<div dir="ltr">On Fri, Sep 6, 2013 at 2:58 PM, Stephen Chang <<a href="mailto:stchang@ccs.neu.edu" target="_blank">stchang@ccs.neu.edu</a>> wrote: <div class="gmail_extra"><div class="gmail_quote"> <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><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: > > (for/list ([x (in-stream S)]) > (define y (f x)) > #:break (g? y) > (h y)) </div>Ok I see, thanks. It does make things easier to read when the break expression becomes too large to fit on one line. So is the reason why #:when and #:unless can't be used the same way because there already exist when and unless forms? </blockquote><div> </div><div>Are you looking for an equivalent of C's "continue" statement? I could imagine clauses for skipping to the next iteration using the keyword #:continue or #:next, though I have no idea if the existing loop architecture would easily accommodate them. I don't think calling them #:when or #:unless makes sense outside the for clauses. </div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> <div><div>>> > Carl Eastlund >> > >> > >> > On Fri, Sep 6, 2013 at 2:31 PM, Stephen Chang <<a href="mailto:stchang@ccs.neu.edu" target="_blank">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" target="_blank">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" target="_blank">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" target="_blank">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" target="_blank">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>