10 KiB
Syntax-rules extensions
SKINT implements all standard features of R7RS syntax-rules, including custom ellipsis, non-final ellipsis patterns, non-binding underscore pattern, and (... tpl) template escapes. It also supports the following extensions:
Support for boxes
Boxes, as defined by SRFI-111 and the future (scheme box) library, are supported natively, and can be parts of both patterns and templates. See examples of their use below.
Simple pattern escape
A pattern of the form (<ellipsis> <pattern>) where <ellipsis> is the current ellipsis is interpreted as if it were <pattern>, but ellipses and underscores in <pattern> lose their special meaning; e.g.:
(define-syntax underscored
(syntax-rules ()
[(_ (... _) (... ...)) (list (... ...) (... _))]))
(underscored 1 2)
; => (2 1)
Note that R7RS prescribes special treatment of the keyword identifier at the beginning of the pattern in a <syntax rule>: it is matched automatically with the head of the use form, but is not considered a pattern variable, so can't be substituted. SKINT's pattern escape extension drops this positional restriction and matches its sub-pattern in a normal way; e.g.:
; in R7RS, x is not a pattern variable here due to its head position:
(let-syntax ([ttt (syntax-rules () [(x y) '(x y)])]) (ttt 123))
; => (x 123)
; x is a pattern variable here, even though it is in the head position:
(let-syntax ([ttt (syntax-rules () [((... x) y) '(x y)])]) (ttt 123))
; => (ttt 123)
; same thing, but with pattern template escaped via template escape to work properly:
((syntax-rules () ; NB: anonymous transformer positioned at the head of the use form
[(_) (let-syntax ([ttt (syntax-rules () [(((... ...) x) y) '(x y)])]) (ttt 123))]))
; => (ttt 123)
The importance of this feature will be clear when we get to circumventing hygiene below.
Named pattern escapes
A pattern of the form (<ellipsis> <predicate name> <pattern>), where <ellipsis> is the current ellipsis, is interpreted as if it were <pattern> as long as the matching S-expression satisfies the constraint specified by <predicate name>. The matching fails if the predicate returns #f. Predicate names are compared to predefined symbols according to free-identifier=? rules. The following named pattern escapes are supported:
(... number? <pattern>)(... exact-integer? <pattern>)(... boolean? <pattern>)(... char? <pattern>)(... string? <pattern>)(... bytevector? <pattern>)(... id? <pattern>)
All but the last predicate have the same meaning as the corresponding Scheme procedures. The id? predicate checks if the corresponding S-expression is either a symbol or a syntax object representing an identifier.
The rationale for adding these escapes is obvious: while syntax-rules-based macros can perform very complex calculations with structured S-expressions, they lack an ability to deal with atomic S-expressions (with the exception of identifiers – they can be recognized, but the technique for that is quite complicated).
Example (also uses box templates):
(define-syntax wrap-by-type
(syntax-rules ()
[(_ (... string? x)) '#&x]
[(_ (... number? x)) '#(x)]
[(_ x) 'x]))
(list (wrap-by-type 42) (wrap-by-type "yes") (wrap-by-type #\c))
; => (#(42) #&"yes" #\c)
Named template escapes
A template of the form (<ellipsis> <converter name> <template+>) where <ellipsis> is the current ellipsis is interpreted as follows: First, <template+> (which can be any nonempty sequence of <template>s), is instantiated recursively, resulting in a list of S-expressions. These S-expressions become arguments to a converter specified by <converter name>. It is a syntax error to apply converters to a wrong type or number of arguments. Converter names are compared to predefined symbols according to free-identifier=? rules. The following named template escapes are supported:
(... number->string <template>)(... string->number <template>)(... list->string <template>)(... string->list <template>)(... list->bytevector <template>)(... bytevector->list <template>)(... length <template>)(... make-list <template> <template>)(... char<=? <template+>)(... <= <template+>)(... + <template+>)(... - <template+>)(... id->string <template>)(... string->id <template> <id template>)
All but the last two converters have the same meaning as the corresponding Scheme procedures. The id->string converter expects either a symbol or a syntax object representing an identifier and produces a string containing a “quote name” of the identifier (the result of applying symbol->string to the original name supplied by the user after all substitutions).
The string->id converter allows one to produce identifiers having the same syntax properties as identifiers explicitly introduced as part of macro definitions or macro uses. The properies are copied from <id template>, which, after all substitutions are performed, should instantiate to an identifier serving as a prototype. The <template> argument should instantiate to a string, which is then converted to a symbol via string->symbol, and then turned into an identifier syntax object as if it was introduced alongside the prototype identifier (same expression, same expansion phase).
