Conditional includes about marionette HOT 2 CLOSED

So I'd opened #58 for this, but it turns out that there were too many issues so that was closed.

The new approach is to convert our implementation to something more akin to a tree-walking interpreter:

• The parser will not expand backticks, or file-inclusion.
• Instead the parser will return an AST comprised of three distinct node-types:
  • File inclusion.
  • Variable assignments.
  • Actual rules.

This means that the evaluator will not solely be passed an array of "Rule" objects, instead it will receive the AST and walk that, switching on node-types:

• Assignments will result in environment-updates.
• Inclusions will result in recursion
• Rules will be processed as they should be.

This will be a significant change in terms of the implementation, however the steps are simple and well-known and the functionality should be unchanged...

from marionette.

That's good feedback, thank-you!

In terms of the parser-changes the first suggestion - add an if/unless clause to the include statement - would be easiest to implement.

I'd be happy to add that once I've finished up the user module in #56.

EDIT: Now I think about it, this might be harder than expected. At parse-time the environment isn't really populated. Conditionals might refer to things that aren't setup yet. Consider:

let h = `hostname`
include "local" if equal( h, "frodo.local" );

To handle that in the parser means interpreting assignments, not just expanding text.

I guess a naive approach would be to include ALWAYS, but add a secret "conditional" statement to all rules contained within an included file. Then evaluate that if present.

That keeps execution and parsing as distinct steps. But it's a bit horrid.

I'll think on it for a day or two.

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