Re: {mark} - a new simple notation that unifies JSON andXML

Hi Michael,

Thanks for sharing your thoughts on the specific issue of content model for element/object.

Here are my thoughts on this specific issue.

There are two options here, each with its pros and cons.

Option 1: Orthogonal approach, allows any value in element/object content, just like for property value

• Very orthogonal design, simpler in syntax and data model;
• Keep the door open for future 'innovative' usecases;

• Current Mark's content model for element/object is primarily designed for or optimized for mixed-content usecase;
• There are few important differences of the element content model vs. generic array:
• null values are stripped;
• no direct nesting of array; (Mark flattens an array when constructing object content)
• consecutive strings are merged into one;
• do not allow number and boolean primitive values;
• So although on the surface, the element content might look like an array, but there are subtle differences. And may require very different tools/API to work with them;
• e.g. we are already used to use CSS selectors to query the content model, and it requires content normalization that is not usually done on array data
  
• Among the 4 differences, I think the last one is up to discussion. But if we also give up the first 3 restrictions, then it could make the primary mixed-content usecase awkward, and people might as stay with their current non-orthogonal HTML or XML format.
• Current Mark's design is more on a conservative approach on what is allowed in the content model.

Anyone, any further thoughts?

That is also a concern that I have when designing Mark. Should it be generalized to allow any value in content? Do we have any solid usecase for storing number and boolean in content? They are probably not needed for normal mixed content usage.

I don't think the absence of a use case should ever be used to justify lack of orthogonality. If design were driven solely by use cases, orthogonality would go out of the window. The design process should aim to produce the most elegant/orthogonal design that satisfies all the use cases, it should not aim to satisfy the known use cases and nothing else.

Or to put it another way: find the space with the smallest boundary that has all the use cases within the space, where the "boundary" is the boundary between things that the system handles and things that it doesn't.

(An extreme example I often use to illustrate this principle: arithmetic expressions X+Y should allow either operand to be a literal zero, even though there is no use case for adding zero to a number, because a specification that allows literal zero is smaller than one that doesn't).

The lack of orthogonality between attribute values and element content was in fact one of the design mistakes in XML that I was trying to correct.

I know XML Schema allows primitive values as element content. But personally, I don't like that, and prefer to use attribute for those usecases.

Why should personal preferences come into it?

You might prefer <price currency="USD" amount="5.00"/> to <price currency="USD">5.00</price>, but others don't.

Michael Kay

Saxonica