You will need to combine EVCASE with Vincent's hash-dot. I can't see why this should evaluate (init state) more than once.
On Jul 29, 2013, at 9:16 PM, Roman Klochkov wrote: > 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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