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proposed XML parser design.

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inspired by expat's callback interface,
but a bit simpler thanks to go interfaces.

also serves as reference notes about XML.

the parser itself is unimplemented.

not in Makefiles, though it does build.

R=r
DELTA=425  (425 added, 0 deleted, 0 changed)
OCL=25077
CL=25080
This commit is contained in:
Russ Cox 2009-02-16 20:14:21 -08:00
parent bbc190b3ee
commit 9e3e61627d
1 changed files with 426 additions and 0 deletions
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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// NOTE(rsc): Actually, this package is just a description
// of an implementation that hasn't been written yet.
// This package implements an XML parser but relies on
// clients to implement the parsing actions.
// An XML document is a single XML element.
//
// An XML element is either a start tag and an end tag,
// like <tag>...</tag>, or a combined start/end tag <tag/>.
// The latter is identical in semantics to <tag></tag>,
// and this parser does not distinguish them.
//
// The start (or combined start/end) tag can have
// name="value" attributes inside the angle brackets after
// the tag name, as in <img src="http://google.com/icon.png" alt="Google">.
// Names are drawn from a fixed set of alphabetic letters;
// Values are strings quoted with single or double quotes.
//
// An element made up of distinct start and end tags can
// contain free-form text and other elements inside it,
// as in <a href="http://www.google.com">Google</a>
// or <b><a href="http://www.google.com">Google</a></b>.
// The former is an <a> element with the text "Google" inside it.
// The latter is a <b> element with that <a> element inside it.
// In general, an element can contain a sequence of elements
// and text inside it. In XML, white space inside an element is
// always counted as text--it is never discarded by the parser.
// XML parsers do translate \r and \r\n into \n in text.
//
// This parser reads an XML document and calls methods on a
// Builder interface object in response to the text.
// It calls the builder's StartElement, Text, and EndElement
// methods, mimicking the structure of the text.
// For example, the simple XML document:
//
// <a href="http://www.google.com">
// <img src="http://www.google.com/icon.png" alt="Google" />
// <br/></a>
//
// results in the following sequence of builder calls:
//
// StartElement("a", []Attr(Attr("href", "http://www.google.com")));
// Text("\n\t");
// StartElement("img", []Attr(Attr("src", "http://www.google.com/icon.png"),
// Attr("alt", "Google")));
// EndElement("img");
// Text("\n");
// StartElement("br", []Attr());
// EndElement("br");
// EndElement("a");
//
// There are, of course, a few more details, but the story so far
// should be enough for the majority of uses. The details are:
//
// * XML documents typically begin with an XML declaration line like
// <?xml version="1.0" encoding="UTF-8"?>.
// This line is strongly recommended, but not strictly required.
// It introduces the XML version and text encoding for the rest
// of the file. XML parsers are required to recognize UTF-8 and
// UTF-16. This parser only recognizes UTF-8 (for now?).
//
// * After the XML declaration comes an optional doctype declaration like
// <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
// "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
// The parser should pass this information on to the client in some
// form, but does not. It discards such lines.
//
// * The XML declaration line is an instance of a more general tag
// called a processing instruction, XML's #pragma. The general form is
// <?target text?>, where target is a name (like "xml") specifying
// the intended recipient of the instruction, and text is the
// instruction itself. This XML parser keeps the <?xml ...?> declaration
// to itself but passes along other processing instructions using
// the ProcInst method. Processing instructions can appear anywhere
// in an XML document. Most clients will simply ignore them.
//
// * An XML comment can appear anywhere in an XML document.
// Comments have the form <!--text-->. The XML parser passes
// them along by calling the Comment method. Again, most clients
// will simply ignore them.
//
// * Text inside an XML element must be escaped to avoid looking like
// a start/end tag. Specifically, the characters < and & must be
// written as &lt; and &amp;. An alternate quoting mechanism is to
// use the construct <![CDATA[...]]>. The quoted text ... can contain
// < characters, but not the sequence ]]>. Ampersands must still be
// escaped. For some reason, the existence of the CDATA quoting mechanism
// infects the processing of ordinary unquoted text, which is not allowed
// to contain the literal sequence ]]>. Instead, it would be written
// escaped, as in ]]&gt;. The parser hides all these considerations
// from the library client -- it reports all text, regardless of original
// form and already unescaped, using the Text method.
