Re: [racket-users] DrRackGit & LibGit2

['John Clements' via users-redirect]

> On Dec 7, 2017, at 2:34 PM, Stephen De Gabrielle  
> wrote:
> 
> I should be clear, I ran the libgit2 tests by hard coding the location of the 
> library and running them manually. I don’t really know what I’m doing so that 
> was the best I could muddle through.

That doesn’t sound ideal. Probably it’s better to deliver the library with the 
package on macos & windows, & ask user to install using package manager on 
linux. Sounds like entertaining work, and I’ve been meaning to take a look at 
this for quite a while now, but I definitely don’t have any time before next 
Tuesday.

John

> S.
> 
> On Thu, 7 Dec 2017 at 22:28, Stephen De Gabrielle  
> wrote:
> Only on macOS, but most tests passed.
> S.
> On Thu, 7 Dec 2017 at 22:15, John Clements  wrote:
> Bradley was a student of mine, so I’m interested generally. Have you checked 
> to see whether adding the libgit2 dependency allows the tests to pass?
> 
> John
> 
> > On Dec 7, 2017, at 10:27 AM, Stephen De Gabrielle  
> > wrote:
> >
> >
> > Hi,
> > https://pkgs.racket-lang.org/package/Drrackgit
> > https://github.com/bbusching/drrackgit
> > https://pkgs.racket-lang.org/package/libgit2
> > https://github.com/bbusching/libgit2
> > https://libgit2.github.com/
> >
> > I was very excited when I found the DrRackGit tool in the Racket packages, 
> > but saddened to find it is currently failing.
> >
> > Bradley, the original developer was kind enough to diagnose the issue ( it 
> > is missing the dependency on the libgit2 library.) and add a compatible 
> > licence, but isn’t able to maintain it at the moment.
> >
> > Is anyone interested in fixing the libgit2 package?
> >
> > Kind regards,
> >
> > Stephen
> >
> > PS I have built the lib2git library on macOS, but I lack the required 
> > experience in FFI and I can’t currently build for Linux or windows.
> >
> >
> >
> >
> >
> >
> > --
> > Kind regards,
> > Stephen
> > --
> > Ealing (London), UK
> >
> > --
> > 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.
> 
> 
> 
> -- 
> Kind regards,
> Stephen
> --
> Ealing (London), UK
> -- 
> Kind regards,
> Stephen
> --
> Ealing (London), UK



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Re: [racket-users] DrRackGit & LibGit2

[Stephen De Gabrielle]

I should be clear, I ran the libgit2 tests by hard coding the location of
the library and running them manually. I don’t really know what I’m doing
so that was the best I could muddle through.
S.

On Thu, 7 Dec 2017 at 22:28, Stephen De Gabrielle 
wrote:

> Only on macOS, but most tests passed.
> S.
> On Thu, 7 Dec 2017 at 22:15, John Clements 
> wrote:
>
>> Bradley was a student of mine, so I’m interested generally. Have you
>> checked to see whether adding the libgit2 dependency allows the tests to
>> pass?
>>
>> John
>>
>> > On Dec 7, 2017, at 10:27 AM, Stephen De Gabrielle <
>> spdegabrie...@gmail.com> wrote:
>> >
>> >
>> > Hi,
>> > https://pkgs.racket-lang.org/package/Drrackgit
>> > https://github.com/bbusching/drrackgit
>> > https://pkgs.racket-lang.org/package/libgit2
>> > https://github.com/bbusching/libgit2
>> > https://libgit2.github.com/
>> >
>> > I was very excited when I found the DrRackGit tool in the Racket
>> packages, but saddened to find it is currently failing.
>> >
>> > Bradley, the original developer was kind enough to diagnose the issue (
>> it is missing the dependency on the libgit2 library.) and add a compatible
>> licence, but isn’t able to maintain it at the moment.
>> >
>> > Is anyone interested in fixing the libgit2 package?
>> >
>> > Kind regards,
>> >
>> > Stephen
>> >
>> > PS I have built the lib2git library on macOS, but I lack the required
>> experience in FFI and I can’t currently build for Linux or windows.
>> >
>> >
>> >
>> >
>> >
>> >
>> > --
>> > Kind regards,
>> > Stephen
>> > --
>> > Ealing (London), UK
>> >
>> > --
>> > 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.
>>
>>
>>
>> --
> Kind regards,
> Stephen
> --
> Ealing (London), UK
>
-- 
Kind regards,
Stephen
--
Ealing (London), UK

