NAME

w3c-css - cascading style sheet parser

SYNOPSIS

include "css.m";

css := load CSS CSS->PATH;

Stylesheet: adt {
   charset:    string;
   imports:    list of ref Import;
   statements: list of ref Statement;
};

Import: adt {
   name:   string;
   media:  list of string;
};

Statement: adt {
   pick{
   Media =>
       media:  list of string;
       rules:  list of ref Statement.Ruleset;
   Page =>
       pseudo: string;
       decls:  list of ref Decl;
   Ruleset =>
       selectors: list of Selector;
       decls:     list of ref Decl;
   }
};

Decl: adt {
   property:   string;
   values:     list of ref Value;
   important:  int;
};

Selector:   type list of (int, Simplesel);   # (combinator, simplesel)
Simplesel:  type list of ref Select;

Select: adt {
   name:   string;
   pick{
   Element or ID or Any or Class or Pseudo =>
       # empty
   Attrib =>
       op:     string; # "=" "~=" "|="
       value:  ref Value;  # optional Ident or String
   Pseudofn =>
       arg:    string;
   }
};

Value: adt {
   sep:    int;    # operator preceding this term
   pick{
   String or
   Number or
   Percentage or
   Url or
   Unicoderange =>
       value:  string;
   Hexcolour =>
       value:  string;             # as given
       rgb:    (int, int, int);    # converted
   RGB =>
       args:   cyclic list of ref Value;  # as given
       rgb:    (int, int, int);           # converted
   Ident =>
       name:   string;
   Unit =>
       value:  string; # int or float
       units:  string; # suffix giving units
   Function =>
       name:   string;
       args:   cyclic list of ref Value;
   }
};

init:       fn(diag: int);
parse:      fn(s: string): (ref Stylesheet, string);
parsedecl:  fn(s: string): (list of ref Decl, string);

DESCRIPTION

Css implements a parser for the World-Wide Web Consortium's Cascading Style Sheet, specification 2.1.

Init must be called before any other operation in the module. If diag is non-zero, the module will print diagnostics on standard output for malformed or unrecognised items that are ignored during parsing (as required by the specification).

Parse takes a complete stylesheet in string s, parses it, and returns a tuple (sheet, err) where sheet refers to a Stylesheet value containing the logical content of s, as described below. On a fatal error, sheet is nil and err is a diagnostic. Most syntactic errors are ignored, as the specification requires.

In some applications there can be auxiliary declarations outside a stylesheet. Parsedecl takes a string s containing a sequence of declarations, and returns a tuple (decls, err) where decls is a list of references to Decl values, each representing a single declaration in s. On a fatal error, decls is nil, and err is a diagnostic.

The adts represent an abstract syntax of the CSS grammar. The concrete syntax is presented below in an extended BNF, derived from the reference grammar, with each section labelled by the name of the corresponding adts. (Compared to the reference grammar in the specification, it abstracts away from the complex rules about where whitespace can appear.)

Stylesheet

stylesheet ::= [ '@charset' STRING ';' ] import* statement*

Limbo lists represent lists of items in the grammar. Nil values denote optional components that are missing. Upper-case names such as IDENT, STRING and NUMBER are terminals; see the CSS specification for their often subtle definitions. They are usually represented by Limbo string values in the adts.

Import

import ::= '@import' (STRING|uri) [medium (',' medium)*] ';'
uri ::= 'url(' STRING ')'

Import.name holds the text of the STRING or uri.

Statement

statement ::= ruleset | media | page
media ::= '@media' medium (',' medium)* '{' ruleset* '}'
medium ::= IDENT
page ::= '@page' [pseudo_page] '{' declaration (';' declaration)* '}'
pseudo_page ::= ':' IDENT
ruleset ::= selector (',' selector)* '{' declaration (';' declaration)* '}'

Statement is not in the reference grammar, but is introduced here to give a name corresponding to the pick adt.

Decl

declaration ::= property ':' expr ['!' 'important'] | /* empty */
property ::= IDENT

Decl.values is a list representing the terms of the expr (see below for details). Decl's field important is non-zero if the optional `important' priority is given.

list of ref Value

expr ::= term (operator term)*
operator ::= '/' | ',' | /* empty */

An expr is always represented as a list of references to Value in some containing structure (where Value represents a term, see below). The operator preceding each term appears as the field sep of the corresponding Value, where a space character represents `empty' (concatenation).

Selector

selector ::= simple_selector (combinator simple_selector)*
combinator ::= '+' | '>' | /* empty */

Selector is just a type synonym for a list of tuples, say (com, simplesel) where the simplesel value represents simple_selector (see below), and the integer com is one of the characters space (representing `empty'), `>' or `+', giving the combinator that preceded the simple selector. (The first in the list is always space.)

Simplesel, Select

simple_selector ::= element_name (hash | class | attrib | pseudo)*
	| (hash | class | attrib | pseudo)+
hash ::= '#' NAME
class ::= '.' IDENT
element_name ::= IDENT | '*'
attrib ::= '[' IDENT [('=' | '|=' | '~=') (IDENT | STRING)] ']'
pseudo ::= ':' ( IDENT | IDENT '(' [IDENT] ')' )

A simple_selector is represented by Simplesel, a list of references to Select values, each representing one element_name or qualifier. An element_name is represented by Select.Element for an IDENT, or Select.Any for `*'. The qualifiers are hash (Select.ID), class (Select.Class), attrib (Select.Attrib, where the comparison operator is the string op), pseudo (either Select.Pseudo if a plain identifier, or Select.Pseudofn for a function with optional parameter).

Value

term ::= ['+' | '-'] (NUMBER | percent | unit) | STRING | IDENT | uri | function | hexcolour | rgb
function ::= IDENT '(' expr ')'
hash ::= '#' NAME
hexcolour ::= '#' HEXDIGIT+
percent ::= NUMBER '%'
unit ::= NUMBER STRING
rgb ::= 'rgb(' term ',' term ',' term ')'
uri ::= 'url(' STRING ')'

Any sign before a Number, Percentage or Unit appears as the first character of value. All the dimensional units (LENGTH, EMS, EXS, ANGLE, TIME, FREQ and others) in the reference grammar are mapped to Value.Unit, with the field units containing the name of the relevant unit (eg, cm, in, etc.) in lower case. Values and names appear shorn of the surrounding punctuation. Value.Hexcolour includes the original sequence of hex digits as a string, and a decoding of it as an rgb triple. The arguments to the CSS rgb function are similarly presented in original and decoded forms, in Value.RGB. Other function references are returned uninterpreted in Value.Function.

SOURCE

/appl/lib/w3c/css.b

SEE ALSO

``Cascading Style Sheets, level 2 revision 1'', http://www.w3.org/TR/CSS21

W3C-CSS(2 )

Rev:  Tue Mar 31 02:42:39 GMT 2015