Re: [Yaml-core] Re: Proposal

[David H. <dav...@bl...>]

Sean O'Dell wrote:
> On Tuesday 07 September 2004 07:42, Clark C. Evans wrote:
> 
>>Clearly, if one 'transforms' the graph on the way in, the 'schema'
>>is being altered, so if one wishes to round-trip, a reverse transform
>>on the way out would be required to convert the data structure back.
>>But this, once again, is an application-specific thingy.
> 
> Schemas, I thought, are for validation and wouldn't alter the graph in any 
> way.  A set of transformation rules would be something different from a 
> schema, and it would probably do nothing more than re-tag nodes, so it should 
> be re-emittable all the way back from the fully loaded state.

As it happens, once you have the mechanisms needed to validate a graph, very
little additional mechanism is needed to transform it. A schema language
essentially parses the graph, yielding a parse tree in which each parse tree
node corresponds to a graph node. At that point you have all the information
needed to fill in unspecified types: for each node, the schema production
tells you what kind of node it "should" be. In simple cases, all you then
need is a mapping from schema productions to tags, which is used to change
any unspecified tags. In more complicated cases, the API could call an
application-provided function depending on the production for each node.

-- 
David Hopwood <dav...@bl...>