<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>I'm not really sure why this works either, but try this:</div><div> </div><div><div>(define-syntax (:match stx)</div><div> (syntax-case stx ()</div><div> [(:match val-expr [pat . more] ...)</div><div> (with-syntax ([(new-pat ...) (for/list ([pat-stx (in-list (syntax->list #'(pat ...)))])</div><div> (datum->syntax pat-stx `(:pat ,(syntax->datum pat-stx))))])</div><div> #'(match val-expr [new-pat . more] ...))]))</div></div><div> </div><div><div>(check-equal? (:match '(42 x) [(list n:num s:sym) (list n s)])</div><div> (match '(42 x) [(:pat (list n:num s:sym)) (list n s)]))</div></div> <div><div>On Dec 29, 2013, at 8:54 AM, Jens Axel Søgaard wrote:</div> <blockquote type="cite"><div>Hi Alexander, I extended your example to allow other patterns than symbols inside :pat. <blockquote type="cite"> (match '(42 x) [(:pat (list n:num s:sym)) (list n s)]) </blockquote> (list 42 x) This works fine. I now want to "hide" the :pat, that is I want to write: (:match '(42 x) [(list n:num s:sym) (list n s)]) Since the syntax of match is: (match val-expr clause ...) and each clause has the form [pat . more] we can rewrite pat to [(:pat pat) . more]. So I tried this: (define-syntax (:match stx) (syntax-case stx () [(_ val-expr [pat . more] ...) #'(match val-expr [(:pat pat) . more] ...)])) This doesn't work however. I am tempted to consider this a bug in match, but I am not sure. #lang racket (require (for-syntax (only-in lang/htdp-intermediate-lambda string-contains?) racket/string racket/match) rackunit) (begin-for-syntax (define (type-str->stx-type-pred type-str) (match type-str ["num" #'number?] ["str" #'string?] ["sym" #'symbol?] ["lst" #'list?] [_ #f])) (define (split str) (string-split str ":")) (define (parse-pat-str pat-str stx) (match (split pat-str) [(list pat-name-str type-str) (with-syntax ([type-pred (type-str->stx-type-pred type-str)] [pat-name (datum->syntax stx (string->symbol pat-name-str))]) #'(? type-pred pat-name))])) (define (id:type? str) (and (string-contains? ":" str) (type-str->stx-type-pred (cadr (split str)))))) (define-match-expander :pat (lambda (stx) (define (rewrite pat) (let* ([pat-sym (syntax->datum pat)] [pat-str (symbol->string pat-sym)]) (if (id:type? pat-str) (parse-pat-str pat-str stx) pat))) (syntax-case stx () [(_ pat) (identifier? #'pat) (rewrite #'pat)] [(_ (pat ...)) (with-syntax ([(p ...) (map rewrite (syntax->list #'(pat ...)))]) (syntax/loc stx (p ...)))] [(_ pat) #'pat]))) (define-syntax (:match stx) (syntax-case stx () [(_ val-expr [pat . more] ...) #'(match val-expr [(:pat pat) . more] ...)])) (check-equal? (match 1 [(:pat n:num) n]) 1) (check-equal? (match 'x [(:pat n:num) n] [_ 2]) 2) (check-equal? (match "string" [(:pat s:str) s]) "string") (check-equal? (match 'x [(:pat s:str) s] [_ 2]) 2) (check-equal? (match (list 1 2 3) [(:pat l:lst) l]) (list 1 2 3)) (check-equal? (match 'x [(:pat l:lst) l] [_ 2]) 2) (check-equal? (match 'x [(:pat l) l]) 'x) (check-equal? (match '(2 x "foo" (3 4)) [(:pat (list n s f l)) (list n s f l)]) '(2 x "foo" (3 4))) (check-equal? (match '(42 x) [(:pat (list n:num s:sym)) (list n s)]) '(42 x)) (match '(42 x) [(:pat (list n:num s:sym)) (list n s)]) ; (:match '(42 x) [(list n:num s:sym) (list n s)]) 2013/12/28 Alexander D.Knauth <<a href="mailto:alexander@knauth.org">alexander@knauth.org</a>>: <blockquote type="cite">I just wrote a match-expander that does something like that: </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">(check-equal? (match 1 [(my-pat n:num) n]) 1) </blockquote><blockquote type="cite">(check-equal? (match 'x [(my-pat n:num) n] [_ 2]) 2) </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">like this: </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">#lang racket </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">(require rackunit) </blockquote><blockquote type="cite">(require (for-syntax </blockquote><blockquote type="cite"> (only-in lang/htdp-intermediate-lambda </blockquote><blockquote type="cite"> string-contains?) </blockquote><blockquote type="cite"> racket/string </blockquote><blockquote