Re: [racket-users] Syntax for hash contracts

[David Storrs]

On Thu, Dec 1, 2016 at 10:18 PM, Alexis King  wrote:

> > On Dec 1, 2016, at 21:43, David Storrs  wrote:
> >
> > The difference between a dictionary and a structure being that
> dictionaries are easily extensible on the fly and structures are not?  I'm
> curious -- what are the elements of that design and what are the reasons?
> It seems like a natural fit -- if Racket supported contracts on the values
> of a contract then you would have the best of dictionaries and structures.
>
> The difference is that a structure has intrinsic meaning while a hash
> with a particular collection of keys has extrinsic meaning.[...] Racket in
> general heavily favors custom, tagged data over reusing
>
data structures


Fair enough.  I come from a Perl background and am used to a slightly more
relaxed system, but I'll try to get used to the new way.


>
> > Hm.  Well, that approach would work.  It's not really what I'm looking
> for, though -- this is data that's coming back from a SQL query and being
> forwarded on to another function for further processing.  It isn't needed
> anywhere else, and creating a struct for this one use feels pretty clumsy
> and heavyweight.  "Hash of field-name-in-table to value-in-field" seemed
> like a really intuitive solution.  It's fine, though.  I can just do a
> manual check.
>
> If you are just handing off this data between two functions as an
> implementation detail, do you need the contract at all? That is, what
> value are you getting from it? Could the arguments be provided as
> keyword arguments, instead?
>

That's a good thought.  Yes, that will work.  I had them as a hash because
I used them in the source function and it was convenient to have them that
way, so I figured I could just hand them off to the sub-function in the
original hash form.  Better to split it up though, I suppose.



> That said, if you wanted a contract that does what you describe, it
> wouldn’t be too difficult to write:
>
>   (define (hash-object/c ctc-dict)
> (make-contract
>  #:name `(hash-object/c
>   ,(for/list ([(k v) (in-dict ctc-dict)])
>  (cons k (contract-name v
>  #:projection
>  (λ (blame)
>(λ (val)
>  (for ([(k v) (in-hash val)])
>(let ([ctc (dict-ref ctc-dict k #f)]
>  [blame (blame-add-context
>  blame (format "value for key ~e of" k))])
>  (when ctc
>(((contract-projection ctc) blame) v
>

...I think your definition of "not too hard" may differ from mine.  :>


> The existing hash contracts are mostly designed to accommodate
> homogenous dictionaries, like (hash/c string? boolean?). I’m not
> entirely sure what the intended use case of hash/dc is, and while I’m
> sure I’d be very glad it exists if I ever needed it, I admit I’ve never
> used it myself. It just allows you to provide a function that determines
> the contract of a value given the key, but it still requires that all
> keys have the same contract, and it does not let you specify which keys
> should be supplied.
>

Okay, that makes more sense.  Thanks for all the explanation.



>
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[racket-users] Re: Methods for least squares

['John Clements' via Racket Users]

> On Dec 1, 2016, at 12:07, Bradley Lucier  wrote:
> 
> On 12/01/2016 02:04 PM, John Clements wrote:
>> 
>> Would it be all right with you if I shared your mail with the mailing list? 
>> A brief reading of this paper shows me the relationship between solving this 
>> problem and approximation of gaussian elimination, and I’d like to ask Jens 
>> Axel Soegaard and Neil Toronto how this relates to the general algorithms 
>> for matrix solution.
> 
> Yes, you can share it.
> 
> The Conjugate Gradient method is applicable to solving $Ax=b$ when $A$ is a 
> symmetric, positive definite matrix.  It's not applicable to the problem with 
> general $A$.
> 
> So, in fact, it's applicable to solving $A^*Ax=A^*b$, the normal equations 
> for least squares, since $A^*A$ is symmetric and positive definite.  The 
> brilliance of the method is that it doesn't actually compute $A^*A$, but only 
> $Ay$ and $A^*y$ for various vectors $y$.
> 
> The method is particularly useful when the linear system $Ax=b$ has inherent 
> error in $A$ and $b$, and so one requires only an approximation to the 
> solution $x$.

Okay, I’ve taken a crack at implementing this… well, I have a toy 
implementation, that doesn’t do any checking and only works for a particular 
matrix shape.  

Short version: yep, it works!

Now, a few questions.

1) What should one use as the initial estimate, x_0 ? The method appears to 
work fine for an initial estimate that is uniformly zero. Is there any reason 
not to use this?

