Re: [racket-users] Re: Let's organize a Scheme European Event at FOSDEM 2019

[amz3]

On Sunday, October 14, 2018 at 4:33:22 AM UTC+2, Claes Wallin (韋嘉誠) wrote:
>
> Looks like the 
> https://fosdem.org/2019/schedule/track/minimalistic_languages/ 
> devroom came through! 
>
> Unfortunately, http://community.schemewiki.org/?FOSDEM2019 has been 
> down for days. Anyone know who is running it, and what the status is 
> on recovering? 
>

I don't know what happened to schemewiki.

Anyway, here is the proposal that was submitted at FOSDEM:

  https://libreplanet.org/wiki/FOSDEM2019-devroom-minimalism

Of course, you are welcome to submit your proposal at: 
https://penta.fosdem.org/submission/FOSDEM19

To create an account if you don't already have one: 
https://penta.fosdem.org/user/new_account

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Re: [racket-users] handin-server: reader used in checker modules

[Jordan Johnson]

> On Oct 17, 2018, at 11:30, Shu-Hung You  > wrote:
>> 
>> Would it help if in the check module you use `=' as the equality test,
>> or specify #e10.25 instead?
> 
> Success!
> 
> Specifying
> (!test (hour-angle 12 20 30) #e10.25)
> produces the behavior I expected, correctly comparing the numbers.

Ehh, I spoke too soon. The analagous student test —
(check-expect (hour-angle 12 20 30) 10.25)
— fails when included in the handin submission.

My working hypothesis is that I just need to configure the handin server’s 
evaluator to have (read-decimal-as-inexact #f). I’m trying to work out how to 
do that from the (check: ...) form, and failing that, will manually create & 
configure the evaluator.

If you can tell me how to do that — or tell me if I’m wrong — I’d much 
appreciate the saved time. But I do expect it’s a problem I can solve on my own 
with enough time.

Thanks,
Jordan


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Re: [racket-users] handin-server: reader used in checker modules

[Jordan Johnson]

On Oct 17, 2018, at 11:30, Shu-Hung You  
wrote:
> 
> Would it help if in the check module you use `=' as the equality test,
> or specify #e10.25 instead?

Success!

Specifying
(!test (hour-angle 12 20 30) #e10.25)
produces the behavior I expected, correctly comparing the numbers.

Thanks, all!

Best,
Jordan

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Re: [racket-users] handin-server: reader used in checker modules

[Jordan Johnson]

>> So, what aspect of the language config would be responsible for 41/4 not 
>> equaling 10.25?
> 
> Welcome to Racket v7.0.0.20.
>> (= 41/4 10.25)
> #t
>> (read-decimal-as-inexact #f)
>> (= 41/4 10.25)
> #t
> 
> Do you perhaps print and read the result? 

Nope. The code that’s generating the error is

(check: :language 'lang/htdp-intermediate
...
(!test (hour-angle 12 20 30) 10.25)
...)

I’m looking into !test now; I don’t yet know how it does its comparison.

jmj

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Re: [racket-users] handin-server: reader used in checker modules

[Shu-Hung You]

On Wed, Oct 17, 2018 at 1:12 PM Jordan Johnson  wrote:
>
>
> On Oct 17, 2018, at 10:35, Matthias Felleisen  wrote:
>
> (check-expect (+ 10 .25) 10.25)
>
> errors with check-expect’s message about not comparing inexact numbers, but 
> in DrRacket+BSL the decimals are treated as exact numbers.
>
> What do I need to change in the checker, in order to obtain the same numeric 
> behavior as in DrRacket?
>
> Set read-decimal-as-inexact might solve the problem.
>
>
> Progress! But not completely solved. By adding
>
> (read-decimal-as-inexact #f)
>
> to the checker module, I now don’t get the check-expect error. Instead, I’m 
> seeing
>
> submit error: your code failed a test: (hour-angle 12 20 30) evaluated to 
> 41/4, expecting 10.25
>
>
> produced by the checker line
>
> (!test (hour-angle 12 20 30) 10.25)
>

Would it help if in the check module you use `=' as the equality test,
or specify #e10.25 instead?

