Experimental DSSSL extensions for XSL

# Experimental DSSSL extensions

This document descibes some experimental extensions to DSSSL that I have implemented in Jade. These are designed so that, with these extensions, DSSSL provides a superset of the semantics XSL for flow object tree construction. Jade has a `-2` option that enables these extensions.

These extensions do not include the additional flow object classes and characteristics that will be needed for XSL; in particular they do not include the HTML/CSS flow object classes.

## Imperative programming

The following features come from R4RS:

- assignment (`set!`) expressions (with restrictions)
- vectors (with restrictions)
- `call-with-current-continuation` (with restrictions)
- `begin` expressions
- multiple expressions in procedure bodies, `cond` clauses
- alternate in `if` expression optional
- it is not an error when nothing matches in `cond` or `case` expression
- `eqv?` and `memv` procedures; these behave as specified in R4RS for vectors but behave the same as `equal?` for strings and lists

  This is so that case expressions can use `eqv?` as required by R4RS without breaking compatibility with existing DSSSL code which assumes case expressions with strings and lists will use `equal?`. R4RS specifies that `eqv?` should return #t when its arguments "should normally be regarded as the same object". R4RS treats strings and lists as mutable and its specification of `eqv?` for strings and lists is consistent with this. So long as DSSSL keeps strings and lists as immutable data-types with value semantics, it is more consistent to define `eqv?` to behave like `equal?` for them.

The use of side-effects is restricted. Assignment to top-level variables is not allowed. There is also the concept that a memory location can be read-only. When a memory location is read-only, it is an error to change that location. An memory location can be recursively marked as read-only; this means that the memory location along with all memory locations reachable from that memory location become read-only. A memory location is recursively marked as read-only when:

- an object stored in that memory location is bound to a top-level variable
- an expression specifying a characteristic is evaluated and a variable that names that memory location occurs free in that expression; for example, this would be an error:

      (let ((x 10pt))
        (make paragraph
          font-size: (begin (set! x 12pt) x)))

- an object stored in that memory location is returned by a `(inherited-C)` or `(actual-C)` procedure
- an object stored in that memory location is passed as the first argument to the `node-list-map` procedure

A continuation created with `call-with-current-continuation` cannot be called if it is read-only, and can only be used to return to a stack frame in the current call chain (sometimes referred to as upwards only).

There's a `void` data type with a single value which can be written as `#v`. This is returned by `cond`, `case` and `if` expressions which don't match.

## Style rules

When a construction rule has a keyword argument list instead of a construct expression it is treated as a style rule. For example,

    (element H1
      font-size: 14pt
      font-weight: 'bold)

The keyword argument list can include a `use:` keyword just as with `style` expressions. See the XSL proposal for the semantics of style rules.

## Extended patterns

The syntax for element patterns is extended. These provide provide a superset of the semantics of XSL patterns. They are allowed both in element construction rules and in contexts where a `match-element?` pattern is currently allowed (eg `select-elements`, `process-matching-children`, `process-first-descendant`).

A pattern is either a single gi or a list. A list consists of a sequence of gis, where each gi can be followed by one or more keyword/value pairs (where the value is always a single datum). A gi can be #t, a string or a symbol. The following keywords are allowed:

`id:`
  followed by a string or symbol
`class:`
  followed by a string or symbol
`repeat:`
  followed by one of the symbols `*`, `+`, `?`
`only:`
  followed by one of the symbols `of-type`, `of-any`
`position:`
  followed by one of the symbols `first-of-type`, `first-of-any`, `last-of-type`, `last-of-any`
`attributes:`
  followed by a list of name/value pairs; for backward compatibility with `match-element?` patterns in the current DSSSL standard the `attributes:` keyword can be omitted; #t and #f can be used as a value to test for presence or absence of attributes.
`children:`
  followed by a pattern; each of the elements in the pattern must occur as child; `repeat:` is not allowed in children patterns; the `children:` qualifier is allowed on any gi in a pattern not just the last element
`priority:`
  followed by an integer; multiple `priority:` qualifiers are allowed in a pattern and will be added together
`importance:`
  followed by an integer; multiple `importance:` qualifiers are allowed and will be added together

Class attribute names are declared using

    (declare-class-attribute "class")

or

    (declare-class-attribute class)

Id attribute names can be declared similarily using `declare-id-attribute`.

Some examples:

    (element (E importance: 42) ...)

    (element (E attributes: (A1 V1)) ...)

    (element (P E children: C) ...)

    (element (P children: C priority: -11
              E children: C attributes: (A1 V1 A2 V2))
     ...)

    (element (P E children: (A children: C B children: C)) ...)

The last is equivalent to the following in XSL syntax:

    <element type="P">
      <target-element type="E">
         <element type="A">
           <element type="C"/>
         </element>
         <element type="B">
           <element type="C"/>
         </element>
      </target-element>
    </element>

## Multiple patterns per rule

An `or-element` construction rule has the syntax

    (or-element (pattern+) expression)

where `pattern` is any pattern that could be allowed in an element construction rule. It is equivalent to a sequence of element construction rules.

For example,

    (or-element (H1 H2 H3)
      font-weight: 'bold)

    (or-element ((H1 TITLE) (H2 TITLE) (H3 TITLE))
      font-weight: 'bold)

is equivalent to

    (element H1
      font-weight: 'bold)

    (element H2
      font-weight: 'bold)

    (element H3
      font-weight: 'bold)

    (element (H1 TITLE)
      font-weight: 'bold)

    (element (H2 TITLE)
      font-weight: 'bold)

    (element (H3 TITLE)
      font-weight: 'bold)

## Flow object macros

A flow object macro can be defined like this:

    (declare-flow-object-macro list-item ((indent 1in)
                                          (marker "\bullet")
                                          #!contents contents)
      (make paragraph
        first-line-start-indent: (- indent)
        start-indent: (+ indent (inherited-start-indent))
        (make line-field
              field-width: indent
              (literal marker))
        contents))

     (root
      (make simple-page-sequence
            (make paragraph
                  (literal "Para 1"))
            (make list-item
                  (literal "Item 1")
                  (make list-item
                        indent: .5in
                        marker: "\black-circle"
                        (literal "Sub item 1.1")))
            (make list-item
                  font-weight: 'bold
                  (literal "Item 2")
                  (make list-item
                        (literal "Sub item 2.1")))
            (make paragraph
                  (literal "Para 2"))))

If the formal argument list includes `#!contents` the flow object behaves like a compound flow object, otherwise like an atomic flow object. Inherited characteristics can be specified; these are applied to a sequence flow object which is automatically wrapped around what is returned by the body of the flow object macro. The preceding formal arguments are the non-inherited characteristics; like keyword arguments they can be specified either as `id` (in which case they default to #f), or as `(id init-expression)`.

Note that flow object macros are quite different from ordinary procedures in that the macro body is not evaluated when the make expression is evaluated, but rather when the flow object is to be added to the tree; this allows `(inherited-c)` and `(actual-c)` procedures to be used in flow object macro characteristics, non-inherited as well as inherited, just as with normal flow objects. It is also possible to use `(inherited-c)` and `(actual-c)` procedures in the body of the flow object macro; they will return the same result as if they were used in the specification of a characteristic on the invocation of the flow object macro.

## Characteristic value conversion

Most characteristic values can now be specified as strings and will be converted appropriately. For boolean valued characteristics any of the strings `"true"`, `"false"`, `"yes"` and `"no"` are acceptable.

## Characteristic names

The question mark that can be omitted from those DSSSL characteristic names that end with a question mark. This is because the question mark is not (and cannot resonably be made) a legal XML name character.