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