2) When does one stop? I have not read the paper carefully, but it appears that 
it’s intended to “halt” in approximately ’n’ steps, where ’n’ is the … number 
of rows? In the one case I tried, my “direction” vector p_i quickly dropped to 
something on the order of [1e-16, 1e-16], and in fact it became perfectly 
stable, in that successive iterations produced precisely the same values for 
the three iteration variables. However, it wouldn’t surprise me to discover 
that there were cases on which the answer oscillated between two minutely 
different values. Can you shed any light on when it’s safe to stop in the 
general case?

3) Do you have any idea how this technique compares to any described in 
Numerical Recipes or implemented in LAPACK ?

John



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Re: [racket-users] Syntax for hash contracts

[Alexis King]

> On Dec 1, 2016, at 21:43, David Storrs  wrote:
> 
> The difference between a dictionary and a structure being that dictionaries 
> are easily extensible on the fly and structures are not?  I'm curious -- what 
> are the elements of that design and what are the reasons?  It seems like a 
> natural fit -- if Racket supported contracts on the values of a contract then 
> you would have the best of dictionaries and structures.

The difference is that a structure has intrinsic meaning while a hash
with a particular collection of keys has extrinsic meaning. One could
represent a point in two-dimensional space as a struct called “point” or
a hash with keys 'x and 'y. The former is intrinsically a point, but the
latter is only a point if you use it like a point; it could just as
easily be a two-dimensional vector (which itself has a myriad of
possible meanings).

Put more practically, structs are tagged, and hashes are untagged. This
might not seem terribly important in your case (and it probably isn’t),
but Racket in general heavily favors custom, tagged data over reusing
data structures, in contrast with Clojure, which takes precisely the
opposite approach. (The design differences between Racket’s contract
combinators and the recent clojure.spec illustrate much of this
difference in philosophy.) I’m sure there are many other people on this
list who can describe the reasoning behind Racket’s design choices here
far better than I can, so I will not try at this time.

> Hm.  Well, that approach would work.  It's not really what I'm looking for, 
> though -- this is data that's coming back from a SQL query and being 
> forwarded on to another function for further processing.  It isn't needed 
> anywhere else, and creating a struct for this one use feels pretty clumsy and 
> heavyweight.  "Hash of field-name-in-table to value-in-field" seemed like a 
> really intuitive solution.  It's fine, though.  I can just do a manual check.

If you are just handing off this data between two functions as an
implementation detail, do you need the contract at all? That is, what
value are you getting from it? Could the arguments be provided as
keyword arguments, instead?

That said, if you wanted a contract that does what you describe, it
wouldn’t be too difficult to write:

  (define (hash-object/c ctc-dict)
(make-contract
 #:name `(hash-object/c
  ,(for/list ([(k v) (in-dict ctc-dict)])
 (cons k (contract-name v
 #:projection
 (λ (blame)
   (λ (val)
 (for ([(k v) (in-hash val)])
   (let ([ctc (dict-ref ctc-dict k #f)]
 [blame (blame-add-context
 blame (format "value for key ~e of" k))])
 (when ctc
   (((contract-projection ctc) blame) v

This contract is not terribly robust or performant (it should ensure the
value is immutable, use the late negative projection and do more work
ahead of time, etc.), but it’s a demonstration of the behavior you want.
You could use it like this:

  (define/contract h
(hash-object/c `([foo . ,string?]
 [bar . ,boolean?]))
(hash 'foo "hello" 'bar #f))

> So, given that these don't work the way I thought, how DO they work?  I still 
> can't understand this documentation -- like, literally *at all*.  I have not 
> managed to write a single non-trivial contract thus far.  Could you please 
> provide some examples of how to use hash contracts and why?
> 
> For example, I don't see how to do something as simple as "this hash must 
> have the following keys".  Or how to say that some keys will be of different 
> types -- e.g., 'foo and "bar".

The existing hash contracts are mostly designed to accommodate
homogenous dictionaries, like (hash/c string? boolean?). I’m not
entirely sure what the intended use case of hash/dc is, and while I’m
sure I’d be very glad it exists if I ever needed it, I admit I’ve never
used it myself. It just allows you to provide a function that determines
the contract of a value given the key, but it still requires that all
keys have the same contract, and it does not let you specify which keys
should be supplied.