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Re: [racket-users] DrRackGit & LibGit2

[Stephen De Gabrielle]

Only on macOS, but most tests passed.
S.
On Thu, 7 Dec 2017 at 22:15, John Clements 
wrote:

> Bradley was a student of mine, so I’m interested generally. Have you
> checked to see whether adding the libgit2 dependency allows the tests to
> pass?
>
> John
>
> > On Dec 7, 2017, at 10:27 AM, Stephen De Gabrielle <
> spdegabrie...@gmail.com> wrote:
> >
> >
> > Hi,
> > https://pkgs.racket-lang.org/package/Drrackgit
> > https://github.com/bbusching/drrackgit
> > https://pkgs.racket-lang.org/package/libgit2
> > https://github.com/bbusching/libgit2
> > https://libgit2.github.com/
> >
> > I was very excited when I found the DrRackGit tool in the Racket
> packages, but saddened to find it is currently failing.
> >
> > Bradley, the original developer was kind enough to diagnose the issue (
> it is missing the dependency on the libgit2 library.) and add a compatible
> licence, but isn’t able to maintain it at the moment.
> >
> > Is anyone interested in fixing the libgit2 package?
> >
> > Kind regards,
> >
> > Stephen
> >
> > PS I have built the lib2git library on macOS, but I lack the required
> experience in FFI and I can’t currently build for Linux or windows.
> >
> >
> >
> >
> >
> >
> > --
> > Kind regards,
> > Stephen
> > --
> > Ealing (London), UK
> >
> > --
> > 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.
>
>
>
> --
Kind regards,
Stephen
--
Ealing (London), UK

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Re: [racket-users] DrRackGit & LibGit2

['John Clements' via users-redirect]

Bradley was a student of mine, so I’m interested generally. Have you checked to 
see whether adding the libgit2 dependency allows the tests to pass?

John

> On Dec 7, 2017, at 10:27 AM, Stephen De Gabrielle  
> wrote:
> 
> 
> Hi, 
> https://pkgs.racket-lang.org/package/Drrackgit
> https://github.com/bbusching/drrackgit  
> https://pkgs.racket-lang.org/package/libgit2
> https://github.com/bbusching/libgit2
> https://libgit2.github.com/
> 
> I was very excited when I found the DrRackGit tool in the Racket packages, 
> but saddened to find it is currently failing.
> 
> Bradley, the original developer was kind enough to diagnose the issue ( it is 
> missing the dependency on the libgit2 library.) and add a compatible licence, 
> but isn’t able to maintain it at the moment.
> 
> Is anyone interested in fixing the libgit2 package? 
> 
> Kind regards,
> 
> Stephen
> 
> PS I have built the lib2git library on macOS, but I lack the required 
> experience in FFI and I can’t currently build for Linux or windows.
> 
> 
> 
> 
> 
> 
> -- 
> Kind regards,
> Stephen
> --
> Ealing (London), UK
> 
> -- 
> You received this message because you are subscribed to the Google Groups 
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[racket-users] Re: Scribble Top-level Evaluation Order of Define

[gfb]

>From the docs for scribble/base:
 

> The scribble/base 
>  language 
> provides functions and forms that can be used from code written either in 
> Racket or with @ expressions. It essentially extends racket/base 
> , except that 
> top-level forms within a module using the scribble/base 
>  language 
> are treated as document content (like scribble/doclang 
> 
> ).


It sounds like it might be doing something special with top-level forms. 
The docs for scribble/doclang say this:

The first three elements in the #%module-begin 
> ’s
>  
> body must be the id, post-process, and exprs arguments.


Expanding your example with the macro stepper, witht "Macro hiding:" 
Disable, ends up with:

(module anonymous-module scribble/base/lang
>   (#%module-begin
>(⋯)
>(define-values (foo) (begin (#%app printf (quote "two")) (quote 
> "three")))
>(⋯)
>(define-values (doc)
>  (#%app values (#%app idY184 lifted.0 (#%app list
>  (quote "\n")
>  (#%app:18 
> check-pre-part:19
>(#%app:20 
> printf (quote "one"))
>   
>  (quote-syntax:19 loc:19))
>  (quote "\n")
>  (#%app:21 
> check-pre-part:22
>(#%app:23 
> printf foo)
>   
>  (quote-syntax:22 loc:22))
>  (quote "\n")
>(#%provide:24 doc)))


So what it's doing is gathering all top-level definitions and moving them 
before the top-level expressions, to then define the document as a 
combination of the expressions. 