> where hour-angle computes exactly.
>
> So, what aspect of the language config would be responsible for 41/4 not 
> equaling 10.25?
>
> Best,
> Jordan
>
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Re: [racket-users] handin-server: reader used in checker modules

[Matthias Felleisen]

> On Oct 17, 2018, at 2:11 PM, Jordan Johnson  wrote:
> 
> 
>> On Oct 17, 2018, at 10:35, Matthias Felleisen  wrote:
>>> (check-expect (+ 10 .25) 10.25)
>>> 
>>> errors with check-expect’s message about not comparing inexact numbers, but 
>>> in DrRacket+BSL the decimals are treated as exact numbers.
>>> 
>>> What do I need to change in the checker, in order to obtain the same 
>>> numeric behavior as in DrRacket?
>> Set read-decimal-as-inexact might solve the problem. 
> 
> Progress! But not completely solved. By adding 
> (read-decimal-as-inexact #f)
> to the checker module, I now don’t get the check-expect error. Instead, I’m 
> seeing
> submit error: your code failed a test: (hour-angle 12 20 30) evaluated to 
> 41/4, expecting 10.25
> 
> produced by the checker line
> (!test (hour-angle 12 20 30) 10.25)
> where hour-angle computes exactly.
> 
> So, what aspect of the language config would be responsible for 41/4 not 
> equaling 10.25?


Hmph: 
 
Welcome to Racket v7.0.0.20.
> (= 41/4 10.25)
#t
> (read-decimal-as-inexact #f)
> (= 41/4 10.25)
#t


Do you perhaps print and read the result? 

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Re: [racket-users] handin-server: reader used in checker modules

[Jordan Johnson]

> On Oct 17, 2018, at 10:35, Matthias Felleisen  wrote:
>>  (check-expect (+ 10 .25) 10.25)
>> 
>> errors with check-expect’s message about not comparing inexact numbers, but 
>> in DrRacket+BSL the decimals are treated as exact numbers.
>> 
>> What do I need to change in the checker, in order to obtain the same numeric 
>> behavior as in DrRacket?
> Set read-decimal-as-inexact might solve the problem. 

Progress! But not completely solved. By adding 
(read-decimal-as-inexact #f)
to the checker module, I now don’t get the check-expect error. Instead, I’m 
seeing
submit error: your code failed a test: (hour-angle 12 20 30) evaluated to 41/4, 
expecting 10.25

produced by the checker line
(!test (hour-angle 12 20 30) 10.25)
where hour-angle computes exactly.

So, what aspect of the language config would be responsible for 41/4 not 
equaling 10.25?

Best,
Jordan

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Re: [racket-users] handin-server: reader used in checker modules

[Matthias Felleisen]

> On Oct 17, 2018, at 1:21 PM, Jordan Johnson  wrote:
> 
> Hi all,
> 
> In my last thread regarding the handin server, Matthew suggested:
>> A workaround is to use `'lang/htdp-beginner` as the language instead of
>> `'(special beginner)`. That happens to avoid a redundant and
>> troublemaking `require`.
> 
> So, I’m using 'lang/htdp-beginner in an assignment’s checker.rkt file, and I 
> am seeing errors that appear to be the result of the handin server using a 
> different reader (or configuration thereof) than DrRacket does for BSL. 
> Specifically,
> 
>   (check-expect (+ 10 .25) 10.25)
> 
> errors with check-expect’s message about not comparing inexact numbers, but 
> in DrRacket+BSL the decimals are treated as exact numbers.
> 
> What do I need to change in the checker, in order to obtain the same numeric 
> behavior as in DrRacket?


Set read-decimal-as-inexact might solve the problem. 

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[racket-users] handin-server: reader used in checker modules

[Jordan Johnson]

Hi all,

In my last thread regarding the handin server, Matthew suggested:
> A workaround is to use `'lang/htdp-beginner` as the language instead of
> `'(special beginner)`. That happens to avoid a redundant and
> troublemaking `require`.

So, I’m using 'lang/htdp-beginner in an assignment’s checker.rkt file, and I am 
seeing errors that appear to be the result of the handin server using a 
different reader (or configuration thereof) than DrRacket does for BSL. 
Specifically,

(check-expect (+ 10 .25) 10.25)

errors with check-expect’s message about not comparing inexact numbers, but in 
DrRacket+BSL the decimals are treated as exact numbers.

What do I need to change in the checker, in order to obtain the same numeric 
behavior as in DrRacket?

Thanks,
Jordan

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Re: [racket-users] Higher-order units

[Philip McGrath]

The way I have found to do this is to both use the `tag` form and
explicitly rename all signature elements, e.g.:

(define-unit twice-collector@
  (import (tag base (prefix base: collector^)))
  (export collector^)

  (define item/c base:item/c)
  (define (collect . xs)
(apply base:collect (append xs xs

However, I also find this unsatisfying, and I would note that there is a
bug that prevents doing this sort of thing with `define-unit/contract`:
https://github.com/racket/racket/issues/2196

-Philip


On Wed, Oct 17, 2018 at 11:29 AM Alexis King  wrote:

