HashML Schemas

HashML places a lot of importance on schemas. A schema describes the supported tags, and places constraints on where they can appear. HashML schemas are written in HashML. The supported tags are described by the schema schema.

Example

A schema looks like this:

#block root
    #content
        #one document

#block document
    #default paragraph
    #content
        #zeroOrMore section
        #zeroOrMore paragraph

#block section
    #default paragraph
    #content
        #zeroOrMore paragraph

#block paragraph
    #head
        #zeroOrMore code

#inline code
    #sugar
        #start `
        #end `
    #arg
        #raw

The first block describes the root (a reserved name indicating the top-level of the document), which must contain exactly one #document. The document can contain any number of #section and #paragraph. The schema also says that if omitted, the default tag inside of a #document should be paragraph.

Schemas use #zeroOrMore as the default tag for #content, so we could rewrite the #block document as:

#block document
    #default paragraph
    #content
        section
        paragraph

Schemas can also define the syntactic sugar for inline tags. The above defines backticks (`) as the sugar for #code. You could also write your own schema in which &hello& means #uppercase[hello], for instance.

Schema validation

HashML documents can be validated by the schema. This means that you can check whether a document respects the tag descriptions in the schema, and whether the restrictions imposed by the #content field of blocks is respected.

Since schemas are themselves HashML documents, there is a schema schema that can validate schemas. The schema schema also validates itself, as it is also a valid schema. This is very useful when writing schemas, and can help you catch all sorts of errors.

Schema validation gives a list of errors, and can tell you exactly why a document isn't compliant with the schema.