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Re: [racket-users] Syntax for hash contracts

[David Storrs]

On Thu, Dec 1, 2016 at 8:59 PM, Alexis King  wrote:

> > On Dec 1, 2016, at 16:29, David Storrs  wrote:
> >
> > - This function returns #t because it is a simple test function intended
> to get the hang of hash contracts...
> > - This function takes one argument...
> > - Which is a hash...
> > - Which has keys 'success, 'file-id, 'scratchdir-path, and 'chunk-hash...
> > - All of which are symbols...
> > - The values will be, respectively:
> > success boolean?
> > file-id (or/c #f exact-positive-integer?)
> > scratchdir-path path-string?
> > chunk-hash string?
>
> The hash/dc contract is not designed to assign contracts to the values
> associated with specific keys; rather, it allows the contract on a value
> to depend generally on the value of the key. You could theoretically use
> the right hand side of the contract to do a case analysis on the value
> of the key, but that would not be pretty. This is mostly intentional,
> though: Racket hashes are designed to be used as dictionaries, not
> structures.
>

The difference between a dictionary and a structure being that dictionaries
are easily extensible on the fly and structures are not?  I'm curious --
what are the elements of that design and what are the reasons?  It seems
like a natural fit -- if Racket supported contracts on the values of a
contract then you would have the best of dictionaries and structures.

(I'm worried that the above sounds snarky, but I'm not sure how to rephrase
it  It's not intended as snark, it's an honest question.)


>
> It sounds like you likely want a struct, not a hash. Probably something
> like this:
>
>   (struct some-name (success file-id scratchdir-path chunk-hash))
>
> …with the following contract:
>
>   (struct/c some-name
> boolean?
> (or/c #f exact-positive-integer?)
> path-string?
> string?)
>
>
Hm.  Well, that approach would work.  It's not really what I'm looking for,
though -- this is data that's coming back from a SQL query and being
forwarded on to another function for further processing.  It isn't needed
anywhere else, and creating a struct for this one use feels pretty clumsy
and heavyweight.  "Hash of field-name-in-table to value-in-field" seemed
like a really intuitive solution.  It's fine, though.  I can just do a
manual check.


So, given that these don't work the way I thought, how DO they work?  I
still can't understand this documentation -- like, literally *at all*.  I
have not managed to write a single non-trivial contract thus far.  Could
you please provide some examples of how to use hash contracts and why?

For example, I don't see how to do something as simple as "this hash must
have the following keys".  Or how to say that some keys will be of
different types -- e.g., 'foo and "bar".


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Re: [racket-users] Syntax for hash contracts

[Alexis King]

> On Dec 1, 2016, at 16:29, David Storrs  wrote:
> 
> - This function returns #t because it is a simple test function intended to 
> get the hang of hash contracts...
> - This function takes one argument...
> - Which is a hash...
> - Which has keys 'success, 'file-id, 'scratchdir-path, and 'chunk-hash...
> - All of which are symbols...
> - The values will be, respectively:
> success boolean?
> file-id (or/c #f exact-positive-integer?)
> scratchdir-path path-string?
> chunk-hash string?

The hash/dc contract is not designed to assign contracts to the values
associated with specific keys; rather, it allows the contract on a value
to depend generally on the value of the key. You could theoretically use
the right hand side of the contract to do a case analysis on the value
of the key, but that would not be pretty. This is mostly intentional,
though: Racket hashes are designed to be used as dictionaries, not
structures.

It sounds like you likely want a struct, not a hash. Probably something
like this:

  (struct some-name (success file-id scratchdir-path chunk-hash))

…with the following contract:

  (struct/c some-name
boolean?
(or/c #f exact-positive-integer?)
path-string?
string?)

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Re: [racket-users] Scribble equivalent of rowspan

[Matthew Flatt]

We currently don't have good enough tests for renderers. The tests in

 scribble-test/tests/scribble/docs

use only the text renderer, and they're mostly meant to check
renderer-independent formatting details. That could be a starting point
and a good way to test the text renderer, though.