//
// * A revision to XML 1.0 introduced the concept of name spaces
// for attribute and tag names. A start tag with an attribute
// xmlns:prefix="URL" introduces `prefix' as a shorthand
// for the name space whose identifier is URL. Inside the element
// with that start tag, an element name or attribute prefix:foo
// (as in <prefix:foo prefix:bar="baz">) is understood to refer
// to name `foo' in the name space denoted by `URL'. Although
// this is a shorthand, there is no canonical expansion. Thus:
//
// <tag xmlns:foo="http://google.com/foo" xmlns:bar="http://google.com/bar">
// <foo:red bar:attr="value">text1</foo:red>
// <bar:red>text2</bar:red>
// </tag>
//
// and
//
// <tag xmlns:bar="http://google.com/foo" xmlns:foo="http://google.com/bar">
// <bar:red foo:attr="value">text1</bar:red>
// <foo:red>text2</foo:red>
// </tag>
//
// are equivalent XML documents, and there is no canonical form.
//
// The special attribute xmlns="URL" sets the default name space
// for unprefixed tags (but not attribute names) to URL.
// Thus:
//
// <tag xmlns="http://google.com/foo" xmlns:bar="http://google.com/bar">
// <red bar:attr="value">text1</red>
// <bar:red>text2</bar:red>
// </tag>
//
// is another XML document equivalent to the first two, and
//
// <tag xmlns:bar="http://google.com/foo" xmlns="http://google.com/bar">
// <bar:red attr="value">text1</bar:red>
// <red>text2</red>
// </tag>
//
// would be equivalent, except that `attr' in attr="value" has no
// associated name space, in contrast to the previous three where it
// is in the http://google.com/bar name space.
//
// The XML parser hides these details from the client by passing
// a Name struct (ns + name pair) for tag and attribute names.
// Tags and attributes without a name space have ns == "".
//
// References:
// Annotated XML spec: http://www.xml.com/axml/testaxml.htm
// XML name spaces: http://www.w3.org/TR/REC-xml-names/
package xml
import (
"io";
"os";
)
// XML name, annotated with name space URL
type Name struct {
ns, name string;
}
// XML attribute (name=value).
type Attr struct {
name Name;
value string;
}
// XML Builder - methods client provides to Parser.
// Parser calls methods on builder as it reads and parses XML.
// If a builder method returns an error, the parse stops.
type Builder interface {
// Called when an element starts.
// Attr is list of attributes given in the tag.
// <name attr.name=attr.value attr1.name=attr1.value ...>
// <name attr.name=attr.value attr1.name=attr1.value ... />
// xmlns and xmlns:foo attributes are handled internally
// and not passed through to StartElement.
StartElement(name Name, attr []Attr) *os.Error;
// Called when an element ends.
// </name>
// <name ... />
EndElement(name Name) *os.Error;
// Called for non-empty character data string inside element.
// Can be called multiple times between elements.
// text
// <![CDATA[text]]>
Text(text []byte) *os.Error;
// Called when a comment is found in the XML.
// <!-- text -->
Comment(text []byte) *os.Error;
// Called for a processing instruction
// <?target text?>
ProcInst(target string, text []byte) *os.Error;
}
// Default builder. Implements no-op Builder methods.
// Embed this in your own Builders to handle the calls
// you don't care about (e.g., Comment, ProcInst).
type BaseBuilder struct {
}
func (b *BaseBuilder) StartElement(name Name, attr []Attr) *os.Error {
return nil;
}
func (b *BaseBuilder) EndElement(name Name) *os.Error {
return nil;
}
func (b *BaseBuilder) Text(text []byte) *os.Error {
return nil;
}
func (b *BaseBuilder) Comment(text []byte) *os.Error {
return nil;
}
func (b *BaseBuilder) ProcInst(target string, text []byte) *os.Error {
return nil;
}
// XML Parser. Calls Builder methods as it parses.
func Parse(r io.Read, b Builder) *os.Error {
return os.NewError("unimplemented");
}
// Channel interface to XML parser: create a new channel,
// go ParseTokens(r, c), and then read from the channel
// until TokenEnd. This variant has the benefit that
// the process reading the channel can be a recursive
// function instead of a set of callbacks, but it has the
// drawback that the channel interface cannot signal an
// error to cause the parser to stop early.
// An XML parsing token.
const (
TokenStartElement = 1 + iota;
TokenEndElement;
TokenText;
TokenComment;
TokenProcInst;
TokenEnd;
)
type Token struct {
Kind int; // TokenStartElement, TokenEndElement, etc.
Name Name; // name (TokenStartElement, TokenEndElement)
Attr []Attr; // attributes (TokenStartElement)
Target string; // target (TokenProcessingInstruction)
Text []byte; // text (TokenCharData, TokenComment, etc.)