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Re: [racket-users] copying prefab structs

[Sam Tobin-Hochstadt]

I would write the latter as

#lang racket
(require racket/struct)
(define (copy-prefab s)
  (apply make-prefab-struct (prefab-struct-key s)
 (struct->list s)))

Sam

On Thu, Dec 7, 2017 at 11:53 AM, David Vanderson
 wrote:
> I would appreciate any advice on the best way to copy a prefab struct that
> could be a sub-struct.  I have been using write/read but felt bad about it.
> Trying again now gave me a new option. Is there a better solution out there?
>
> (define (copy-writable s)
>   (read (open-input-string (with-output-to-string (lambda () (write s))
>
> (define (copy-prefab s)
>   (call-with-values (lambda () (vector->values (struct->vector s) 1))
> (lambda args (apply make-prefab-struct
> (prefab-struct-key s) args
>
> Thanks,
> Dave
>
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Re: [racket-users] structs vs classes

[Greg Hendershott]

p.s. The package supplies a `define-struct-updaters` macro separately.
This is handy because you can use it on any structs -- including those
from other modules you don't control. If you do this frequently for
your own structs, you could of course write a little macro to `(begin
(struct id ) (define-struct-updaters id))`.

On Thu, Dec 7, 2017 at 2:19 PM, Greg Hendershott
 wrote:
> On Thu, Dec 7, 2017 at 11:08 AM, David Storrs  wrote:
>> assembled.  There's no reason not to use functional update, but I
>> haven't seen a straightforward way to do that since there is no
>> 'set-db-chunk-foo' as opposed to 'set-db-chunk-foo!'.   I guess I need
>> to use 'struct-copy'?  At that point it seems more sensible to have a
>
> There is a struct-update package:
>
>   https://pkgd.racket-lang.org/pkgn/package/struct-update
>
> Doc:
>
>   http://docs.racket-lang.org/struct-update/index.html

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Re: [racket-users] structs vs classes

[Greg Hendershott]

On Thu, Dec 7, 2017 at 11:08 AM, David Storrs  wrote:
> assembled.  There's no reason not to use functional update, but I
> haven't seen a straightforward way to do that since there is no
> 'set-db-chunk-foo' as opposed to 'set-db-chunk-foo!'.   I guess I need
> to use 'struct-copy'?  At that point it seems more sensible to have a

There is a struct-update package:

  https://pkgd.racket-lang.org/pkgn/package/struct-update

Doc:

  http://docs.racket-lang.org/struct-update/index.html

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[racket-users] DrRackGit & LibGit2

[Stephen De Gabrielle]

Hi,
https://pkgs.racket-lang.org/package/Drrackgit
https://github.com/bbusching/drrackgit
https://pkgs.racket-lang.org/package/libgit2
https://github.com/bbusching/libgit2
https://libgit2.github.com/

I was very excited when I found the DrRackGit tool in the Racket packages,
but saddened to find it is currently failing.

Bradley, the original developer was kind enough to diagnose the issue ( it
is missing the dependency on the libgit2 library
.) and add a compatible licence, but isn’t
able to maintain it at the moment.

Is anyone interested in fixing the libgit2
 package?

Kind regards,

Stephen

PS I have built the lib2git library on macOS, but I lack the required
experience in FFI and I can’t currently build for Linux or windows.






-- 
Kind regards,
Stephen
--
Ealing (London), UK

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Re: [racket-users] structs vs classes

[Shu-Hung You]

I'm not sure whether the following is a good idea, but it more or less
works. The impersonate-struct function allows one to redirect the some
or all of accessor and mutator functions on *individual struct
instances*. If the actual constructor is hidden from the user and only
a ``smart'' constructor is exported, then we can put impersonators on
all struct instances.