At Thu, 1 Dec 2016 19:00:33 -0600, Philip McGrath wrote:
> Thanks for this. I think I see in general where the changes need to be, and
> I've taken some first steps toward making them, though it will probably be
> a week or so before I have time to totally dive in.
> 
> One additional question: is there a good/light-weight way to test changes
> to the renderers? This is having me dive into much lower-level parts of
> Scribble than I've looked at previously (though maybe I was headed there
> eventually). It seems like it would be ideal to avoid manually checking the
> rendering of a dummy document, but it isn't immediately obvious to me how
> to write more rackunit-like test cases.
> 
> -Philip
> 
> On Wed, Nov 30, 2016 at 7:56 AM, Matthew Flatt  wrote:
> 
> > Hi Philip,
> >
> > There's nothing like rowspan currently.
> >
> > If you want to try adding something, I think you'll end up changing the
> > "scribble-lib" package in several places: in "core.rkt" to adjust the
> > contract for tables, in "base.rkt" to adjust the contract for
> > `tabular`, and in "html-render.rkt", "latex-render.rkt",
> > "markdown-render.rkt", and "text-render.rkt" to adjust the various
> > renderers. The rendering part is the main work, naturally.
> >
> > If you decide not to try, I can take a look in a few days.
> >
> > Matthew
> >
> > At Tue, 29 Nov 2016 21:47:03 -0600, Philip McGrath wrote:
> > > A very rookie question: I am trying to figure out how to specify the
> > > equivalent of HTML's rowspan attribute for tabular from scribble/base:
> > that
> > > is, to have a cell which spans more than one row. In LaTeX, I think I
> > would
> > > use "multirow" (but I'm no LaTeX expert).
> > >
> > > I know so know about 'cont, but I believe that only lets a cell span
> > > multiple columns: I'm looking for the a vertical version. Is there a
> > > built-in way to do this? Or otherwise, if it's not too daunting to
> > > implement myself, could someone point me in the right direction?
> > >
> > > Here's some ASCII art of what I'm going for, if that's clearer:
> > >
> > > -
> > > | | Date: | Nov 29 | Dec 30 |
> > > | |---|||
> > > | | Time: | 12:18  |  3:06  |
> > > |---|
> > > | | Expected: |   3|   1|
> > > | Apples  |---|||
> > > | | Actual:   |   2|   1|
> > > |---|
> > > | Pears & | Expected: |   4|   9|
> > > | |---|||
> > > |  Plums  | Actual:   |   6|   3|
> > > -
> > >
> > > --
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> > > "Racket Users" group.
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> > > email to racket-users+unsubscr...@googlegroups.com.
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Re: [racket-users] Scribble equivalent of rowspan

[David Storrs]

On Thu, Dec 1, 2016 at 5:00 PM, Philip McGrath 
wrote:

> Thanks for this. I think I see in general where the changes need to be,
> and I've taken some first steps toward making them, though it will probably
> be a week or so before I have time to totally dive in.
>
> One additional question: is there a good/light-weight way to test changes
> to the renderers? This is having me dive into much lower-level parts of
> Scribble than I've looked at previously (though maybe I was headed there
> eventually). It seems like it would be ideal to avoid manually checking the
> rendering of a dummy document, but it isn't immediately obvious to me how
> to write more rackunit-like test cases.
>

If you're thinking about manually checking it then you should be able to
generate a reference document, right?

You could try generating the document to a string (or to a file and then
read it in) and then running a regex against it, or comparing it to the
reference.

Alternatively, you could hash the reference and generated documents and
verify that the hashes are identical.  That might be a higher standard of
'correct' than you're looking for, though.

Dave

-Philip

On Wed, Nov 30, 2016 at 7:56 AM, Matthew Flatt  wrote:

> Hi Philip,
>
> There's nothing like rowspan currently.
>
> If you want to try adding something, I think you'll end up changing the
> "scribble-lib" package in several places: in "core.rkt" to adjust the
> contract for tables, in "base.rkt" to adjust the contract for
> `tabular`, and in "html-render.rkt", "latex-render.rkt",
> "markdown-render.rkt", and "text-render.rkt" to adjust the various
> renderers. The rendering part is the main work, naturally.
>
> If you decide not to try, I can take a look in a few days.
>
> Matthew
>
> At Tue, 29 Nov 2016 21:47:03 -0600, Philip McGrath wrote:
> > A very rookie question: I am trying to figure out how to specify the
> > equivalent of HTML's rowspan attribute for tabular from scribble/base:
> that
> > is, to have a cell which spans more than one row. In LaTeX, I think I
> would
> > use "multirow" (but I'm no LaTeX expert).
> >
> > I know so know about 'cont, but I believe that only lets a cell span
> > multiple columns: I'm looking for the a vertical version. Is there a
> > built-in way to do this? Or otherwise, if it's not too daunting to
> > implement myself, could someone point me in the right direction?
> >
> > Here's some ASCII art of what I'm going for, if that's clearer:
> >
> > -
> > | | Date: | Nov 29 | Dec 30 |
> > | |---|||
> > | | Time: | 12:18  |  3:06  |
> > |---|
> > | | Expected: |   3|   1|
> > | Apples  |---|||
> > | | Actual:   |   2|   1|
> > |---|
> > | Pears & | Expected: |   4|   9|
> > | |---|||
> > |  Plums  | Actual:   |   6|   3|
> > -
> >
> > --
> > You received this message because you are subscribed to the Google Groups
> > "Racket Users" group.
> > To unsubscribe from this group and stop receiving emails from it, send an
> > email to racket-users+unsubscr...@googlegroups.com.
> > For more options, visit https://groups.google.com/d/optout.
>
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[racket-users] Syntax for hash contracts

