Thanks for all the good answers as usual. BTW, rkt is very "practical" in the sense it makes me think about programming in a way that I normally do not in my more mundane chores.
On Tue, Dec 13, 2011 at 9:29 AM, Richard Cleis <rcl...@mac.com> wrote: > I was thinking that a person mailing from 'fundingmatters' is asking a more > practical question, for which 'compose' is the answer :O) > > #lang racket/base > > (define some-list '(1 2 3)) > (define proc-1 +) > (define proc-2 sqrt) > (define (proc-n v) (* v 10)) > > (define f (compose proc-n proc-2 proc-1)) > > (apply f some-list) > > rac > > > > On Dec 12, 2011, at 5:08 PM, Danny Yoo wrote: > >>> Just for the readability, is it possible to write >>> >>> some-list >>> (proc-1 some-list) >>> (proc-2 (val-of-last-expression)) >>> ... >>> (proc-n (val-of-last-expression)) ;where n is 'largeish' >>> >>> instead of >>> >>> (proc-n ... (proc-2 (proc-1 some-list)))))))))))) >> >> >> Possible. The syntax in Racket is programmable enough that adding it >> to the language isn't so bad. >> >> >> Let's say that we'd like to extend Racket's syntax so that it supports >> a (begin/it ...) form that acts like begin, but it also binds the >> value of the last expression. How do we do this? >> >> >> First, let's reserve the name "it" the value of the very last >> expression. We can create a context-sensitive keyword by using >> define-syntax-parameter: >> >> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; >> (define-syntax-parameter it >> (lambda (stx) >> (raise-syntax-error #f "it should be used only in the context of a >> begin/it" stx))) >> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; >> >> We'll have it so if we use 'it' by accident, then it'll raise a syntax >> error with the right source location error. >> >> Next, we can introduce a begin/it form that works like begin, except >> that it binds intermediate results: >> >> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; >> (define-syntax (begin/it stx) >> (syntax-case stx () >> [(_ b) >> #'b] >> >> [(_ b b-rest ...) >> (syntax/loc stx >> (let ([b-val b]) >> (syntax-parameterize ([it >> (make-rename-transformer #'b-val)]) >> (begin/it b-rest ...))))])) >> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; >> >> >> >> Here's the complete source, with an example of its use: >> >> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; >> #lang racket/base >> >> (require (for-syntax racket/base) >> racket/stxparam) >> >> (define-syntax-parameter it >> (lambda (stx) >> (raise-syntax-error #f "it should be used only in the context of a >> begin/it" stx))) >> >> (define-syntax (begin/it stx) >> (syntax-case stx () >> [(_ b) >> #'b] >> [(_ b b-rest ...) >> (syntax/loc stx >> (let ([b-val b]) >> (syntax-parameterize ([it >> (make-rename-transformer #'b-val)]) >> (begin/it b-rest ...))))])) >> >> >> (define (f x) >> (* x x)) >> (define (g x) (* x x x)) >> >> >> ;; Exercise: >> (begin/it >> (+ 1 2) >> (f it) ;; ... Um, didn't mean to swear there. >> (g it)) >> >> (g (f (+ 1 2))) >> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; >> _________________________________________________ >> For list-related administrative tasks: >> http://lists.racket-lang.org/listinfo/users > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
