[racket] users Digest, Vol 99, Issue 77

For emina's problem, shouldn't the implementation of typed/racket have a
clue?


On Wed, Nov 20, 2013 at 5:19 AM, <users-request at racket-lang.org> wrote:

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> 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 <forumangus at gmail.com>
> To: Racket Newsgroup <users at 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 <olopierpa at gmail.com>
> To: Bo Gus <forumangus at gmail.com>
> Cc: Racket Newsgroup <users at 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 <forumangus at 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 <forumangus at gmail.com>
> To: Racket Newsgroup <users at 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 <olopierpa at gmail.com> wrote:
>
>> On Wed, Nov 20, 2013 at 12:09 PM, Bo Gus <forumangus at 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.prager at gmail.com>
> To: Bo Gus <forumangus at gmail.com>, Racket Users <users at 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.prager at gmail.com>
> To: Bo Gus <forumangus at gmail.com>, Racket Users <users at 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.prager at 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 <robby at eecs.northwestern.edu>
> To: Emina Torlak <emina at eecs.berkeley.edu>
> Cc: users <users at racket-lang.org>, Sam Tobin-Hochstadt <
> samth at 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 <emina at 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 <
>> samth at 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 <emina at 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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