[David Storrs]

Hi folks,

I'm having trouble understanding the docs on hash contracts (
https://docs.racket-lang.org/reference/data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fhash..rkt%29._hash%2Fc%29%29
)

What I'm trying to express is:

- This function returns #t because it is a simple test function intended to
get the hang of hash contracts...
- This function takes one argument...
- Which is a hash...
- Which has keys 'success, 'file-id, 'scratchdir-path, and 'chunk-hash...
- All of which are symbols...
- The values will be, respectively:
success boolean?
file-id (or/c #f exact-positive-integer?)
scratchdir-path path-string?
chunk-hash string?

My first attempt was:

(define/contract (x h)
  (-> (hash/dc [file-id symbol?]  [a (file-id) (or/c #f
exact-positive-integer?)]
   [scratchdir-path symbol?]  [b (scratchdir-path) path-string?]
   [chunk-hash symbol?]   [c (chunk-hash) string?]
   [success symbol?]  [d (success) boolean?])
  #t)
  #t)


My second attempt was:
(define/contract (x h)
  (-> (hash/dc [file-id symbol?
scratchdir-path symbol?
chunk-hash symbol?
success symbol?]
   [a (file-id) (or/c #f exact-positive-integer?)
b (scratchdir-path) path-string?
c (chunk-hash) string?
d (success) boolean?])
  #t)
  #t)

Both of these have syntax issues and I can't make heads or tails of the
docs.  Help, please?

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Re: [racket-users] running dr racket on chromebook without ubuntu

[Vincent St-Amour]

Is your filesystem mounted with "noexec"?

Vincent


On Mon, 28 Nov 2016 09:14:01 -0600,
yehoshua zaman wrote:
> 
> I have asus chromebook c100p. i am in developer mode. I installed dr racket 
> while in developer mode by entering crosh, then typing shell then finding the 
> location of downloads and while in that location typing racket.sh (i renamed 
> it to racket.sh) after that i went into the directory of racket and am trying 
> to run drracket by typing ./drracket it says bash: ./drracket permission 
> denied. i tried chmod +x drracket and chmod u+x drracket still it says 
> permission denied. the rights of the file are -rwxr-xr-x
> 
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RE: [racket-users] hyperlink in code:comment

[Jos Koot]

Thank you very much.
I don't understand why I missed that part of the docs.
Jos 

-Original Message-
From: stchang...@gmail.com [mailto:stchang...@gmail.com] On Behalf Of Stephen 
Chang
Sent: miércoles, 30 de noviembre de 2016 19:56
To: Jos Koot
Cc: Racket Users
Subject: Re: [racket-users] hyperlink in code:comment

Each code formatting form, like `racketblock`, allows specifying an
escape identifier for use within that form, though all forms default
to `unsyntax` if unspecified, I believe. Some forms, like `codeblock`
do not support escapes.

The docs here [1] has more details, and specifically the docs for
racketblock [2] has more examples.


