guile/doc/ref/sxml.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C) 2013 Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node SXML
@section SXML

@menu
* sxml apply-templates::  A more XSLT-like approach to SXML transformations
* sxml fold::            Fold-based SXML transformation operators
* sxml simple::          Convenient XML parsing and serializing
* sxml ssax::            Functional-style XML parsing for Scheme
* sxml ssax input-parse::  The SSAX tokenizer, optimized for Guile
* sxml transform::       A higher-order SXML transformation operator, @code{pre-post-order}
* sxml xpath::           XPath for SXML
@end menu

@node sxml apply-templates
@subsection (sxml apply-templates)
@subsubsection Overview
Pre-order traversal of a tree and creation of a new tree:

@smallexample 
	apply-templates:: tree x <templates> -> <new-tree>
@end smallexample

where

@smallexample 
 <templates> ::= (<template> ...)
 <template>  ::= (<node-test> <node-test> ... <node-test> . <handler>)
 <node-test> ::= an argument to node-typeof? above
 <handler>   ::= <tree> -> <new-tree>
@end smallexample

This procedure does a @emph{normal}, pre-order traversal of an SXML
tree. It walks the tree, checking at each node against the list of
matching templates.

If the match is found (which must be unique, i.e., unambiguous), the
corresponding handler is invoked and given the current node as an
argument. The result from the handler, which must be a @code{<tree>},
takes place of the current node in the resulting tree. The name of the
function is not accidental: it resembles rather closely an
@code{apply-templates} function of XSLT.

@subsubsection Usage
@anchor{sxml apply-templates apply-templates}@defun apply-templates tree templates
@end defun

@node sxml fold
@subsection (sxml fold)
@subsubsection Overview
@code{(sxml fold)} defines a number of variants of the @dfn{fold}
algorithm for use in transforming SXML trees. Additionally it defines
the layout operator, @code{fold-layout}, which might be described as a
context-passing variant of SSAX's @code{pre-post-order}.

@subsubsection Usage
@anchor{sxml fold foldt}@defun foldt fup fhere tree
The standard multithreaded tree fold.

@var{fup} is of type [a] -> a. @var{fhere} is of type object -> a.

@end defun

@anchor{sxml fold foldts}@defun foldts fdown fup fhere seed tree
The single-threaded tree fold originally defined in SSAX. @xref{sxml
ssax,,(sxml ssax)}, for more information.

@end defun

@anchor{sxml fold foldts*}@defun foldts* fdown fup fhere seed tree
A variant of @ref{sxml fold foldts,,foldts} that allows pre-order tree
rewrites. Originally defined in Andy Wingo's 2007 paper,
@emph{Applications of fold to XML transformation}.

@end defun

@anchor{sxml fold fold-values}@defun fold-values proc list . seeds
A variant of @ref{SRFI-1 Fold and Map,fold} that allows multi-valued
seeds. Note that the order of the arguments differs from that of
@code{fold}.

@end defun

@anchor{sxml fold foldts*-values}@defun foldts*-values fdown fup fhere tree . seeds
A variant of @ref{sxml fold foldts*,,foldts*} that allows multi-valued
seeds. Originally defined in Andy Wingo's 2007 paper, @emph{Applications
of fold to XML transformation}.

@end defun

@anchor{sxml fold fold-layout}@defun fold-layout tree bindings params layout stylesheet
A traversal combinator in the spirit of SSAX's @ref{sxml transform
pre-post-order,,pre-post-order}.

@code{fold-layout} was originally presented in Andy Wingo's 2007 paper,
@emph{Applications of fold to XML transformation}.

@example 
bindings := (<binding>...)
binding  := (<tag> <bandler-pair>...)
          | (*default* . <post-handler>)
          | (*text* . <text-handler>)
tag      := <symbol>
handler-pair := (pre-layout . <pre-layout-handler>)
          | (post . <post-handler>)
          | (bindings . <bindings>)
          | (pre . <pre-handler>)
          | (macro . <macro-handler>)
@end example

@table @var
@item pre-layout-handler
A function of three arguments:

@table @var
@item kids
the kids of the current node, before traversal

@item params
the params of the current node

@item layout
the layout coming into this node

@end table

@var{pre-layout-handler} is expected to use this information to return a
layout to pass to the kids. The default implementation returns the
layout given in the arguments.

