<div dir="ltr">Right, that's the issue with needing the (let () result) in my define-syntax-rule version. I still didn't need split-for-body, which doesn't guarantee there are no definitions in the post ... part. All it guarantees to eliminate are #:final and #:break. </div><div class="gmail_extra"> <div>Carl Eastlund</div> <div class="gmail_quote">On Fri, Sep 6, 2013 at 12:09 PM, Matthew Flatt <<a href="mailto:mflatt@cs.utah.edu" target="_blank">mflatt@cs.utah.edu</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> The issue is `begin` splicing. The `result` form could be a `begin` form that contains definitions that are referenced by a preceding forms. For example, given (define (fish? v) (or (red? v) (blue? v))) (begin (define (red? v) ....) (define (blue? v) ....) 5) With `begin` splicing, that turns into (define (fish? v) (or (red? v) (blue? v))) (define (red? v) ....) (define (blue? v) ....) 5 which is different than (define (fish? v) (or (red? v) (blue? v))) (let () (define (red? v) ....) (define (blue? v) ....) 5) <div><div class="h5"> At Fri, 6 Sep 2013 11:15:50 -0400, Carl Eastlund wrote: > Is this function ever particularly necessary? Its intended use seems to be > like so: > > (define-syntax (for/print stx) > (syntax-parse stx > [(_ clauses . body) > (with-syntax ([([pre ...] [post ...]) (split-for-body #'body)]) > (syntax > (for clauses > pre ... > (printf "~v/n" (let () post ...)))))])) > > That way any #:break or #:final from the body ends up in pre ..., where the > enclosing for loop will interpret them, and post ... will only include > normal definitions and expressions. > > But it seems to me there's a much easier way that should always work: > > (define-syntax-rule (for/print clauses pre ... result) > (for clauses > pre ... > (printf "~v\n" result))) > > This not only puts all #:break and #:final clauses in pre ..., it should > guarantee result is an expression. Perhaps one should still write (let () > result) in case result is (begin defn expr), but that's still simpler than > using split-for-body. > > My question is -- have I overlooked some clever subtlety here that makes > split-for-body necessary, or is it usually easier to just decompose pre ... > result rather than bothering with split-for-body? > > Carl Eastlund </div></div>> _________________________ > Racket Developers list: > <a href="http://lists.racket-lang.org/dev" target="_blank">http://lists.racket-lang.org/dev</a> </blockquote></div> </div>