[1]: http://docs.racket-lang.org/scribble/scribble_manual_code.html
[2]:
http://docs.racket-lang.org/scribble/scribble_manual_code.html?#%28form._%28%28lib._scribble%2Fmanual..rkt%29._racketblock%29%29

On Tue, Nov 29, 2016 at 3:42 PM, Jos Koot  wrote:
> Thanks, I'll try that.
> It is not clear to me where I can use @#,
> But certainly your response will be a great help for me.
> Thanks again, Jos
>
> -Original Message-
> From: stchang...@gmail.com [mailto:stchang...@gmail.com] On Behalf Of Stephen 
> Chang
> Sent: martes, 29 de noviembre de 2016 21:31
> To: Jos Koot
> Cc: Racket Users
> Subject: Re: [racket-users] hyperlink in code:comment
>
> You can use the escape identifier:
>
> #lang scribble/manual
>
> @(require scribble/eval
>   (for-label racket))
>
> @interaction[
>  (define a (list 1))
>  (define b (list 1))
>  (code:comment @#,para{a and b are not @racket[eq?], but they are
> @racket[equal?]:})
>  (eq? a b)
>  (equal? a b)]
>
> On Tue, Nov 29, 2016 at 2:56 PM, Jos Koot  wrote:
>> A simplified fragment of scribble code:
>>
>> #lang scribble/manual
>>
>> @(require scribble/eval)
>>
>> @interaction[
>>  (define a (list 1))
>>  (define b (list 1))
>>  (code:comment "a and b are not eq, but they are equal:")
>>  (eq? a b)
>>  (equal? a b)]
>>
>> Within the comment I would like to hyperlink 'eq' to the doc of [eq?]
>> and 'equal' to the doc of [equal?]
>>
>> Is this possible?
>> If so how?
>> I have tried it without success.
>>
>> Thanks, Jos
>>
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[racket-users] Re: Confused about bitmaps drawing to canvas

[Hersh Krishna]

On Thursday, December 1, 2016 at 6:40:01 AM UTC-6, Alex Harsanyi wrote:
> Without seeing the code, this is just a blind guess, but try "(send canvas 
> refresh)" in the implementation for your "set-bitmap" method.
> 
> Best Regards,
> Alex.
> 
> On Thursday, December 1, 2016 at 4:28:02 PM UTC+8, Hersh Krishna wrote:
> > So I'm making a frame which draws a bitmap that can be replaced at runtime 
> > so if you call (send my-frame set-bitmap *bitmap*) it will switch from 
> > displaying whatever bitmap you had previously to the bitmap you passed it. 
> > (its doing a lot of other stuff and I want the interface to this canvas 
> > that's drawing the bitmap to be through the frame) At least that's the 
> > theory. In practice, whenever I call (send my-frame show #t) the bitmap 
> > that's being drawn is locked permanently. Doesn't matter if the 
> > paint-callback fetches the bitmap dynamically or if I draw in the bitmap's 
> > dc. I know the bitmap is getting updated or switched. Is there something 
> > I'm missing or a technique I should be using?

That seams to have done the trick. Sorry about that.

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[racket-users] Re: Confused about bitmaps drawing to canvas

[Alex Harsanyi]

Without seeing the code, this is just a blind guess, but try "(send canvas 
refresh)" in the implementation for your "set-bitmap" method.

Best Regards,
Alex.

On Thursday, December 1, 2016 at 4:28:02 PM UTC+8, Hersh Krishna wrote:
> So I'm making a frame which draws a bitmap that can be replaced at runtime so 
> if you call (send my-frame set-bitmap *bitmap*) it will switch from 
> displaying whatever bitmap you had previously to the bitmap you passed it. 
> (its doing a lot of other stuff and I want the interface to this canvas 
> that's drawing the bitmap to be through the frame) At least that's the 
> theory. In practice, whenever I call (send my-frame show #t) the bitmap 
> that's being drawn is locked permanently. Doesn't matter if the 
> paint-callback fetches the bitmap dynamically or if I draw in the bitmap's 
> dc. I know the bitmap is getting updated or switched. Is there something I'm 
> missing or a technique I should be using?

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[racket-users] Confused about bitmaps drawing to canvas

[Hersh Krishna]

So I'm making a frame which draws a bitmap that can be replaced at runtime so 
if you call (send my-frame set-bitmap *bitmap*) it will switch from displaying 
whatever bitmap you had previously to the bitmap you passed it. (its doing a 
lot of other stuff and I want the interface to this canvas that's drawing the 
bitmap to be through the frame) At least that's the theory. In practice, 
whenever I call (send my-frame show #t) the bitmap that's being drawn is locked 
permanently. Doesn't matter if the paint-callback fetches the bitmap 
dynamically or if I draw in the bitmap's dc. I know the bitmap is getting 
updated or switched. Is there something I'm missing or a technique I should be 
using?

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