@item post-handler
A function of five arguments:

@table @var
@item tag
the current tag being processed

@item params
the params of the current node

@item layout
the layout coming into the current node, before any kids were processed

@item klayout
the layout after processing all of the children

@item kids
the already-processed child nodes

@end table

@var{post-handler} should return two values, the layout to pass to the
next node and the final tree.

@item text-handler
@var{text-handler} is a function of three arguments:

@table @var
@item text
the string

@item params
the current params

@item layout
the current layout

@end table

@var{text-handler} should return two values, the layout to pass to the
next node and the value to which the string should transform.

@end table

@end defun

@node sxml simple
@subsection (sxml simple)
@subsubsection Overview
A simple interface to XML parsing and serialization.

@subsubsection Usage
@anchor{sxml simple xml->sxml}@defun xml->sxml [port]
Use SSAX to parse an XML document into SXML. Takes one optional
argument, @var{port}, which defaults to the current input port.

@end defun

@anchor{sxml simple sxml->xml}@defun sxml->xml tree [port]
Serialize the sxml tree @var{tree} as XML. The output will be written to
the current output port, unless the optional argument @var{port} is
present.

@end defun

@anchor{sxml simple sxml->string}@defun sxml->string sxml
Detag an sxml tree @var{sxml} into a string. Does not perform any
formatting.

@end defun

@node sxml ssax
@subsection (sxml ssax)
@subsubsection Overview
@subheading Functional XML parsing framework
@subsubheading SAX/DOM and SXML parsers with support for XML Namespaces and validation
This is a package of low-to-high level lexing and parsing procedures
that can be combined to yield a SAX, a DOM, a validating parser, or a
parser intended for a particular document type. The procedures in the
package can be used separately to tokenize or parse various pieces of
XML documents. The package supports XML Namespaces, internal and
external parsed entities, user-controlled handling of whitespace, and
validation. This module therefore is intended to be a framework, a set
of "Lego blocks" you can use to build a parser following any discipline
and performing validation to any degree. As an example of the parser
construction, this file includes a semi-validating SXML parser.

The present XML framework has a "sequential" feel of SAX yet a
"functional style" of DOM. Like a SAX parser, the framework scans the
document only once and permits incremental processing. An application
that handles document elements in order can run as efficiently as
possible. @emph{Unlike} a SAX parser, the framework does not require an
application register stateful callbacks and surrender control to the
parser. Rather, it is the application that can drive the framework --
calling its functions to get the current lexical or syntax element.
These functions do not maintain or mutate any state save the input port.
Therefore, the framework permits parsing of XML in a pure functional
style, with the input port being a monad (or a linear, read-once
parameter).

Besides the @var{port}, there is another monad -- @var{seed}. Most of
the middle- and high-level parsers are single-threaded through the
@var{seed}. The functions of this framework do not process or affect the
@var{seed} in any way: they simply pass it around as an instance of an
opaque datatype. User functions, on the other hand, can use the seed to
maintain user's state, to accumulate parsing results, etc. A user can
freely mix his own functions with those of the framework. On the other
hand, the user may wish to instantiate a high-level parser:
@code{SSAX:make-elem-parser} or @code{SSAX:make-parser}. In the latter
case, the user must provide functions of specific signatures, which are
called at predictable moments during the parsing: to handle character
data, element data, or processing instructions (PI). The functions are
always given the @var{seed}, among other parameters, and must return the
new @var{seed}.

From a functional point of view, XML parsing is a combined
pre-post-order traversal of a "tree" that is the XML document itself.
This down-and-up traversal tells the user about an element when its
start tag is encountered. The user is notified about the element once
more, after all element's children have been handled. The process of XML
parsing therefore is a fold over the raw XML document. Unlike a fold
over trees defined in [1], the parser is necessarily single-threaded --
obviously as elements in a text XML document are laid down sequentially.
The parser therefore is a tree fold that has been transformed to accept
an accumulating parameter [1,2].

