<div dir="ltr"><div><div class="gmail_extra"><div class="gmail_quote">On Fri, Sep 6, 2013 at 2:43 PM, Stephen Chang <<a href="mailto:stchang@ccs.neu.edu" target="_blank">stchang@ccs.neu.edu</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im">On Fri, Sep 6, 2013 at 2:36 PM, Carl Eastlund <<a href="mailto:cce@ccs.neu.edu">cce@ccs.neu.edu</a>> wrote: > They have to be separate internal definition contexts in order for #:break > and #:final to be able to stop execution before the definitions themselves > get run. </div>Hmm, ok. I guess my question was why does one need #:final and #:break in the body like that? </blockquote><div> </div><div>It's useful when the condition for loop termination and the value of the loop body both depend on some derived computation from the sequence elements: </div><div>(for/list ([x (in-stream S)]) </div><div> (define y (f x)) </div><div> #:break (g? y) </div><div> (h y)) </div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> > Carl Eastlund <div class="HOEnZb"><div class="h5"> > > > On Fri, Sep 6, 2013 at 2:31 PM, Stephen Chang <<a href="mailto:stchang@ccs.neu.edu">stchang@ccs.neu.edu</a>> wrote: >> >> > "Among the bodys, besides stopping the iteration and preventing later >> > body evaluations, a #:break guard-expr or #:final guard-expr clause starts a >> > new internal-definition context." >> >> I had the same thought process as Carl. I now understand the behavior >> but I don't understand why it's needed? It seems kind of arbitrary >> since no other form allows multiple internal def contexts in the body >> like this. Is there a practical example? >> >> On Fri, Sep 6, 2013 at 12:58 PM, Carl Eastlund <<a href="mailto:cce@ccs.neu.edu">cce@ccs.neu.edu</a>> wrote: >> > Okay, I see what's going on here. It's very subtle though, and probably >> > deserves some explanation in split-for-body's documentation. >> > >> > My first thought on seeing my non-fix version break here is that I can >> > make >> > split-for-body break the same way. The problem is that my non-fix >> > separates >> > the definition of fish? from the definitions of red? and blue?, which it >> > depends on. I can make split-for-body separate them the same way, by >> > putting a #:break or #:final clause in between the definition of fish? >> > and >> > the begin form. >> > >> > The problem with doing so is a subtle point about for loops that is only >> > mentioned in the last sentence of the last paragraph of the >> > documentation of >> > for itself: >> > >> > "Among the bodys, besides stopping the iteration and preventing later >> > body >> > evaluations, a #:break guard-expr or #:final guard-expr clause starts a >> > new >> > internal-definition context." >> > >> > So that's what split-for-body is preserving, the boundaries between >> > internal >> > definition contexts. That's not at all what I had expected it was >> > doing; I >> > had no idea the body of a for loop constituted multiple such contexts. >> > >> > Anyway, thanks for the clarification, I now understand why abstractions >> > over >> > for loops need to use split-for-body. >> > >> > Carl Eastlund >> > >> > >> > On Fri, Sep 6, 2013 at 12:38 PM, Matthew Flatt <<a href="mailto:mflatt@cs.utah.edu">mflatt@cs.utah.edu</a>> >> > wrote: >> >> >> >> Sorry that I forgot to add the `let` while turning the code you sent >> >> into a full example. Here's another try. >> >> >> >> #lang racket/base >> >> (require (for-syntax racket/base >> >> syntax/parse >> >> syntax/for-body)) >> >> >> >> (define-syntax (for/print/good stx) >> >> (syntax-parse stx >> >> [(_ clauses . body) >> >> (with-syntax ([([pre ...] [post ...]) (split-for-body stx >> >> #'body)]) >> >> (syntax >> >> (for clauses >> >> pre ... >> >> (printf "~v\n" (let () post ...)))))])) >> >> >> >> (define-syntax-rule (for/print/fixed/not clauses pre ... result) >> >> (for clauses >> >> pre ... >> >> (printf "~v\n" (let () result)))) >> >> >> >> (for/print/fixed/not ([i 1]) >> >> (define (fish? v) (or (red? v) (blue? v))) >> >> (begin >> >> (define (red? v) (eq? v 'red)) >> >> (define (blue? v) (eq? v 'blue)) >> >> (fish? i))) >> >> >> >> At Fri, 6 Sep 2013 12:30:17 -0400, Carl Eastlund wrote: >> >> > 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: >> >> > >> >> > (define-syntax-rule (for/print/fixed clauses pre .. result) >> >> > (for clauses >> >> > pre ... >> >> > (printf "~v\n" (let () result)))) >> >> > >> >> > Carl Eastlund >> >> > >> >> > >> >> > On Fri, Sep 6, 2013 at 12:25 PM, Matthew Flatt <<a href="mailto:mflatt@cs.utah.edu">mflatt@cs.utah.edu</a>> >> >> > wrote: >> >> > >> >> > > >> >> > > #lang racket/base >> >> > > (require (for-syntax racket/base >> >> > > syntax/parse >> >> > > syntax/for-body)) >> >> > > >> >> > > (define-syntax (for/print/good stx) >> >> > > (syntax-parse stx >> >> > > [(_ clauses . body) >> >> > > (with-syntax ([([pre ...] [post ...]) (split-for-body stx >> >> > > #'body)]) >> >> > > (syntax >> >> > > (for clauses >> >> > > pre ... >> >> > > (printf "~v\n" (let () post ...)))))])) >> >> > > >> >> > > (define-syntax-rule (for/print/bad clauses pre ... result) >> >> > > (for clauses >> >> > > pre ... >> >> > > (printf "~v\n" result))) >> >> > > >> >> > > ;; Try changing to for/print/bad: >> >> > > (for/print/good ([i 1]) >> >> > > (define (fish? v) (or (red? v) (blue? v))) >> >> > > (begin >> >> > > (define (red? v) (eq? v 'red)) >> >> > > (define (blue? v) (eq? v 'blue)) >> >> > > (fish? i))) >> >> > > >> >> > > >> >> > > At Fri, 6 Sep 2013 12:17:56 -0400, Carl Eastlund wrote: >> >> > > > 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 >> >> > > > <<a href="mailto:mflatt@cs.utah.edu">mflatt@cs.utah.edu</a>> >> >> > > 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: >> >> > > > > > <a href="http://lists.racket-lang.org/dev" target="_blank">http://lists.racket-lang.org/dev</a> >> >> > > > > >> >> > > > > >> >> > > >> >> > > >> >> >> > >> > >> > _________________________ >> > Racket Developers list: >> > <a href="http://lists.racket-lang.org/dev" target="_blank">http://lists.racket-lang.org/dev</a> >> > >> > </div></div></blockquote></div> </div></div></div>