@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 SXML is a native representation of XML in terms of standard Scheme data types: lists, symbols, and strings. For example, the simple XML fragment: @example Alfie @end example may be represented with the following SXML: @example (parrot (@@ (type "African Grey)) (name "Alfie")) @end example SXML is very general, and is capable of representing all of XML. Formally, this means that SXML is a conforming implementation of the @uref{XML Information Set,http://www.w3.org/TR/xml-infoset/} standard. Guile includes several facilities for working with XML and SXML: parsers, serializers, and transformers. @menu * SXML Overview:: XML, as it was meant to be * Reading and Writing XML:: Convenient XML parsing and serializing * SSAX:: Custom functional-style XML parsers * Transforming SXML:: Munging SXML with @code{pre-post-order} * SXML Tree Fold:: Fold-based SXML transformations * SXPath:: XPath for SXML * sxml apply-templates:: A more XSLT-like approach to SXML transformations * sxml ssax input-parse:: The SSAX tokenizer, optimized for Guile @end menu @node SXML Overview @subsection SXML Overview (This section needs to be written; volunteers welcome.) @node Reading and Writing XML @subsection Reading and Writing XML The @code{(sxml simple)} module presents a basic interface for parsing XML from a port into the Scheme SXML format, and for serializing it back to text. @example (use-modules (sxml simple)) @end example @deffn {Scheme Procedure} xml->sxml [string-or-port] [#:namespaces='()] @ [#:declare-namespaces?=#t] [#:trim-whitespace?=#f] @ [#:entities='()] [#:default-entity-handler=#f] @ [#:doctype-handler=#f] Use SSAX to parse an XML document into SXML. Takes one optional argument, @var{string-or-port}, which defaults to the current input port. Returns the resulting SXML document. If @var{string-or-port} is a port, it will be left pointing at the next available character in the port. @end deffn As is normal in SXML, XML elements parse as tagged lists. Attributes, if any, are placed after the tag, within an @code{@@} element. The root of the resulting XML will be contained in a special tag, @code{*TOP*}. This tag will contain the root element of the XML, but also any prior processing instructions. @example (xml->sxml "") @result{} (*TOP* (foo)) (xml->sxml "text") @result{} (*TOP* (foo "text")) (xml->sxml "text") @result{} (*TOP* (foo (@@ (kind "bar")) "text")) (xml->sxml "") @result{} (*TOP* (*PI* xml "version=\"1.0\"") (foo)) @end example All namespaces in the XML document must be declared, via @code{xmlns} attributes. SXML elements built from non-default namespaces will have their tags prefixed with their URI. Users can specify custom prefixes for certain namespaces with the @code{#:namespaces} keyword argument to @code{xml->sxml}. @example (xml->sxml "text") @result{} (*TOP* (http://example.org/ns1:foo "text")) (xml->sxml "text" #:namespaces '((ns1 . "http://example.org/ns1"))) @result{} (*TOP* (ns1:foo "text")) (xml->sxml "" #:namespaces '((ns2 . "http://example.org/ns2"))) @result{} (*TOP* (foo (ns2:baz))) @end example By default, namespaces passed to @code{xml->sxml} are treated as if they were declared on the root element. Passing a false @code{#:declare-namespaces?} argument will disable this behavior, requiring in-document declarations of namespaces before use.. @example (xml->sxml "" #:namespaces '((ns2 . "http://example.org/ns2"))) @result{} (*TOP* (foo (ns2:baz))) (xml->sxml "" #:namespaces '((ns2 . "http://example.org/ns2")) #:declare-namespaces? #f) @result{} error: undeclared namespace: `bar' @end example By default, all whitespace in XML is significant. Passing the @code{#:trim-whitespace?