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 _________________________ Racket Developers list: http://lists.racket-lang.org/dev