#lang racket

(provide
 smart-db-chunk-approach-1
 smart-db-chunk-approach-2

 (except-out
  (struct-out db-chunk)
  make-db-chunk))

(struct db-chunk (scratchdir-path chunkdir-path) #:mutable
  #:constructor-name make-db-chunk
  #:transparent ;; for impersonators
  ;; and perhaps guards
  )


;; approach 1: save path-string? in struct fields and redirect accessors
(define (smart-db-chunk-approach-1 . args)
  (impersonate-struct
   (apply make-db-chunk args)

   db-chunk-scratchdir-path
   (λ (chnk v) (if (path? v) (path->string v) v))
   db-chunk-chunkdir-path
   (λ (chnk v) (if (path? v) (path->string v) v

(define s1 (smart-db-chunk-approach-1 (string->path "/usr") "/var"))
(string? (db-chunk-scratchdir-path s1))



;; approach 2: redirect mutators and only save string? in struct fields
(define (smart-db-chunk-approach-2 . args)
  (impersonate-struct
   (apply make-db-chunk args)

   set-db-chunk-scratchdir-path!
   (λ (chnk v) (if (path? v) (path->string v) v))
   set-db-chunk-chunkdir-path!
   (λ (chnk v) (if (path? v) (path->string v) v

(define s2 (smart-db-chunk-approach-2 "/usr" "/var"))
(set-db-chunk-chunkdir-path! s2 (string->path "/bin"))
(string? (db-chunk-scratchdir-path s2))

--Shu-Hung

On Thu, Dec 7, 2017 at 10:10 AM, David Storrs  wrote:
> Wow.  Thank you, Matthias.  That's impressive.
>
> On Thu, Dec 7, 2017 at 10:46 AM, Matthias Felleisen
>  wrote:
>>
>> On Dec 6, 2017, at 10:45 PM, David Storrs  wrote:
>>
>> I have a struct that looks like this (simplified for concision):
>>
>> (struct db-chunk (scratchdir-path chunkdir-path) #:mutable)
>>
>> I'd like to ensure that:
>>
>> 1) At least one of the fields must be set at creation
>> 2) Both fields will accept only a path-string? value
>> 3) Both fields will return string? when accessed, as that makes it
>> easier to insert into the DB when the time comes.
>>
>> I can put a #:guard parameter on db-chunk that will check #1 and #2
>> and (if necessary) transform the input into a string in order to
>> satisfy #3, but that only works at initialization.  From there I would
>> need to redefine the mutator:
>>
>> (struct db-chunk (scratchdir-path chunkdir-path) #:mutable)
>>
>> (let ([old-mut set-db-chunk-scratchdir-path!])
>>  (set! set-db-chunk-scratchdir-path!
>>  (lambda (chnk val)
>>(old-mut chnk (if (path? val) (path->string val) val))
>>
>> Alternatively, I could define my struct and then make a bunch of
>> custom manipulation functions, but then (a) I've got these unguarded
>> functions floating around and (b) I've given up a major advantage of
>> structs, which is their concision.
>>
>>
>> Consider the introduction of a syntax transformer that refines struct
>> according to your needs. The sketch below is a bit brittle but if you
>> need to strengthen it, I am sure our virtual macrologists can help.
>>
>> — Matthias
>>
>>
>>
>>
>> #lang racket
>>
>> ;; ---
>> ;; in some module/file:
>>
>> #;
>> (provide
>>  ;; SYNTAX(struct/ccc Id [Id ...] MINUS Id ...)
>>  ;; SEMANTICS (struct/ccc a [b ...] MINUS c ...) is like
>>  ;;  (struct a [b ...]) but hides the struct-defined functions
>>  ;;  c ...
>>  struct/ccc)
>>
>> (define-syntax (struct/ccc stx)
>>   (syntax-case stx (MINUS PLUS)
>> [(struct/ccc s [a ...] (MINUS x ...) (PLUS [f ex] ...))
>>  ;; ==>
>>  (let ((my-require (datum->syntax stx '(require 'server
>>#`(begin
>>(module server #,(datum->syntax stx 'racket)
>>  (provide
>>   (except-out (struct-out s) x ...))
>>  (struct s [a ...])
>>  (set! f ex) ...)
>>#,my-require))]))
>>
>> ;; ---
>> ;; require the module here
>>
>> (struct/ccc s [a b c]
>> (MINUS s-a)
>> (PLUS [s-b (let ((old s-b)) (λ (x) (define b (old x)) (* 2
>> b)))]))
>> (define s0 (s 1 2 3))
>> (s-b s0)
>
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[racket-users] copying prefab structs

[David Vanderson]

I would appreciate any advice on the best way to copy a prefab struct that
could be a sub-struct.  I have been using write/read but felt bad about it.
Trying again now gave me a new option. Is there a better solution out there?