} keyword argument to @code{xml->sxml} will trim whitespace in front, behind and between elements, treating it as ``unsignificant''. Whitespace in text fragments is left alone. @example (xml->sxml "\n Alfie the parrot! \n") @result{} (*TOP* (foo "\n" (bar " Alfie the parrot! ") "\n")) (xml->sxml "\n Alfie the parrot! \n" #:trim-whitespace? #t) @result{} (*TOP* (foo (bar " Alfie the parrot! "))) @end example Parsed entities may be declared with the @code{#:entities} keyword argument, or handled with the @code{#:default-entity-handler}. By default, only the standard @code{<}, @code{>}, @code{&}, @code{'} and @code{"} entities are defined, as well as the @code{&#@var{N};} and @code{&#x@var{N};} (decimal and hexadecimal) numeric character entities. @example (xml->sxml "&") @result{} (*TOP* (foo "&")) (xml->sxml " ") @result{} error: undefined entity: nbsp (xml->sxml " ") @result{} (*TOP* (foo "\xa0")) (xml->sxml " " #:entities '((nbsp . "\xa0"))) @result{} (*TOP* (foo "\xa0")) (xml->sxml "  &foo;" #:default-entity-handler (lambda (port name) (case name ((nbsp) "\xa0") (else (format (current-warning-port) "~a:~a:~a: undefined entitity: ~a\n" (or (port-filename port) "") (port-line port) (port-column port) name) (symbol->string name))))) @print{} :0:17: undefined entitity: foo @result{} (*TOP* (foo "\xa0 foo")) @end example By default, @code{xml->sxml} skips over the @code{} declaration, if any. This behavior can be overridden with the @code{#:doctype-handler} argument, which should be a procedure of three arguments: the @dfn{docname} (a symbol), @dfn{systemid} (a string), and the internal doctype subset (as a string or @code{#f} if not present). The handler should return keyword arguments as multiple values, as if it were calling its continuation with keyword arguments. The continuation accepts the @code{#:entities} and @code{#:namespaces} keyword arguments, in the same format that @code{xml->sxml} itself takes. These entities and namespaces will be prepended to those given to the @code{xml->sxml} invocation. @example (define (handle-foo docname systemid internal-subset) (case docname ((foo) (values #:entities '((greets . "Hello, world!")))) (else (values)))) (xml->sxml "

&greets;

" #:doctype-handler handle-foo) @result{} (*TOP* (p (i "Hello, world!"))) @end example If the document has no doctype declaration, the @var{doctype-handler} is invoked with @code{#f} for the three arguments. In the future, the continuation may accept other keyword arguments, for example to validate the parsed SXML against the doctype. @deffn {Scheme Procedure} 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 deffn @deffn {Scheme Procedure} sxml->string sxml Detag an sxml tree @var{sxml} into a string. Does not perform any formatting. @end deffn @node SSAX @subsection SSAX: A Functional XML Parsing Toolkit Guile's XML parser is based on Oleg Kiselyov's powerful XML parsing toolkit, SSAX. @subsubsection History Back in the 1990s, when the world was young again and XML was the solution to all of its problems, there were basically two kinds of XML parsers out there: DOM parsers and SAX parsers. A DOM parser reads through an entire XML document, building up a tree of ``DOM objects'' representing the document structure. They are very easy to use, but sometimes you don't actually want all of the information in a document; building an object tree is not necessary if all you want to do is to count word frequencies in a document, for example. SAX parsers were created to give the programmer more control on the parsing process. A programmer gives the SAX parser a number of ``callbacks'': functions that will be called on various features of the XML stream as they are encountered. SAX parsers are more efficient, but much harder to user, as users typically have to manually maintain a stack of open elements. Kiselyov realized that the SAX programming model could be made much simpler if the callbacks were formulated not as a linear fold across the features of the XML stream, but as a @emph{tree fold} over the structure implicit in the XML. In this way, the user has a very convenient, functional-style interface that can still generate optimal parsers. The @code{xml->sxml} interface from the @code{(sxml simple)} module is a DOM-style parser built using SSAX, though it returns SXML instead of DOM objects. @subsubsection