> Imagine I have the following signature:
>
> (define-signature collector^
>   [(contracted
> [item/c contract?]
> [collect (-> item/c ... item/c)])])
>
> Writing a unit that exports this signature is easy. For example, here’s
> one that sums its arguments:
>
> (define-unit sum-collector@
>   (import)
>   (export collector^)
>
>   (define item/c number?)
>
>   (define (collect . xs)
> (apply + xs)))
>
> But imagine I want to instead write a higher-order unit that implements
> collector^ by wrapping an existing collector^ implementation. For example,
> I could write a unit that applies the “base” collector to a doubled list:
>
> (define-unit twice-collector@
>   (import (rename collector^ [base:collect collect]))
>   (export collector^)
>
>   (define (collect . xs)
> (apply base:collect (append xs xs
>
> However, this fails with the following error message:
>
> define-unit: import item/c is exported
>
> The error message is not wrong — item/c is indeed re-exported by
> twice-collector@, but this is because twice-collector@ doesn’t care about
> item/c, only collect. However, I can accept that this program is rejected,
> since it isn’t clear how to link twice-collector@ with a base collector;
> there would be two different units that export collector^ in a single
> compound unit. Therefore, it seems that the intended solution would be to
> tag the import:
>
> (define-unit twice-collector@
>   (import (tag base (rename collector^ [base:collect collect])))
>   (export collector^)
>
>   (define (collect . xs)
> (apply base:collect (append xs xs
>
> However, this program is still rejected with the same error message. In
> this case, I can’t understand why.
>
> One alternative is to avoid using the import mechanism altogether and
> instead define twice-collector@ as a function, creating a sort of “unit
> mixin”:
>
> (define (twice-collector@ base-collector@)
>   (unit
> (import)
> (export collector^)
>
> (define-values/invoke-unit base-collector@
>   (import)
>   (export (rename collector^ [base:collect collect])))
>
> (define (collect . xs)
>   (apply base:collect (append xs xs)
>
> This works, but it seems a little bit silly, since it effectively performs
> the linking manually and runs around the linker.
>
> What is the “correct” way to write higher-order units? I looked for ideas
> in the original units paper (Units: Cool Modules for HOT Languages), and it
> seems to just use the function approach I mentioned above, but the system
> it describes is meaningfully different from the one implemented in Racket,
> and I can’t find very much information about the design of the Racket
> system.
>
> Alexis
>
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[racket-users] Higher-order units

[Alexis King]

Imagine I have the following signature:

(define-signature collector^
  [(contracted
[item/c contract?]
[collect (-> item/c ... item/c)])])

Writing a unit that exports this signature is easy. For example, here’s one 
that sums its arguments:

(define-unit sum-collector@
  (import)
  (export collector^)

  (define item/c number?)

  (define (collect . xs)
(apply + xs)))

But imagine I want to instead write a higher-order unit that implements 
collector^ by wrapping an existing collector^ implementation. For example, I 
could write a unit that applies the “base” collector to a doubled list:

(define-unit twice-collector@
  (import (rename collector^ [base:collect collect]))
  (export collector^)

  (define (collect . xs)
(apply base:collect (append xs xs

However, this fails with the following error message:

define-unit: import item/c is exported

The error message is not wrong — item/c is indeed re-exported by 
twice-collector@, but this is because twice-collector@ doesn’t care about 
item/c, only collect. However, I can accept that this program is rejected, 
since it isn’t clear how to link twice-collector@ with a base collector; there 
would be two different units that export collector^ in a single compound unit. 
Therefore, it seems that the intended solution would be to tag the import:

(define-unit twice-collector@
  (import (tag base (rename collector^ [base:collect collect])))
  (export collector^)

  (define (collect . xs)
(apply base:collect (append xs xs

However, this program is still rejected with the same error message. In this 
case, I can’t understand why.

One alternative is to avoid using the import mechanism altogether and instead 
define twice-collector@ as a function, creating a sort of “unit mixin”:

(define (twice-collector@ base-collector@)
  (unit
(import)
(export collector^)

(define-values/invoke-unit base-collector@
  (import)
  (export (rename collector^ [base:collect collect])))

(define (collect . xs)
  (apply base:collect (append xs xs)

This works, but it seems a little bit silly, since it effectively performs the 
linking manually and runs around the linker.

What is the “correct” way to write higher-order units? I looked for ideas in 
the original units paper (Units: Cool Modules for HOT Languages), and it seems 
to just use the function approach I mentioned above, but the system it 
describes is meaningfully different from the one implemented in Racket, and I 
can’t find very much information about the design of the Racket system.

Alexis

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Re: [racket-users] Web server hits "Sorry, this page has expired. Please go back."

