we merge this into develop? Do we get to try the design and see? When is it appropriate to quibble about names or details such as what happens when two let expressions have the same name (@var, I guess it is), etc.

Similarly I wonder whether we could aspire to providing, with a design, some worked examples / functional demos, which we can use for testing and validation both of our conceptual models (addressing questions such as @aj-stein-nist) and of the tools we make.

Nice to see continued headway - thanks @david-waltermire-nist

[wendellpiez]

let/@var and let/@expression are okay but why not (e.g.)

let/@var and let/@expr
let/@variable and let/@expression

Also I wonder if @eval or @evaluate might be more accurate than @expr insofar as what is being passed is not an expression, but a binding to the results of an expression.

However these are not blocking questions inasmuch as others might also have suggestions for syntax improvements, so presumably later chances to discuss.

(I would actually like to see a process whereby the group could come up with 'best names' and syntax.)

[david-waltermire]

Change @expression to @expr based on the Metaschema community call discussion today.