Formally, the denotational semantics of the parser can be expressed as

@smallexample 
 parser:: (Start-tag -> Seed -> Seed) ->
	   (Start-tag -> Seed -> Seed -> Seed) ->
	   (Char-Data -> Seed -> Seed) ->
	   XML-text-fragment -> Seed -> Seed
 parser fdown fup fchar "<elem attrs> content </elem>" seed
  = fup "<elem attrs>" seed
	(parser fdown fup fchar "content" (fdown "<elem attrs>" seed))

 parser fdown fup fchar "char-data content" seed
  = parser fdown fup fchar "content" (fchar "char-data" seed)

 parser fdown fup fchar "elem-content content" seed
  = parser fdown fup fchar "content" (
	parser fdown fup fchar "elem-content" seed)
@end smallexample

Compare the last two equations with the left fold

@smallexample 
 fold-left kons elem:list seed = fold-left kons list (kons elem seed)
@end smallexample

The real parser created by @code{SSAX:make-parser} is slightly more
complicated, to account for processing instructions, entity references,
namespaces, processing of document type declaration, etc.

The XML standard document referred to in this module
is@uref{http://www.w3.org/TR/1998/REC-xml-19980210.html}

The present file also defines a procedure that parses the text of an XML
document or of a separate element into SXML, an S-expression-based model
of an XML Information Set. SXML is also an Abstract Syntax Tree of an
XML document. SXML is similar but not identical to DOM; SXML is
particularly suitable for Scheme-based XML/HTML authoring, SXPath
queries, and tree transformations. See SXML.html for more details. SXML
is a term implementation of evaluation of the XML document [3]. The
other implementation is context-passing.

The present frameworks fully supports the XML Namespaces
Recommendation:@uref{http://www.w3.org/TR/REC-xml-names/} Other links:

@table @asis
@item [1]
Jeremy Gibbons, Geraint Jones, "The Under-appreciated Unfold," Proc.
ICFP'98, 1998, pp. 273-279.

@item [2]
Richard S. Bird, The promotion and accumulation strategies in
transformational programming, ACM Trans. Progr. Lang. Systems,
6(4):487-504, October 1984.

@item [3]
Ralf Hinze, "Deriving Backtracking Monad Transformers," Functional
Pearl. Proc ICFP'00, pp. 186-197.

@end table

@subsubsection Usage
@anchor{sxml ssax current-ssax-error-port}@defun current-ssax-error-port 
@end defun

@anchor{sxml ssax with-ssax-error-to-port}@defun with-ssax-error-to-port port thunk
@end defun

@anchor{sxml ssax xml-token?}@defun xml-token? _
@verbatim 
 -- Scheme Procedure: pair? x
     Return `#t' if X is a pair; otherwise return `#f'.

 
@end verbatim

@end defun

@anchor{sxml ssax xml-token-kind}@defspec xml-token-kind token
@end defspec

@anchor{sxml ssax xml-token-head}@defspec xml-token-head token
@end defspec

@anchor{sxml ssax make-empty-attlist}@defun make-empty-attlist 
@end defun

@anchor{sxml ssax attlist-add}@defun attlist-add attlist name-value
@end defun

@anchor{sxml ssax attlist-null?}@defun attlist-null? _
@verbatim 
 -- Scheme Procedure: null? x
     Return `#t' iff X is the empty list, else `#f'.

 
@end verbatim

@end defun

@anchor{sxml ssax attlist-remove-top}@defun attlist-remove-top attlist
@end defun

@anchor{sxml ssax attlist->alist}@defun attlist->alist attlist
@end defun

@anchor{sxml ssax attlist-fold}@defun attlist-fold kons knil lis1
@end defun

@anchor{sxml ssax define-parsed-entity!}@defun define-parsed-entity! entity str
Define a new parsed entity. @var{entity} should be a symbol.

Instances of &@var{entity}; in XML text will be replaced with the string
@var{str}, which will then be parsed.

@end defun

@anchor{sxml ssax reset-parsed-entity-definitions!}@defun reset-parsed-entity-definitions! 
Restore the set of parsed entity definitions to its initial state.