type="cite"> racket/match)) </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">(define-match-expander my-pat </blockquote><blockquote type="cite"> (lambda (stx) </blockquote><blockquote type="cite"> (syntax-case stx () </blockquote><blockquote type="cite"> [(my-pat pat) </blockquote><blockquote type="cite"> (let* ([pat-sym (syntax->datum #'pat)] </blockquote><blockquote type="cite"> [pat-str (symbol->string pat-sym)]) </blockquote><blockquote type="cite"> (cond [(not (string-contains? ":" pat-str)) </blockquote><blockquote type="cite"> #'pat] </blockquote><blockquote type="cite"> [else </blockquote><blockquote type="cite"> (parse-pat-str pat-str stx)]))]))) </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">(define-for-syntax (parse-pat-str pat-str stx) </blockquote><blockquote type="cite"> (let ([split-pat (string-split pat-str ":")]) </blockquote><blockquote type="cite"> (match split-pat </blockquote><blockquote type="cite"> [(list pat-name-str type-str) </blockquote><blockquote type="cite"> (with-syntax ([type-pred (type-str->stx-type-pred type-str)] </blockquote><blockquote type="cite"> [pat-name (datum->syntax stx (string->symbol </blockquote><blockquote type="cite">pat-name-str))]) </blockquote><blockquote type="cite"> #'(? type-pred pat-name))]))) </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">(define-for-syntax (type-str->stx-type-pred type-str) </blockquote><blockquote type="cite"> (match type-str </blockquote><blockquote type="cite"> ["num" #'number?] </blockquote><blockquote type="cite"> ["str" #'string?] </blockquote><blockquote type="cite"> ["lst" #'list?])) </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">(check-equal? (match 1 [(my-pat n:num) n]) 1) </blockquote><blockquote type="cite">(check-equal? (match 'x [(my-pat n:num) n] [_ 2]) 2) </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">(check-equal? (match "string" [(my-pat s:str) s]) "string") </blockquote><blockquote type="cite">(check-equal? (match 'x [(my-pat s:str) s] [_ 2]) 2) </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">(check-equal? (match (list 1 2 3) [(my-pat l:lst) l]) (list 1 2 3)) </blockquote><blockquote type="cite">(check-equal? (match 'x [(my-pat l:lst) l] [_ 2]) 2) </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">On Dec 26, 2013, at 2:45 PM, Jens Axel Søgaard wrote: </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">The match pattern (? number? n) matches number and </blockquote><blockquote type="cite">binds it to n. </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">(match 1 [(? number? n) n]) </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite"> 1 </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">I'd like to write (match 1 [n:num n]) instead. </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">Since there is no define-identifier-match-expander I have </blockquote><blockquote type="cite">tried to make (match 1 [(n:num) n]) work. I need a hint. </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">Here is a non-working attempt: </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">(define-match-expander n:num </blockquote><blockquote type="cite"> (λ(stx) </blockquote><blockquote type="cite"> (syntax-case stx () </blockquote><blockquote type="cite"> [(id) </blockquote><blockquote type="cite"> (with-syntax ([n (syntax/loc #'id n)]) </blockquote><blockquote type="cite"> #'(? number? n))]))) </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">(check-equal? (match 1 [(n:num) n]) 1) </blockquote><blockquote type="cite">(check-equal? (match 'x [(n:num) n] [_ 2]) 2) </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">/Jens Axel </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">-- </blockquote><blockquote type="cite">Jens Axel Søgaard </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite">____________________ </blockquote><blockquote type="cite"> Racket Users list: </blockquote><blockquote type="cite"> <a href="http://lists.racket-lang.org/users">http://lists.racket-lang.org/users</a> </blockquote><blockquote type="cite"> </blockquote><blockquote type="cite"> </blockquote> -- -- Jens Axel Søgaard </div></blockquote></div> </body></html>