(I was 2/3 there when I noticed Matthew's response.) Klaus, fwiw this is NOT cbn. A cbn let will run the thunks only when they show up in strict positions. Not every identifier shows up in strict positions.
On Jul 29, 2015, at 7:37 AM, Matthew Flatt wrote: > At Wed, 29 Jul 2015 06:28:48 -0700 (PDT), Klaus Ostermann wrote: >> I'd like to have a macro "let-cbn" which does this: >> >> (let-cbn ((x1 e1) ...) body) >> >> is transformed to >> >> (let ((x1 (thunk e1)) ...) newbody) >> >> where newbody is the result of replacing every occurence of x1... by (x1)... >> . >> >> What is the best way to do that in Racket? > > I'd expand > > (let-cbn ((x1 e1) ...) body) > > to > > (let ((x1 (thunk e1)) ...) > (let-syntax ([x1 (make-thunk-call #'x1)] ...) > body)) > > where `make-thunk-call` generates a macro that expands to a call of the > given argument: > > (define-for-syntax (make-thunk-call id-stx) > (lambda (stx) > (with-syntax ([id id-stx]) > (syntax-case stx () > [(_ arg ...) (syntax/loc stx (id arg ...))] > [_ #'(id)])))) > > A complete example is below. > > ---------------------------------------- > > #lang racket/base > (require (for-syntax racket/base)) > > (define-for-syntax (make-thunk-call id-stx) > (lambda (stx) > (with-syntax ([id id-stx]) > (syntax-case stx () > [(_ arg ...) (syntax/loc stx (id arg ...))] > [_ #'(id)])))) > > (define-syntax-rule (thunk e0 e ...) > (lambda () e0 e ...)) > > (define-syntax-rule (let-cbn ((x1 e1) ...) body0 body ...) > (let ((x1 (thunk e1)) ...) > (let-syntax ([x1 (make-thunk-call #'x1)] ...) > body0 body ...))) > > (let-cbn ([x (begin (displayln "here") 1)]) > (list x x)) > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to racket-users+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
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