Err *os.Error; // error (TokenEnd)
}
type ChanBuilder chan Token;
func (c ChanBuilder) StartElement(name Name, attr []Attr) *os.Error {
var t Token;
t.Kind = TokenStartElement;
t.Name = name;
t.Attr = attr;
c <- t;
return nil;
}
func (c ChanBuilder) EndElement(name Name) *os.Error {
var t Token;
t.Kind = TokenEndElement;
t.Name = name;
c <- t;
return nil;
}
func (c ChanBuilder) Text(text []byte) *os.Error {
var t Token;
t.Kind = TokenText;
t.Text = text;
c <- t;
return nil;
}
func (c ChanBuilder) Comment(text []byte) *os.Error {
var t Token;
t.Kind = TokenComment;
t.Text = text;
c <- t;
return nil;
}
func (c ChanBuilder) ProcInst(target string, text []byte) *os.Error {
var t Token;
t.Kind = TokenProcInst;
t.Target = target;
t.Text = text;
c <- t;
return nil;
}
func ParseToChan(r io.Read, c chan Token) {
var t Token;
t.Kind = TokenEnd;
t.Err = Parse(r, ChanBuilder(c));
c <- t;
}
// scribbled notes based on XML spec.
// document is
// xml decl?
// doctype decl?
// element
//
// if xml decl is present, must be first. after that,
// can have comments and procinsts scattered throughout,
// even after the element is done.
//
// xml decl is:
//
// <\?xml version='[a-zA-Z0-9_.:\-]+'( encoding='[A-Za-z][A-Za-z0-9._\-]*')?
// ( standalone='(yes|no)')? ?\?>
//
// spaces denote [ \r\t\n]+.
// written with '' above but can use "" too.
//
// doctype decl might as well be <!DOCTYPE[^>]*>
//
// procinst is <\?name( .*?)\?>. name cannot be [Xx][Mm][Ll].
//
// comment is <!--(.*?)-->.
//
// tags are:
// <name( attrib)* ?> start tag
// <name( attrib)* ?/> combined start/end tag
// </name ?> end tag
// (the " ?" is an optional space, not a literal question mark.)
//
// plain text is [^<&]* except cannot contain "]]>".
// can also have escaped characters:
// &#[0-9]+;
// &#x[0-9A-Fa-f]+;
// &name;
//
// can use <![CDATA[.*?]]> to avoid escaping < characters.
//
// must rewrite \r and \r\n into \n in text.
//
// names are Unicode. valid chars listed below.
//
// attrib is name="value" or name='value'.
// can have spaces around =.
// attribute value text is [^<&"]* for appropriate ".
// can also use the &...; escape sequences above.
// cannot use <![CDATA[...]]>.
//
// xmlns attributes are name=value where name has form xmlns:name
// (i.e., xmlns:123 is not okay, because 123 is not a name; xmlns:a123 is ok).
// sub-name must not start with : either.
//
// name is first(second)*.
//
// first is
//
// 003A 04D0-04EB 0A59-0A5C 0C35-0C39 0F49-0F69 1E00-1E9B
// 0041-005A 04EE-04F5 0A5E 0C60-0C61 10A0-10C5 1EA0-1EF9
// 005F 04F8-04F9 0A72-0A74 0C85-0C8C 10D0-10F6 1F00-1F15
// 0061-007A 0531-0556 0A85-0A8B 0C8E-0C90 1100 1F18-1F1D
// 00C0-00D6 0559 0A8D 0C92-0CA8 1102-1103 1F20-1F45