[Luke Whittlesey]

Interesting conversation.. I want to add that #lang web-server does have
file-boxes (
https://docs.racket-lang.org/web-server/stateless.html?q=web-server#%28part._lang%2Ffile-box%29)
for when one might want to preserve data like you would in the store. To
run across multiple machines I would imagine one could also write a
file-box-ish approach to a database, but I haven't gotten there yet.
I think one of the exciting aspects for me regarding stateless
continuations is the idea of running on multiple servers with the only
shared state being cookies and file-boxes. I imagine being able to easily
scale with something like Amazon Lambda. Although I haven't tried that yet
either, so maybe there are roadblocks there.

On Sun, Oct 14, 2018 at 7:55 PM George Neuner  wrote:

> Hi Philip,
>
> Sorry for the delay in responding ... busy day.
>
>
> On 10/14/2018 7:09 AM, Philip McGrath wrote:
>
> On Sun, Oct 14, 2018 at 5:30 AM George Neuner 
> wrote:
>
>> You are absolutely correct that in typical usage globals are not closed
>> over ... normally they are considered mutable shared "communication"
>> state.
>>
>> However, I was speaking more generally:  to be persisted long duration,
>> and be made portable across separate runtime instances, the continuation
>> must close over AND deep copy ANY VALUE the current call chain references.
>> You don't (necessarily) want to restart the continuation a week later, on a
>> different machine, with different values than when it started.
>>
>
> I am definitely not an expert on continuations or PL generally, so I could
> be very wrong. My understanding, though, has been that continuations not
> closing over non-local variables is a property of continuations in general:
> not only in a web context, and not only of the serializable continuations
> from `#lang web-server`. (I used those in my example just because it is
> easy to see what values they do in fact close over.)
>
>
> In the most basic form, the continuation is just "what to do next" - e.g.,
> a pointer to the next code to be executed.  That is [more or less] all that
> is required for a local (within a function) escape continuation because
> [barring stack corruption] all of the EC's stack context is guaranteed to
> exist if/when the continuation is invoked, and that state is needed anyway
> because the program will continue on regardless of how it leaves the scope
> of the EC.
>
> For a non-local EC, you need to return to the stack frame that was current
> when the continuation was created [the rest of the stack can be
> discarded].  In any case, an EC can very compact - just a couple of
> pointers.
>
> [Aside: a Lisp-style exception requires that the entire call stack be kept
> until you know where execution will continue.  You need to return to the
> EC's call frame (to find the exception handler), but Lisp allows exceptions
> to introspect/modify the environment and then to return control to the
> point of the exception (where the intrinsic EC was invoked).  Scheme
> doesn't have this capability.]
>
>
> General continuations - such as can implement coroutines - need to capture
> much more state.  The entire stack state of the current function call chain
> at the point where the continuation is created must be preserved in order
> to reenter the continuation.
> [this is where stateful and stateless part company - more below]
>
>
> Winding continuations are an extension of general continuations to the
> requirements of dynamic-wind.  In addition to whatever is required for a
> general continuation, a winding continuation has preamble code that must be
> executed whenever a continuation enters the winding scope, and postamble
> code to be executed whenever (and how ever) execution leaves the winding
> scope.
>
>
> The long winded point here is that continuations have no canon
> implementation - if you want more functionality from them, you need to
> capture/preserve more state.
>
>
>
> The web-server is multi-threaded, and the stack state of each thread is
> unique - so for each thread in which a continuation is created in the
> stateful server, the thread's call stack must be preserved.
>
> The stateless server is a different case.  CPS code doesn't use a call
> stack ... or rather its "stack" only ever contains one useful frame - the
> current one.  So there's only one call frame per thread that needs to be
> preserved rather than a (maybe large) stack of them.  This is why stateless
> server continuations - in general - use much less memory than stateful
> server continuations.
>
> [The above is a bit simplistic because Scheme supports nested functions
> with non-local variable access.  [Scheme also supports 1st class closures
> based on nested functions, but that is not what this is about.]   To
> support non-local access without a stack, CPS code needs to construct
> shared environments on the fly as the program executes to communicate
> values from producers to consumers, and these environments (where
> applicable)

Re: [racket-users] How to download image using net/http-client?

[George Neuner]

On 10/17/2018 9:06 AM, George Neuner wrote:


But be aware that you may run into images that do *^NOT* have 
Content-Length  set.  Many sites assume images are only for display by 
the browser.


Sorry.  Should have been  'do not'

Gorge

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Re: [racket-users] How to download image using net/http-client?

[George Neuner]

On 10/16/2018 4:02 PM, Philippe Mechaï wrote:
Indeed the issue is with your call to pipe-content-length which is to 
be used with pipes and is not the Content-Length header returned when 
doing HTTP requests.
The Content-Length can be found in the list of headers returned by the 
call to http-sendrecv (second value or b in your let-values call). You 
can parse these using the net/head module.



But be aware that you may run into images that do have Content-Length  
set.  Many sites assume images are only for display by the browser.


George

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