It will evaluate (state 'init) and so on in runtime a lot of times. Of course, I can write a case-like syntax which will process all case-values with given function.
Something like (case+ x #:selector state ((init stage) (initialize)) ((process) (function1)) ((fail) (function2))) #. if once written, allows avoid writing a lot of such ad-hoc syntaxes. And it can be used to make version- and platform-independent code like #ifdef in C Понедельник, 29 июля 2013, 21:03 -04:00 от Matthias Felleisen <[email protected]>: > >Try evcase >On Jul 29, 2013, at 8:48 PM, Roman Klochkov wrote: >> >>#. calcuclates its argument and place it in the program code. >>Now I have to write something like >> >>(case state-number >> ((0 1) ; state in init, stage1 >> (initialize)) >> ((2) ; state = process >> (function1) >> ((3) ; state = abort >> (function2))) >> >>I want to have >> >>(define (state x) >> (let rec ([states '(init stage1 process fail)] >> [pos 0]) >> (cond >> [(null? states) #f] >> [(eq? (car states) x) pos] >> [else (rec (cdr states) (add1 pos))]))) >> >>(case state-number >> ((#.(state 'init) #.(state 'stage1)) >> (initialize)) >> ((#.(state 'process)) >> (function1) >> ((#.(state 'fail)) >> (function2))) >> >>So the code becomes self-documenting. Like using enum values in C or Pascal. >> >>Понедельник, 29 июля 2013, 13:32 -04:00 от Asumu Takikawa < [email protected] >>>: >>>On 2013-07-29 06:25:21 +0400, Roman Klochkov wrote: >>>> Is there simple way to calculate during compilation. Something like #. >>>> in Common Lisp. >>> >>>Can you tell us what #. means in Common Lisp? >>> >>>I am guessing that you want to do something like the following: >>> >>> #lang racket >>> (require (for-syntax syntax/parse)) >>> >>> ;; functions to compute costly constants >>> ;; at compile-time >>> (begin-for-syntax >>> (define (compute-x) ...) >>> (define (compute-y) ...) >>> (define (compute-z) ...)) >>> >>> ;; macro that defines x, y, z assuming >>> ;; suitable definitions of compute-x, etc. >>> (define-syntax (define-constants stx) >>> (syntax-parse stx >>> [(_ x:id y:id z:id) >>> #`(begin (define x #,(compute-x)) >>> (define y #,(compute-y)) >>> (define z #,(compute-z)))])) >>> >>> (define-constants my-x my-y my-z) >>> >>>This will work if the `compute-` functions produce flat data that can be >>>embedded in syntax, like numbers. >>> >>>Cheers, >>>Asumu >> >> >>-- >>Roman Klochkov >> >>---------------------------------------------------------------------- >> >>-- >>Roman Klochkov >>____________________ >> Racket Users list: >> http://lists.racket-lang.org/users > -- Roman Klochkov
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