// 00D8-00F6 0561-0586 0A8F-0A91 0CAA-0CB3 1105-1107 1F48-1F4D
// 00F8-00FF 05D0-05EA 0A93-0AA8 0CB5-0CB9 1109 1F50-1F57
// 0100-0131 05F0-05F2 0AAA-0AB0 0CDE 110B-110C 1F59
// 0134-013E 0621-063A 0AB2-0AB3 0CE0-0CE1 110E-1112 1F5B
// 0141-0148 0641-064A 0AB5-0AB9 0D05-0D0C 113C 1F5D
// 014A-017E 0671-06B7 0ABD 0D0E-0D10 113E 1F5F-1F7D
// 0180-01C3 06BA-06BE 0AE0 0D12-0D28 1140 1F80-1FB4
// 01CD-01F0 06C0-06CE 0B05-0B0C 0D2A-0D39 114C 1FB6-1FBC
// 01F4-01F5 06D0-06D3 0B0F-0B10 0D60-0D61 114E 1FBE
// 01FA-0217 06D5 0B13-0B28 0E01-0E2E 1150 1FC2-1FC4
// 0250-02A8 06E5-06E6 0B2A-0B30 0E30 1154-1155 1FC6-1FCC
// 02BB-02C1 0905-0939 0B32-0B33 0E32-0E33 1159 1FD0-1FD3
// 0386 093D 0B36-0B39 0E40-0E45 115F-1161 1FD6-1FDB
// 0388-038A 0958-0961 0B3D 0E81-0E82 1163 1FE0-1FEC
// 038C 0985-098C 0B5C-0B5D 0E84 1165 1FF2-1FF4
// 038E-03A1 098F-0990 0B5F-0B61 0E87-0E88 1167 1FF6-1FFC
// 03A3-03CE 0993-09A8 0B85-0B8A 0E8A 1169 2126
// 03D0-03D6 09AA-09B0 0B8E-0B90 0E8D 116D-116E 212A-212B
// 03DA 09B2 0B92-0B95 0E94-0E97 1172-1173 212E
// 03DC 09B6-09B9 0B99-0B9A 0E99-0E9F 1175 2180-2182
// 03DE 09DC-09DD 0B9C 0EA1-0EA3 119E 3007
// 03E0 09DF-09E1 0B9E-0B9F 0EA5 11A8 3021-3029
// 03E2-03F3 09F0-09F1 0BA3-0BA4 0EA7 11AB 3041-3094
// 0401-040C 0A05-0A0A 0BA8-0BAA 0EAA-0EAB 11AE-11AF 30A1-30FA
// 040E-044F 0A0F-0A10 0BAE-0BB5 0EAD-0EAE 11B7-11B8 3105-312C
// 0451-045C 0A13-0A28 0BB7-0BB9 0EB0 11BA 4E00-9FA5
// 045E-0481 0A2A-0A30 0C05-0C0C 0EB2-0EB3 11BC-11C2 AC00-D7A3
// 0490-04C4 0A32-0A33 0C0E-0C10 0EBD 11EB
// 04C7-04C8 0A35-0A36 0C12-0C28 0EC0-0EC4 11F0
// 04CB-04CC 0A38-0A39 0C2A-0C33 0F40-0F47 11F9
//
// second is first plus
//
// 002D 06DD-06DF 09E6-09EF 0B56-0B57 0D3E-0D43 0F3E
// 002E 06E0-06E4 0A02 0B66-0B6F 0D46-0D48 0F3F
// 0030-0039 06E7-06E8 0A3C 0B82-0B83 0D4A-0D4D 0F71-0F84
// 00B7 06EA-06ED 0A3E 0BBE-0BC2 0D57 0F86-0F8B
// 02D0 06F0-06F9 0A3F 0BC6-0BC8 0D66-0D6F 0F90-0F95
// 02D1 0901-0903 0A40-0A42 0BCA-0BCD 0E31 0F97
// 0300-0345 093C 0A47-0A48 0BD7 0E34-0E3A 0F99-0FAD
// 0360-0361 093E-094C 0A4B-0A4D 0BE7-0BEF 0E46 0FB1-0FB7
// 0387 094D 0A66-0A6F 0C01-0C03 0E47-0E4E 0FB9
// 0483-0486 0951-0954 0A70-0A71 0C3E-0C44 0E50-0E59 20D0-20DC
// 0591-05A1 0962-0963 0A81-0A83 0C46-0C48 0EB1 20E1
// 05A3-05B9 0966-096F 0ABC 0C4A-0C4D 0EB4-0EB9 3005
// 05BB-05BD 0981-0983 0ABE-0AC5 0C55-0C56 0EBB-0EBC 302A-302F
// 05BF 09BC 0AC7-0AC9 0C66-0C6F 0EC6 3031-3035
// 05C1-05C2 09BE 0ACB-0ACD 0C82-0C83 0EC8-0ECD 3099
// 05C4 09BF 0AE6-0AEF 0CBE-0CC4 0ED0-0ED9 309A
// 0640 09C0-09C4 0B01-0B03 0CC6-0CC8 0F18-0F19 309D-309E
// 064B-0652 09C7-09C8 0B3C 0CCA-0CCD 0F20-0F29 30FC-30FE
// 0660-0669 09CB-09CD 0B3E-0B43 0CD5-0CD6 0F35
// 0670 09D7 0B47-0B48 0CE6-0CEF 0F37
// 06D6-06DC 09E2-09E3 0B4B-0B4D 0D02-0D03 0F39