sxml-match: Pattern Matching of SXML
(sxml match) module provides syntactic forms for pattern
matching of SXML trees, in a “by example” style reminiscent of the
pattern matching of the
systems. See SXML, for more information on SXML.
The following example25 provides a brief illustration, transforming a music album catalog language into HTML.
(define (album->html x) (sxml-match x [(album (@ (title ,t)) (catalog (num ,n) (fmt ,f)) ...) `(ul (li ,t) (li (b ,n) (i ,f)) ...)]))
Three macros are provided:
Compared to a standard s-expression pattern matcher (see Pattern Matching),
sxml-match provides the following benefits:
The present module is a descendant of WebIt!, and was inspired by an s-expression pattern matcher developed by Erik Hilsdale, Dan Friedman, and Kent Dybvig at Indiana University.
case-like form for pattern matching of XML
Match input-expression, an SXML tree, according to the given clauses
(one or more), each consisting of a pattern and one or more expressions to be
evaluated if the pattern match succeeds. Optionally, each clause within
sxml-match may include a guard expression.
The pattern notation is based on that of Scheme’s
syntax-case macro systems. The grammar for the
is given below:
match-form ::= (sxml-match input-expression clause+) clause ::= [node-pattern action-expression+] | [node-pattern (guard expression*) action-expression+] node-pattern ::= literal-pattern | pat-var-or-cata | element-pattern | list-pattern literal-pattern ::= string | character | number | #t | #f attr-list-pattern ::= (@ attribute-pattern*) | (@ attribute-pattern* . pat-var-or-cata) attribute-pattern ::= (tag-symbol attr-val-pattern) attr-val-pattern ::= literal-pattern | pat-var-or-cata | (pat-var-or-cata default-value-expr) element-pattern ::= (tag-symbol attr-list-pattern?) | (tag-symbol attr-list-pattern? nodeset-pattern) | (tag-symbol attr-list-pattern? nodeset-pattern? . pat-var-or-cata) list-pattern ::= (list nodeset-pattern) | (list nodeset-pattern? . pat-var-or-cata) | (list) nodeset-pattern ::= node-pattern | node-pattern ... | node-pattern nodeset-pattern | node-pattern ... nodeset-pattern pat-var-or-cata ::= (unquote var-symbol) | (unquote [var-symbol*]) | (unquote [cata-expression -> var-symbol*])
Within a list or element body pattern, ellipses may appear only once, but may be followed by zero or more node patterns.
Guard expressions cannot refer to the return values of catamorphisms.
Ellipses in the output expressions must appear only in an expression context; ellipses are not allowed in a syntactic form.
The sections below illustrate specific aspects of the
The example below illustrates the pattern matching of an XML element:
(sxml-match '(e (@ (i 1)) 3 4 5) [(e (@ (i ,d)) ,a ,b ,c) (list d a b c)] [,otherwise #f])
Each clause in
sxml-match contains two parts: a pattern and one or more
expressions which are evaluated if the pattern is successfully match. The
example above matches an element
e with an attribute
i and three
Pattern variables are must be “unquoted” in the pattern. The above expression
binds d to
1, a to
3, b to
4, and c
syntax-rules, ellipses may be used to specify a repeated pattern.
Note that the pattern
item ... specifies zero-or-more matches of the
The use of ellipses in a pattern is illustrated in the code fragment below,
where nested ellipses are used to match the children of repeated instances of an
a element, within an element
(define x '(d (a 1 2 3) (a 4 5) (a 6 7 8) (a 9 10))) (sxml-match x [(d (a ,b ...) ...) (list (list b ...) ...)])
The above expression returns a value of
((1 2 3) (4 5) (6 7 8) (9 10)).
Within the body of an
sxml-match form, a slightly extended version of
quasiquote is provided, which allows the use of ellipses. This is illustrated
in the example below.
(sxml-match '(e 3 4 5 6 7) [(e ,i ... 6 7) `("start" ,(list 'wrap i) ... "end")] [,otherwise #f])
The general pattern is that
`(something ,i ...) is rewritten as
A nodeset pattern is designated by a list in the pattern, beginning the identifier list. The example below illustrates matching a nodeset.
(sxml-match '("i" "j" "k" "l" "m") [(list ,a ,b ,c ,d ,e) `((p ,a) (p ,b) (p ,c) (p ,d) (p ,e))])
This example wraps each nodeset item in an HTML paragraph element. This example can be rewritten and simplified through using ellipsis:
(sxml-match '("i" "j" "k" "l" "m") [(list ,i ...) `((p ,i) ...)])
This version will match nodesets of any length, and wrap each item in the nodeset in an HTML paragraph element.