@end defun

@anchor{sxml ssax ssax:uri-string->symbol}@defun ssax:uri-string->symbol uri-str
@end defun

@anchor{sxml ssax ssax:skip-internal-dtd}@defun ssax:skip-internal-dtd port
@end defun

@anchor{sxml ssax ssax:read-pi-body-as-string}@defun ssax:read-pi-body-as-string port
@end defun

@anchor{sxml ssax ssax:reverse-collect-str-drop-ws}@defun ssax:reverse-collect-str-drop-ws fragments
@end defun

@anchor{sxml ssax ssax:read-markup-token}@defun ssax:read-markup-token port
@end defun

@anchor{sxml ssax ssax:read-cdata-body}@defun ssax:read-cdata-body port str-handler seed
@end defun

@anchor{sxml ssax ssax:read-char-ref}@defun ssax:read-char-ref port
@end defun

@anchor{sxml ssax ssax:read-attributes}@defun ssax:read-attributes port entities
@end defun

@anchor{sxml ssax ssax:complete-start-tag}@defun ssax:complete-start-tag tag-head port elems entities namespaces
@end defun

@anchor{sxml ssax ssax:read-external-id}@defun ssax:read-external-id port
@end defun

@anchor{sxml ssax ssax:read-char-data}@defun ssax:read-char-data port expect-eof? str-handler seed
@end defun

@anchor{sxml ssax ssax:xml->sxml}@defun ssax:xml->sxml port namespace-prefix-assig
@end defun

@anchor{sxml ssax ssax:make-parser}@defspec ssax:make-parser  . kw-val-pairs
@end defspec

@anchor{sxml ssax ssax:make-pi-parser}@defspec ssax:make-pi-parser orig-handlers
@end defspec

@anchor{sxml ssax ssax:make-elem-parser}@defspec ssax:make-elem-parser my-new-level-seed my-finish-element my-char-data-handler my-pi-handlers
@end defspec

@node sxml ssax input-parse
@subsection (sxml ssax input-parse)
@subsubsection Overview
A simple lexer.

The procedures in this module surprisingly often suffice to parse an
input stream. They either skip, or build and return tokens, according to
inclusion or delimiting semantics. The list of characters to expect,
include, or to break at may vary from one invocation of a function to
another. This allows the functions to easily parse even
context-sensitive languages.

EOF is generally frowned on, and thrown up upon if encountered.
Exceptions are mentioned specifically. The list of expected characters
(characters to skip until, or break-characters) may include an EOF
"character", which is to be coded as the symbol, @code{*eof*}.

The input stream to parse is specified as a @dfn{port}, which is usually
the last (and optional) argument. It defaults to the current input port
if omitted.

If the parser encounters an error, it will throw an exception to the key
@code{parser-error}. The arguments will be of the form @code{(@var{port}
@var{message} @var{specialising-msg}*)}.

The first argument is a port, which typically points to the offending
character or its neighborhood. You can then use @code{port-column} and
@code{port-line} to query the current position. @var{message} is the
description of the error. Other arguments supply more details about the
problem.

@subsubsection Usage
@anchor{sxml ssax input-parse peek-next-char}@defun peek-next-char [port]
@end defun

@anchor{sxml ssax input-parse assert-curr-char}@defun assert-curr-char expected-chars comment [port]
@end defun

@anchor{sxml ssax input-parse skip-until}@defun skip-until arg [port]
@end defun

@anchor{sxml ssax input-parse skip-while}@defun skip-while skip-chars [port]
@end defun

@anchor{sxml ssax input-parse next-token}@defun next-token prefix-skipped-chars break-chars [comment] [port]
@end defun

@anchor{sxml ssax input-parse next-token-of}@defun next-token-of incl-list/pred [port]
@end defun

@anchor{sxml ssax input-parse read-text-line}@defun read-text-line [port]
@end defun

@anchor{sxml ssax input-parse read-string}@defun read-string n [port]
@end defun

@anchor{sxml ssax input-parse find-string-from-port?}@defun find-string-from-port? _ _ . _
Looks for @var{str} in @var{<input-port>}, optionally within the first
@var{max-no-char} characters.

