Should I let my users write BnfExpressions to extend my grammar?

Question

Preface

I'm designing a templating language (please skip the don't/why?? speech). One of the major goals of this language is to be extensible. There are 2 main elements in my language. "Tags" and "Directives". Tags, for the most part, map 1-to-1 to HTML tags, with the exception of a few of them which do a bit more. "Directives" are things like conditionals and for/while loops.

"Tags" are simple, because they all follow the same structure: tag_name(arg1=val1, arg2=val2) { content }.

Directives are more complicated because they look like this: directive(anything-can-go-here) { content }.

Question

I'm trying to decide how to let users (other devs using the templating language) write their own directive -- i.e., define the "anything can go here" part.

I'm using Irony to parse my language. Should I give them the same level of access, and define their own BnfExpression (requiring them to learn a bit of Irony/EBNF), or should I just toss them a string (the stuff between ( and )) and let them parse it however they want?

My Concerns

The former is perhaps more powerful and easier to work with (once you know it), but it would also potentially give them too much power (they wouldn't have to stop at the next ) -- well, unless I stripped out the inner string first and then basically let them define a sub-grammar to parse only that).

Not sure how to approach this.

More About Tags

For reference, users can define "tags" like this:

class ANode : HtmlNode
{
    public ANode(AttrDict attrs, params TagNode[] children)
        : base("a", attrs, children) { }
}

Would be the most basic example, which just extends the HtmlNode I've already defined. A more complicated one might define some default attributes or do something fancy with the child elements, or generate multiple html tags.

Answer 1

There are many factors that should go into this decision. Here are some things to consider:

Requirements: Are there use cases that require this capability be present, or would be substantially easier to deliver if you provide this capability?

Schedule: Adding this feature may delay the completion of your project. How much money do you (or your company) lose if the product launch is delayed?

Cost / Benefit Tradeoff: It will cost you something to provide this capability. You have to write code, you have to write test cases, your testers have to perform these tests, you need to write end-user documentation. Once the product is deployed, this capability will need to be supported. You can estimate these costs, and balance them with any benefits that you will get from providing this capability. Will you be able to sell your product at a higher price, or will you be able to sell more of them?