For emina's problem, shouldn't the implementation of typed/racket have a clue?
On Wed, Nov 20, 2013 at 5:19 AM, <users-requ...@racket-lang.org> wrote: > Send users mailing list submissions to > users@racket-lang.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://lists.racket-lang.org/users/listinfo > or, via email, send a message with subject or body 'help' to > users-requ...@racket-lang.org > > You can reach the person managing the list at > users-ow...@racket-lang.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of users digest..." > > [Racket Users list: > http://lists.racket-lang.org/users ] > > Today's Topics: > > 1. Why does this tail recursive list length function fail at cdr > l at end of list? (Bo Gus) > 2. Re: Why does this tail recursive list length function fail at > cdr l at end of list? (Pierpaolo Bernardi) > 3. Re: Why does this tail recursive list length function fail at > cdr l at end of list? (Bo Gus) > 4. Re: Why does this tail recursive list length function fail at > cdr l at end of list? (Daniel Prager) > 5. Re: Why does this tail recursive list length function fail at > cdr l at end of list? (Daniel Prager) > 6. Re: obtaining the location of an identifier (Robby Findler) > > > ---------- Forwarded message ---------- > From: Bo Gus <foruman...@gmail.com> > To: Racket Newsgroup <users@racket-lang.org> > Cc: > Date: Wed, 20 Nov 2013 11:09:28 +0000 > Subject: [racket] Why does this tail recursive list length function fail > at cdr l at end of list? > My tail recursive implementation of length is like this: > > (define (length2 l) > (define (l-iter l count) > (if (null? 1) count > (l-iter (cdr l) (+ count 1)))) > (l-iter l 0)) > > If I call (length2 '(1 2 3)) and step through the code in racket, count > increments to 3 but then on the (if (null? l) count line instead of > returning out of the function it goes onto the next line (l-iter (cdr l) (+ > count l)))) and of course fails at cder of an empty list. > > Racket error message is: > > mcdr: contract violation > expected: mpair? > given: '() > > > I am fairly new to scheme. What am I doing wrong? > > Angus > > > > ---------- Forwarded message ---------- > From: Pierpaolo Bernardi <olopie...@gmail.com> > To: Bo Gus <foruman...@gmail.com> > Cc: Racket Newsgroup <users@racket-lang.org> > Date: Wed, 20 Nov 2013 12:27:40 +0100 > Subject: Re: [racket] Why does this tail recursive list length function > fail at cdr l at end of list? > On Wed, Nov 20, 2013 at 12:09 PM, Bo Gus <foruman...@gmail.com> wrote: > > My tail recursive implementation of length is like this: > > > > (define (length2 l) > > (define (l-iter l count) > > (if (null? 1) count > > (l-iter (cdr l) (+ count 1)))) > > (l-iter l 0)) > > > I am fairly new to scheme. What am I doing wrong? > > You wrote (null? 1), which is always false. Note 1 instead of l. > > Cheers > > > > ---------- Forwarded message ---------- > From: Bo Gus <foruman...@gmail.com> > To: Racket Newsgroup <users@racket-lang.org> > Cc: > Date: Wed, 20 Nov 2013 11:33:18 +0000 > Subject: Re: [racket] Why does this tail recursive list length function > fail at cdr l at end of list? > I noticed that just after posting. > > I changed to: > (define (length2 lst) > (define (l-iter l count) > (if (null? l) count > (l-iter (cdr l) (+ count 1)))) > (l-iter lst 0)) > > Must get better at spotting obvious mistakes... > > > On 20 November 2013 11:27, Pierpaolo Bernardi <olopie...@gmail.com> wrote: > >> On Wed, Nov 20, 2013 at 12:09 PM, Bo Gus <foruman...@gmail.com> wrote: >> > My tail recursive implementation of length is like this: >> > >> > (define (length2 l) >> > (define (l-iter l count) >> > (if (null? 1) count >> > (l-iter (cdr l) (+ count 1)))) >> > (l-iter l 0)) >> >> > I am fairly new to scheme. What am I doing wrong? >> >> You wrote (null? 1), which is always false. Note 1 instead of l. >> >> Cheers >> > > > > ---------- Forwarded message ---------- > From: Daniel Prager <daniel.a.pra...@gmail.com> > To: Bo Gus <foruman...@gmail.com>, Racket Users <users@racket-lang.org> > Cc: > Date: Wed, 20 Nov 2013 22:52:17 +1100 > Subject: Re: [racket] Why does this tail recursive list length function > fail at cdr l at end of list? > Replace the expression (null? 1) with (null? l). You appear to have typed > 1 (one) where you meant l (ell). > As a general rule-of-thumb a name such as lst or a-list will reduce the > risk of this kind of error. > > Dan > > > ---------- Forwarded message ---------- > From: Daniel Prager <daniel.a.pra...@gmail.com> > To: Bo Gus <foruman...