@end defun

@node sxml transform
@subsection (sxml transform)
@subsubsection Overview
@heading SXML expression tree transformers
@subheading Pre-Post-order traversal of a tree and creation of a new tree
@smallexample 
pre-post-order:: <tree> x <bindings> -> <new-tree>
@end smallexample

where

@smallexample 
 <bindings> ::= (<binding> ...)
 <binding> ::= (<trigger-symbol> *preorder* . <handler>) |
               (<trigger-symbol> *macro* . <handler>) |
		(<trigger-symbol> <new-bindings> . <handler>) |
		(<trigger-symbol> . <handler>)
 <trigger-symbol> ::= XMLname | *text* | *default*
 <handler> :: <trigger-symbol> x [<tree>] -> <new-tree>
@end smallexample

The pre-post-order function visits the nodes and nodelists
pre-post-order (depth-first). For each @code{<Node>} of the form
@code{(@var{name} <Node> ...)}, it looks up an association with the
given @var{name} among its @var{<bindings>}. If failed,
@code{pre-post-order} tries to locate a @code{*default*} binding. It's
an error if the latter attempt fails as well. Having found a binding,
the @code{pre-post-order} function first checks to see if the binding is
of the form

@smallexample 
	(<trigger-symbol> *preorder* . <handler>)
@end smallexample

If it is, the handler is 'applied' to the current node. Otherwise, the
pre-post-order function first calls itself recursively for each child of
the current node, with @var{<new-bindings>} prepended to the
@var{<bindings>} in effect. The result of these calls is passed to the
@var{<handler>} (along with the head of the current @var{<Node>}). To be
more precise, the handler is _applied_ to the head of the current node
and its processed children. The result of the handler, which should also
be a @code{<tree>}, replaces the current @var{<Node>}. If the current
@var{<Node>} is a text string or other atom, a special binding with a
symbol @code{*text*} is looked up.

A binding can also be of a form

@smallexample 
	(<trigger-symbol> *macro* . <handler>)
@end smallexample

This is equivalent to @code{*preorder*} described above. However, the
result is re-processed again, with the current stylesheet.

@subsubsection Usage
@anchor{sxml transform SRV:send-reply}@defun SRV:send-reply . fragments
Output the @var{fragments} to the current output port.

The fragments are a list of strings, characters, numbers, thunks,
@code{#f}, @code{#t} -- and other fragments. The function traverses the
tree depth-first, writes out strings and characters, executes thunks,
and ignores @code{#f} and @code{'()}. The function returns @code{#t} if
anything was written at all; otherwise the result is @code{#f} If
@code{#t} occurs among the fragments, it is not written out but causes
the result of @code{SRV:send-reply} to be @code{#t}.

@end defun

@anchor{sxml transform foldts}@defun foldts fdown fup fhere seed tree
@end defun

@anchor{sxml transform post-order}@defun post-order tree bindings
@end defun

@anchor{sxml transform pre-post-order}@defun pre-post-order tree bindings
@end defun

@anchor{sxml transform replace-range}@defun replace-range beg-pred end-pred forest
@end defun

@node sxml xpath
@subsection (sxml xpath)
@subsubsection Overview
@heading SXPath: SXML Query Language
SXPath is a query language for SXML, an instance of XML Information set
(Infoset) in the form of s-expressions. See @code{(sxml ssax)} for the
definition of SXML and more details. SXPath is also a translation into
Scheme of an XML Path Language, @uref{http://www.w3.org/TR/xpath,XPath}.
XPath and SXPath describe means of selecting a set of Infoset's items or
their properties.

To facilitate queries, XPath maps the XML Infoset into an explicit tree,
and introduces important notions of a location path and a current,
context node. A location path denotes a selection of a set of nodes
relative to a context node. Any XPath tree has a distinguished, root
node -- which serves as the context node for absolute location paths.
Location path is recursively defined as a location step joined with a
location path. A location step is a simple query of the database
relative to a context node. A step may include expressions that further
filter the selected set. Each node in the resulting set is used as a
context node for the adjoining location path. The result of the step is
a union of the sets returned by the latter location paths.

The SXML representation of the XML Infoset (see SSAX.scm) is rather
suitable for querying as it is. Bowing to the XPath specification, we
will refer to SXML information items as 'Nodes':

@example 
 	<Node> ::= <Element> | <attributes-coll> | <attrib>
 		   | "text string" | <PI>
@end example

This production can also be described as

@example 
	<Node> ::= (name . <Nodeset>) | "text string"
@end example

An (ordered) set of nodes is just a list of the constituent nodes:

@example 
 	<Nodeset> ::= (<Node> ...)
@end example

Nodesets, and Nodes other than text strings are both lists. A <Nodeset>
however is either an empty list, or a list whose head is not a symbol. A
symbol at the head of a node is either an XML name (in which case it's a
tag of an XML element), or an administrative name such as '@@'. This
uniform list representation makes processing rather simple and elegant,
while avoiding confusion. The multi-branch tree structure formed by the
mutually-recursive datatypes <Node> and <Nodeset> lends itself well to
processing by functional languages.