Please note that identifiers generated with string->id are not autorenamed with other “free” template
identifiers; their syntactic identity is defined entirely by that of <id template> id, which might have already being renamed by the time string->id converter is applied.
Examples:
; generated and plain versions of pi and e are syntactically the same
(define-syntax pi-e-example
(syntax-rules ()
[(_)
(let ([(... string->id "pi" e) 3.14] [e 2.72])
(+ pi (... string->id "e" pi)))]))
(pi-e-example)
; => 5.86
(let-syntax
([define-math-constants
(syntax-rules ()
[((... ref-id))
(begin (define (... string->id "pi" ref-id) 3.14)
(define (... string->id "e" ref-id) 2.72))])])
(define-math-constants)
(+ pi e))
; => 5.86
Note that in the last example the escaped keyword ref-id at the beginning of the pattern was used to bring in the define-math-constants from the macro use, and then used as a prototype id for the introduced pi and e identifiers. This allows them to capture the corresponding identifier names typed in by the user in (+ pi e). Without a simple pattern escape, this keyword would not be treated as a pattern variable.
Here are some more examples:
(define-syntax loop-until-break
(syntax-rules ()
[((... ref-id) e ...)
(call/cc
(lambda ((... string->id "break" ref-id))
(let loop () e ... (loop))))]))
(let ([n 10] [steps 0] [break write])
(loop-until-break
; break here refers to the escape continuation
(when (= n 4) (break steps))
(set! n (- n 1))
(set! steps (+ steps 1))))
; => 6
To demonstrate combined use of different converters, here is a thin macro layer over tagged vectors:
(define-syntax define-variant
(syntax-rules ()
[((... ref-id) name () ([field0 index0] ...))
(begin
(define-syntax name
(lambda (field0 ...)
(vector 'name field0 ...)))
(define-syntax
(... string->id
(... string-append (... id->string name) "?") ref-id)
(lambda (object)
(and (vector? object)
(= (vector-length object) (... length (name field0 ...)))
(eq? (vector-ref object 0) 'name))))
(define-syntax
(... string->id
(... string-append (... id->string name) "->"
(... id->string field0)) ref-id)
(lambda (object)
(vector-ref object index0)))
...)]
[((... ref-id) name (field0 field ...) (pair ...))
(ref-id name (field ...)
(pair ... [field0 (... length (name pair ...))]))]
[((... ref-id) name (field0 ...))
(ref-id name (field0 ...) ())]))
(define-syntax variant-case
(syntax-rules (else)
[((... ref-id) (a . d) clause ...)
(let ([var (a . d)]) (ref-id var clause ...))]
[((... ref-id) var) (error "no variant-case clause matches" var)]
[((... ref-id) var (else exp1 exp2 ...)) (body exp1 exp2 ...)]
[((... ref-id) var [name (field ...) exp1 exp2 ...] clause ...)
(if ((... string->id (... string-append (... id->string name) "?") ref-id) var)
(let ([field ((... string->id (... string-append (... id->string name) "->"
(... id->string field)) ref-id)
var)] ...)
exp1 exp2 ...)
(ref-id var clause ...))]))
(let ()
(define-variant point (x y))
(define-variant kons (a d))
(define-syntax pair->a car)
(define-syntax pair->d cdr)
(define (println v)
(variant-case v ; nb: pair? is already defined
[point (x y) (format #t "[point x=~s y=~s]~%" x y)]
[kons (a d) (format #t "[kons a=~s d=~s]~%" a d)]
[pair (a d) (format #t "[pair a=~s d=~s]~%" a d)]
[else (format #t "[unknown ~s]~%" v)]))
(define p (point 1 4))
(define k (kons 'a 'd))
(define c (cons 'a 'd))
(format #t "p = ~s k = ~s c = ~s~%" p k c)
(println p)
(println k)
(println c))
; prints:
; p = #(point 1 4) k = #(kons a d) c = (a . d)
; [point x=1 y=4]
; [kons a=a d=d]
; [pair a=a d=d]
Why stop here?
The above collection of named escapes is selected as an almost minimal one. Its purpose is not to make syntax-rules-based macro programming more convenient, but to extend its core abilities in dealing with non-structural S-expressions. It is possible to recognize them and work with them via convertion to/from structural form if a need arises. Arithmetics is limited to what one can do using lists as Peano numbers; also, for numbers and chars, access to ordering is provided to support simple ranges. One can imitate string-append without a dedicated converter, but this unnecessarily complicates generation of identifiers, which is a major use case.