@gmail.com>, Racket Users <users@racket-lang.org> > Cc: > Date: Wed, 20 Nov 2013 22:56:25 +1100 > Subject: Re: [racket] Why does this tail recursive list length function > fail at cdr l at end of list? > Oops - dup. > > Didn't see Pierpaolo's reply. > > Dan > > > On Wed, Nov 20, 2013 at 10:52 PM, Daniel Prager <daniel.a.pra...@gmail.com > > wrote: > >> Replace the expression (null? 1) with (null? l). You appear to have typed >> 1 (one) where you meant l (ell). >> As a general rule-of-thumb a name such as lst or a-list will reduce the >> risk of this kind of error. >> >> Dan >> > > > > -- > *Daniel Prager* > Agile/Lean Coaching, Software Development and Leadership > Twitter: @agilejitsu <https://twitter.com/agilejitsu> > Blog: agile-jitsu.blogspot.com > > > ---------- Forwarded message ---------- > From: Robby Findler <ro...@eecs.northwestern.edu> > To: Emina Torlak <em...@eecs.berkeley.edu> > Cc: users <users@racket-lang.org>, Sam Tobin-Hochstadt < > sa...@cs.indiana.edu> > Date: Wed, 20 Nov 2013 07:19:35 -0600 > Subject: Re: [racket] obtaining the location of an identifier > I didn't try it, but you it might work to use local-expand and then find > all the binding forms and then rewrite them to use boxes (or some other > source of uniqueness you might have around). > > Robby > > > On Wed, Nov 20, 2013 at 12:53 AM, Emina Torlak <em...@eecs.berkeley.edu>wrote: > >> This is how my solution currently works, but unfortunately, it's not >> quite right. Here is a small example that demonstrates why, assuming the >> implementation based on free-id-table: >> >> (define (cell init) >> (let ([x init]) >> (case-lambda >> [() x] >> [(v) (set! x v)]))) >> >> (define foo (cell 0)) >> (define bar (cell 1)) >> >> > (foo 2) >> > (bar 3) >> > (foo) >> 2 >> > (bar) >> 3 >> > (dict->list global) >> '((.#<syntax:18:17 x> . 3)) >> >> In the above scenario, I need the global map to contain two bindings: >> one for the location of 'foo.x' and the other for the location of 'bar.x.' >> >> >> Emina >> >> >> >> On Tue, Nov 19, 2013 at 10:31 PM, Sam Tobin-Hochstadt < >> sa...@cs.indiana.edu> wrote: >> >>> I think that just identifiers and `free-id-table`s should work here. >>> Here's your example: >>> >>> #lang racket >>> >>> (require syntax/id-table (only-in racket [set! #%set!])) >>> >>> (define global (make-free-id-table)) >>> >>> (define-syntax-rule (location-of id) #'id) >>> >>> (define-syntax-rule (set! id expr) >>> (let ([v expr]) >>> (dict-set! global (location-of id) v) >>> (#%set! id v))) >>> >>> > (define x 1) >>> > (set! x 2) >>> > (set! x 3) >>> > (for/list ([(k v) (in-dict global)]) (list k v)) >>> '((.#<syntax:4:8 x> 3)) >>> >>> Also at https://gist.github.com/samth/7558673 >>> >>> Sam >>> >>> On Mon, Nov 18, 2013 at 2:43 PM, Emina Torlak <em...@eecs.berkeley.edu> >>> wrote: >>> > I'm using Racket to implement a language for which I need to track >>> state >>> > updates---in particular, variable mutation using set!. For example, >>> > consider this module definition: >>> > >>> > #lang racket >>> > >>> > (require (only-in racket [set! #%set!])) >>> > >>> > (define global (make-hash)) >>> > >>> > (define-syntax-rule (location-of id) >>> > (#%variable-reference id)) ; doesn't quite do the right thing >>> > >>> > (define-syntax-rule (set! id expr) >>> > (let ([val expr]) >>> > (hash-set! global (location-of id) val) >>> > (#%set! id val))) >>> > >>> > When I evaluate the following sequence of forms against the above >>> > definition, I would like the global hash map to contain just one >>> binding >>> > that maps the location for 'x' to the value 2. With the above >>> > implementation I get two map entries, since variable-reference doesn't >>> quite >>> > do what I hoped it did: >>> > >>> >> (define x 0) >>> >> (set! x 1) >>> >> (set! x 2) >>> >> x >>> > 2 >>> >> global >>> > '#hash((#<variable-reference> . 1) (#<variable-reference> . 2)) >>> > >>> > Is there another construct in Racket that I could use for this >>> purpose? If >>> > not, can something like this be implemented and how much work would it >>> > entail? >>> > >>> > I have a purely macro-based solution that works for the most part, but >>> it's >>> > fragile and there are corner cases for which it is just wrong. So, >>> before >>> > trying to fix that, I was wondering if there is a nicer way to solve >>> it by >>> > somehow getting handles for variable locations that are comparable >>> using eq? >>> > or equal? >>> > >>> > Thanks! >>> > >>> > Emina >>> > >>> > >>> > ____________________ >>> > Racket Users list: >>> > http://lists.racket-lang.org/users >>> > >>> >> >> >> ____________________ >> Racket Users list: >> http://lists.racket-lang.org/users >> >> > >
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