A location path is in fact a composite query over an XPath tree or its
branch. A singe step is a combination of a projection, selection or a
transitive closure. Multiple steps are combined via join and union
operations. This insight allows us to @emph{elegantly} implement XPath
as a sequence of projection and filtering primitives -- converters --
joined by @dfn{combinators}. Each converter takes a node and returns a
nodeset which is the result of the corresponding query relative to that
node. A converter can also be called on a set of nodes. In that case it
returns a union of the corresponding queries over each node in the set.
The union is easily implemented as a list append operation as all nodes
in a SXML tree are considered distinct, by XPath conventions. We also
preserve the order of the members in the union. Query combinators are
high-order functions: they take converter(s) (which is a Node|Nodeset ->
Nodeset function) and compose or otherwise combine them. We will be
concerned with only relative location paths [XPath]: an absolute
location path is a relative path applied to the root node.

Similarly to XPath, SXPath defines full and abbreviated notations for
location paths. In both cases, the abbreviated notation can be
mechanically expanded into the full form by simple rewriting rules. In
case of SXPath the corresponding rules are given as comments to a sxpath
function, below. The regression test suite at the end of this file shows
a representative sample of SXPaths in both notations, juxtaposed with
the corresponding XPath expressions. Most of the samples are borrowed
literally from the XPath specification, while the others are adjusted
for our running example, tree1.

@subsubsection Usage
@anchor{sxml xpath nodeset?}@defun nodeset? x
@end defun

@anchor{sxml xpath node-typeof?}@defun node-typeof? crit
@end defun

@anchor{sxml xpath node-eq?}@defun node-eq? other
@end defun

@anchor{sxml xpath node-equal?}@defun node-equal? other
@end defun

@anchor{sxml xpath node-pos}@defun node-pos n
@end defun

@anchor{sxml xpath filter}@defun filter pred?
@verbatim 
 -- Scheme Procedure: filter pred list
     Return all the elements of 2nd arg LIST that satisfy predicate
     PRED.  The list is not disordered - elements that appear in the
     result list occur in the same order as they occur in the argument
     list. The returned list may share a common tail with the argument
     list. The dynamic order in which the various applications of pred
     are made is not specified.

          (filter even? '(0 7 8 8 43 -4)) => (0 8 8 -4)

 
@end verbatim

@end defun

@anchor{sxml xpath take-until}@defun take-until pred?
@end defun

@anchor{sxml xpath take-after}@defun take-after pred?
@end defun

@anchor{sxml xpath map-union}@defun map-union proc lst
@end defun

@anchor{sxml xpath node-reverse}@defun node-reverse node-or-nodeset
@end defun

@anchor{sxml xpath node-trace}@defun node-trace title
@end defun

@anchor{sxml xpath select-kids}@defun select-kids test-pred?
@end defun

@anchor{sxml xpath node-self}@defun node-self pred?
@verbatim 
 -- Scheme Procedure: filter pred list
     Return all the elements of 2nd arg LIST that satisfy predicate
     PRED.  The list is not disordered - elements that appear in the
     result list occur in the same order as they occur in the argument
     list. The returned list may share a common tail with the argument
     list. The dynamic order in which the various applications of pred
     are made is not specified.

          (filter even? '(0 7 8 8 43 -4)) => (0 8 8 -4)

 
@end verbatim

@end defun

@anchor{sxml xpath node-join}@defun node-join . selectors
@end defun

@anchor{sxml xpath node-reduce}@defun node-reduce . converters
@end defun

@anchor{sxml xpath node-or}@defun node-or . converters
@end defun

@anchor{sxml xpath node-closure}@defun node-closure test-pred?
@end defun

@anchor{sxml xpath node-parent}@defun node-parent rootnode
@end defun

@anchor{sxml xpath sxpath}@defun sxpath path
@end defun