Matching the “rest” of a nodeset is achieved by using a
. rest) pattern
at the end of an element or nodeset pattern.
This is illustrated in the example below:
(sxml-match '(e 3 (f 4 5 6) 7) [(e ,a (f . ,y) ,d) (list a y d)])
The above expression returns
(3 (4 5 6) 7).
Sometimes it is useful to bind a list of attributes present in the element being
matched, but which do not appear in the pattern. This is achieved by using a
. rest) pattern at the end of the attribute list pattern. This is
illustrated in the example below:
(sxml-match '(a (@ (z 1) (y 2) (x 3)) 4 5 6) [(a (@ (y ,www) . ,qqq) ,t ,u ,v) (list www qqq t u v)])
The above expression matches the attribute
y and binds a list of the
remaining attributes to the variable qqq. The result of the above
(2 ((z 1) (x 3)) 4 5 6).
This type of pattern also allows the binding of all attributes:
(sxml-match '(a (@ (z 1) (y 2) (x 3))) [(a (@ . ,qqq)) qqq])
It is possible to specify a default value for an attribute which is used if the attribute is not present in the element being matched. This is illustrated in the following example:
(sxml-match '(e 3 4 5) [(e (@ (z (,d 1))) ,a ,b ,c) (list d a b c)])
1 is used when the attribute
z is absent from the
Guards may be added to a pattern clause via the
guard keyword. A guard
expression may include zero or more expressions which are evaluated only if the
pattern is matched. The body of the clause is only evaluated if the guard
expressions evaluate to
The use of guard expressions is illustrated below:
(sxml-match '(a 2 3) ((a ,n) (guard (number? n)) n) ((a ,m ,n) (guard (number? m) (number? n)) (+ m n)))
The example below illustrates the use of explicit recursion within an
sxml-match form. This example implements a simple calculator for the
basic arithmetic operations, which are represented by the XML elements
(define simple-eval (lambda (x) (sxml-match x [,i (guard (integer? i)) i] [(plus ,x ,y) (+ (simple-eval x) (simple-eval y))] [(times ,x ,y) (* (simple-eval x) (simple-eval y))] [(minus ,x ,y) (- (simple-eval x) (simple-eval y))] [(div ,x ,y) (/ (simple-eval x) (simple-eval y))] [,otherwise (error "simple-eval: invalid expression" x)])))
Using the catamorphism feature of
sxml-match, a more concise version of
simple-eval can be written. The pattern
,[x] recursively invokes
the pattern matcher on the value bound in this position.
(define simple-eval (lambda (x) (sxml-match x [,i (guard (integer? i)) i] [(plus ,[x] ,[y]) (+ x y)] [(times ,[x] ,[y]) (* x y)] [(minus ,[x] ,[y]) (- x y)] [(div ,[x] ,[y]) (/ x y)] [,otherwise (error "simple-eval: invalid expression" x)])))
It is also possible to explicitly name the operator in the “cata” position.
,[id*] recurs to the top of the current
,[cata -> id*] recurs to
cata must evaluate to a
procedure which takes one argument, and returns as many values as there are
Named catamorphism patterns allow processing to be split into multiple, mutually recursive procedures. This is illustrated in the example below: a transformation that formats a “TV Guide” into HTML.
(define (tv-guide->html g) (define (cast-list cl) (sxml-match cl [(CastList (CastMember (Character (Name ,ch)) (Actor (Name ,a))) ...) `(div (ul (li ,ch ": " ,a) ...))])) (define (prog p) (sxml-match p [(Program (Start ,start-time) (Duration ,dur) (Series ,series-title) (Description ,desc ...)) `(div (p ,start-time (br) ,series-title (br) ,desc ...))] [(Program (Start ,start-time) (Duration ,dur) (Series ,series-title) (Description ,desc ...) ,[cast-list -> cl]) `(div (p ,start-time (br) ,series-title (br) ,desc ...) ,cl)])) (sxml-match g [(TVGuide (@ (start ,start-date) (end ,end-date)) (Channel (Name ,nm) ,[prog -> p] ...) ...) `(html (head (title "TV Guide")) (body (h1 "TV Guide") (div (h2 ,nm) ,p ...) ...))]))
These forms generalize the
let* forms of Scheme to allow
an XML pattern in the binding position, rather than a simple variable.
For example, the expression below:
(sxml-match-let ([(a ,i ,j) '(a 1 2)]) (+ i j))
binds the variables i and j to
2 in the XML
This example is taken from a paper by
Krishnamurthi et al. Their paper was the first to show the usefulness of the
syntax-rules style of pattern matching for transformation of XML, though
the language described, XT3D, is an XML language.