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.
Carl Eastlund On Fri, Sep 6, 2013 at 12:09 PM, Matthew Flatt <mfl...@cs.utah.edu> wrote: > 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) > > 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 > > _________________________ > > Racket Developers list: > > http://lists.racket-lang.org/dev > >
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