<div dir="ltr"><div>You're proving that (let () ...) is necessary, which I have explicitly agreed with since the original email, but you have not yet demonstrated that split-for-body is necessary. Here is the fix I have described twice already, now explicitly put into the define-syntax-rule solution:<br>
</div><div><br>(define-syntax-rule (for/print/fixed clauses pre .. result)<br></div><div> (for clauses<br></div><div> pre ...<br></div><div> (printf "~v\n" (let () result))))<br></div></div><div class="gmail_extra">
<br clear="all"><div>Carl Eastlund</div>
<br><br><div class="gmail_quote">On Fri, Sep 6, 2013 at 12:25 PM, Matthew Flatt <span dir="ltr"><<a href="mailto:mflatt@cs.utah.edu" target="_blank">mflatt@cs.utah.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
#lang racket/base<br>
(require (for-syntax racket/base<br>
syntax/parse<br>
syntax/for-body))<br>
<br>
(define-syntax (for/print/good stx)<br>
<div class="im"> (syntax-parse stx<br>
[(_ clauses . body)<br>
</div> (with-syntax ([([pre ...] [post ...]) (split-for-body stx #'body)])<br>
<div class="im"> (syntax<br>
(for clauses<br>
pre ...<br>
</div> (printf "~v\n" (let () post ...)))))]))<br>
<br>
(define-syntax-rule (for/print/bad clauses pre ... result)<br>
<div class="im"> (for clauses<br>
pre ...<br>
(printf "~v\n" result)))<br>
<br>
</div>;; Try changing to for/print/bad:<br>
(for/print/good ([i 1])<br>
<div class="im"> (define (fish? v) (or (red? v) (blue? v)))<br>
(begin<br>
</div> (define (red? v) (eq? v 'red))<br>
(define (blue? v) (eq? v 'blue))<br>
(fish? i)))<br>
<div class="HOEnZb"><div class="h5"><br>
<br>
At Fri, 6 Sep 2013 12:17:56 -0400, Carl Eastlund wrote:<br>
> Right, that's the issue with needing the (let () result) in my<br>
> define-syntax-rule version. I still didn't need split-for-body, which<br>
> doesn't guarantee there are no definitions in the post ... part. All it<br>
> guarantees to eliminate are #:final and #:break.<br>
><br>
> Carl Eastlund<br>
><br>
><br>
> On Fri, Sep 6, 2013 at 12:09 PM, Matthew Flatt <<a href="mailto:mflatt@cs.utah.edu">mflatt@cs.utah.edu</a>> wrote:<br>
><br>
> > The issue is `begin` splicing. The `result` form could be a `begin`<br>
> > form that contains definitions that are referenced by a preceding<br>
> > forms.<br>
> ><br>
> > For example, given<br>
> ><br>
> > (define (fish? v) (or (red? v) (blue? v)))<br>
> > (begin<br>
> > (define (red? v) ....)<br>
> > (define (blue? v) ....)<br>
> > 5)<br>
> ><br>
> > With `begin` splicing, that turns into<br>
> ><br>
> > (define (fish? v) (or (red? v) (blue? v)))<br>
> > (define (red? v) ....)<br>
> > (define (blue? v) ....)<br>
> > 5<br>
> ><br>
> > which is different than<br>
> ><br>
> > (define (fish? v) (or (red? v) (blue? v)))<br>
> > (let ()<br>
> > (define (red? v) ....)<br>
> > (define (blue? v) ....)<br>
> > 5)<br>
> ><br>
> > At Fri, 6 Sep 2013 11:15:50 -0400, Carl Eastlund wrote:<br>
> > > Is this function ever particularly necessary? Its intended use seems to<br>
> > be<br>
> > > like so:<br>
> > ><br>
> > > (define-syntax (for/print stx)<br>
> > > (syntax-parse stx<br>
> > > [(_ clauses . body)<br>
> > > (with-syntax ([([pre ...] [post ...]) (split-for-body #'body)])<br>
> > > (syntax<br>
> > > (for clauses<br>
> > > pre ...<br>
> > > (printf "~v/n" (let () post ...)))))]))<br>
> > ><br>
> > > That way any #:break or #:final from the body ends up in pre ..., where<br>
> > the<br>
> > > enclosing for loop will interpret them, and post ... will only include<br>
> > > normal definitions and expressions.<br>
> > ><br>
> > > But it seems to me there's a much easier way that should always work:<br>
> > ><br>
> > > (define-syntax-rule (for/print clauses pre ... result)<br>
> > > (for clauses<br>
> > > pre ...<br>
> > > (printf "~v\n" result)))<br>
> > ><br>
> > > This not only puts all #:break and #:final clauses in pre ..., it should<br>
> > > guarantee result is an expression. Perhaps one should still write (let<br>
> > ()<br>
> > > result) in case result is (begin defn expr), but that's still simpler<br>
> > than<br>
> > > using split-for-body.<br>
> > ><br>
> > > My question is -- have I overlooked some clever subtlety here that makes<br>
> > > split-for-body necessary, or is it usually easier to just decompose pre<br>
> > ...<br>
> > > result rather than bothering with split-for-body?<br>
> > ><br>
> > > Carl Eastlund<br>
> > > _________________________<br>
> > > Racket Developers list:<br>
> > > <a href="http://lists.racket-lang.org/dev" target="_blank">http://lists.racket-lang.org/dev</a><br>
> ><br>
> ><br>
<br>
</div></div></blockquote></div><br></div>