Implementation @code{(sxml ssax)} 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. SSAX 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 " content " seed = fup "" seed (parser fdown fup fchar "content" (fdown "" 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 @deffn {Scheme Procedure} current-ssax-error-port @end deffn @deffn {Scheme Procedure} with-ssax-error-to-port port thunk @end deffn @deffn {Scheme Procedure} xml-token? _ @verbatim -- Scheme Procedure: pair? x Return `#t' if X is a pair; otherwise return `#f'. @end verbatim @end deffn @deffn {Scheme Syntax} xml-token-kind token @end deffn @deffn {Scheme Syntax} xml-token-head token @end deffn @deffn {Scheme Procedure} make-empty-attlist @end deffn @deffn {Scheme Procedure} attlist-add attlist name-value @end deffn @deffn {Scheme Procedure} attlist-null? x Return @code{#t} if @var{x} is the empty list, else @code{#f}. @end deffn @deffn {Scheme Procedure} attlist-remove-top attlist @end deffn @deffn {Scheme Procedure} attlist->alist attlist @end deffn @deffn {Scheme Procedure} attlist-fold kons knil lis1 @end deffn @deffn {Scheme Procedure} 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 deffn @deffn {Scheme Procedure} reset-parsed-entity-definitions! Restore the set of parsed entity definitions to its initial state. @end deffn @deffn {Scheme Procedure} ssax:uri-string->symbol uri-str @end deffn @deffn {Scheme Procedure} ssax:skip-internal-dtd port @end deffn @deffn {Scheme Procedure} ssax:read-pi-body-as-string port @end deffn @deffn {Scheme Procedure} ssax:reverse-collect-str-drop-ws fragments @end deffn @deffn {Scheme Procedure} ssax:read-markup-token port @end deffn @deffn {Scheme Procedure} ssax:read-cdata-body port str-handler seed @end deffn @deffn {Scheme Procedure} ssax:read-char-ref port @end deffn @deffn {Scheme Procedure} ssax:read-attributes port entities @end deffn @deffn {Scheme Procedure} ssax:complete-start-tag tag-head port elems entities namespaces @end deffn @deffn {Scheme Procedure} ssax:read-external-id port @end deffn @deffn {Scheme Procedure} ssax:read-char-data port expect-eof? str-handler seed @end deffn @deffn {Scheme Procedure} ssax:xml->sxml port namespace-prefix-assig @end deffn @deffn {Scheme Syntax} ssax:make-parser . kw-val-pairs @end deffn @deffn {Scheme Syntax} ssax:make-pi-parser orig-handlers @end deffn @deffn {Scheme Syntax} ssax:make-elem-parser my-new-level-seed my-finish-element my-char-data-handler my-pi-handlers @end deffn @node Transforming SXML @subsection Transforming SXML @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:: x -> @end smallexample where @smallexample ::= ( ...) ::= ( *preorder* . ) | ( *macro* . ) | ( . ) | ( . ) ::= XMLname | *text* | *default* :: x [] -> @end smallexample The pre-post-order function visits the nodes and nodelists pre-post-order (depth-first). For each @code{} of the form @code{(@var{name} ...)}, it looks up an association with the given @var{name} among its @var{}. 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 ( *preorder* . ) @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{} prepended to the @var{} in effect. The result of these calls is passed to the @var{} (along with the head of the current @var{}). 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{}, replaces the current @var{}. If the current @var{} 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 ( *macro* . ) @end smallexample This is equivalent to @code{*preorder*} described above. However, the result is re-processed again, with the current stylesheet. @subsubsection Usage @deffn {Scheme Procedure} 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 deffn @deffn {Scheme Procedure} foldts fdown fup fhere seed tree @end deffn @deffn {Scheme Procedure} post-order tree bindings @end deffn @deffn {Scheme Procedure} pre-post-order tree bindings @end deffn @deffn {Scheme Procedure} replace-range beg-pred end-pred forest @end deffn @node SXML Tree Fold @subsection SXML Tree 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 @deffn {Scheme Procedure} foldt fup fhere tree The standard multithreaded tree fold. @var{fup} is of type [a] -> a. @var{fhere} is of type object -> a. @end deffn @deffn {Scheme Procedure} foldts fdown fup fhere seed tree The single-threaded tree fold originally defined in SSAX. @xref{SSAX}, for more information. @end deffn @deffn {Scheme Procedure} foldts* fdown fup fhere seed tree A variant of @code{foldts} that allows pre-order tree rewrites. Originally defined in Andy Wingo's 2007 paper, @emph{Applications of fold to XML transformation}. @end deffn @deffn {Scheme Procedure} fold-values proc list . seeds A variant of @code{fold} that allows multi-valued seeds. Note that the order of the arguments differs from that of @code{fold}. @xref{SRFI-1 Fold and Map}. @end deffn @deffn {Scheme Procedure} foldts*-values fdown fup fhere tree . seeds A variant of @code{foldts*} that allows multi-valued seeds. Originally defined in Andy Wingo's 2007 paper, @emph{Applications of fold to XML transformation}. @end deffn @deffn {Scheme Procedure} fold-layout tree bindings params layout stylesheet A traversal combinator in the spirit of @code{pre-post-order}. @xref{Transforming SXML}. @code{fold-layout} was originally presented in Andy Wingo's 2007 paper, @emph{Applications of fold to XML transformation}. @example bindings := (...) binding := ( ...) | (*default* . ) | (*text* . ) tag := handler-pair := (pre-layout . ) | (post . ) | (bindings . ) | (pre . ) | (macro . ) @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 deffn @node SXPath @subsection SXPath @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 ::= | | | "text string" | @end example This production can also be described as @example ::= (name . ) | "text string" @end example An (ordered) set of nodes is just a list of the constituent nodes: @example ::= ( ...) @end example Nodesets, and Nodes other than text strings are both lists. A 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 and 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 @deffn {Scheme Procedure} nodeset? x @end deffn @deffn {Scheme Procedure} node-typeof? crit @end deffn @deffn {Scheme Procedure} node-eq? other @end deffn @deffn {Scheme Procedure} node-equal? other @end deffn @deffn {Scheme Procedure} node-pos n @end deffn @deffn {Scheme Procedure} 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 deffn @deffn {Scheme Procedure} take-until pred? @end deffn @deffn {Scheme Procedure} take-after pred? @end deffn @deffn {Scheme Procedure} map-union proc lst @end deffn @deffn {Scheme Procedure} node-reverse node-or-nodeset @end deffn @deffn {Scheme Procedure} node-trace title @end deffn @deffn {Scheme Procedure} select-kids test-pred? @end deffn @deffn {Scheme Procedure} 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 deffn @deffn {Scheme Procedure} node-join . selectors @end deffn @deffn {Scheme Procedure} node-reduce . converters @end deffn @deffn {Scheme Procedure} node-or . converters @end deffn @deffn {Scheme Procedure} node-closure test-pred? @end deffn @deffn {Scheme Procedure} node-parent rootnode @end deffn @deffn {Scheme Procedure} sxpath path @end deffn @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 @deffn {Scheme Procedure} peek-next-char [port] @end deffn @deffn {Scheme Procedure} assert-curr-char expected-chars comment [port] @end deffn @deffn {Scheme Procedure} skip-until arg [port] @end deffn @deffn {Scheme Procedure} skip-while skip-chars [port] @end deffn @deffn {Scheme Procedure} next-token prefix-skipped-chars break-chars [comment] [port] @end deffn @deffn {Scheme Procedure} next-token-of incl-list/pred [port] @end deffn @deffn {Scheme Procedure} read-text-line [port] @end deffn @deffn {Scheme Procedure} read-string n [port] @end deffn @deffn {Scheme Procedure} find-string-from-port? _ _ . _ Looks for @var{str} in @var{}, optionally within the first @var{max-no-char} characters. @end deffn @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 -> @end smallexample where @smallexample ::= (