(define (copy-writable s)
  (read (open-input-string (with-output-to-string (lambda () (write s))

(define (copy-prefab s)
  (call-with-values (lambda () (vector->values (struct->vector s) 1))
(lambda args (apply make-prefab-struct
(prefab-struct-key s) args

Thanks,
Dave

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Re: [racket-users] structs vs classes

[David Storrs]

Wow.  Thank you, Matthias.  That's impressive.

On Thu, Dec 7, 2017 at 10:46 AM, Matthias Felleisen
 wrote:
>
> On Dec 6, 2017, at 10:45 PM, David Storrs  wrote:
>
> I have a struct that looks like this (simplified for concision):
>
> (struct db-chunk (scratchdir-path chunkdir-path) #:mutable)
>
> I'd like to ensure that:
>
> 1) At least one of the fields must be set at creation
> 2) Both fields will accept only a path-string? value
> 3) Both fields will return string? when accessed, as that makes it
> easier to insert into the DB when the time comes.
>
> I can put a #:guard parameter on db-chunk that will check #1 and #2
> and (if necessary) transform the input into a string in order to
> satisfy #3, but that only works at initialization.  From there I would
> need to redefine the mutator:
>
> (struct db-chunk (scratchdir-path chunkdir-path) #:mutable)
>
> (let ([old-mut set-db-chunk-scratchdir-path!])
>  (set! set-db-chunk-scratchdir-path!
>  (lambda (chnk val)
>(old-mut chnk (if (path? val) (path->string val) val))
>
> Alternatively, I could define my struct and then make a bunch of
> custom manipulation functions, but then (a) I've got these unguarded
> functions floating around and (b) I've given up a major advantage of
> structs, which is their concision.
>
>
> Consider the introduction of a syntax transformer that refines struct
> according to your needs. The sketch below is a bit brittle but if you
> need to strengthen it, I am sure our virtual macrologists can help.
>
> — Matthias
>
>
>
>
> #lang racket
>
> ;; ---
> ;; in some module/file:
>
> #;
> (provide
>  ;; SYNTAX(struct/ccc Id [Id ...] MINUS Id ...)
>  ;; SEMANTICS (struct/ccc a [b ...] MINUS c ...) is like
>  ;;  (struct a [b ...]) but hides the struct-defined functions
>  ;;  c ...
>  struct/ccc)
>
> (define-syntax (struct/ccc stx)
>   (syntax-case stx (MINUS PLUS)
> [(struct/ccc s [a ...] (MINUS x ...) (PLUS [f ex] ...))
>  ;; ==>
>  (let ((my-require (datum->syntax stx '(require 'server
>#`(begin
>(module server #,(datum->syntax stx 'racket)
>  (provide
>   (except-out (struct-out s) x ...))
>  (struct s [a ...])
>  (set! f ex) ...)
>#,my-require))]))
>
> ;; ---
> ;; require the module here
>
> (struct/ccc s [a b c]
> (MINUS s-a)
> (PLUS [s-b (let ((old s-b)) (λ (x) (define b (old x)) (* 2
> b)))]))
> (define s0 (s 1 2 3))
> (s-b s0)

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Re: [racket-users] structs vs classes

[David Storrs]

On Wed, Dec 6, 2017 at 11:23 PM, Philip McGrath
 wrote:
>
>> 1) At least one of the fields must be set at creation
>
>
> What does it mean for one of the fields not to be set at creation? Is there
> implicitly some not-initialized value like #f that is also valid for the
> constructor? Do you want your accessor functions to raise an error if
> someone tries to access a non-initialized field?

Our system pushes chunks of data around.  A chunk is a small file on
disk that lives in the '.chunks/' directory.  If it's being actively
worked with then it gets copied to the '.scratch/' directory.  This
struct tracks where the file currently exists; you wouldn't create one
of these if the chunk isn't on disk (there are other mechanisms for
that), so at least one and possibly both of the paths must be set.

Yes, the fields will default to #f, although throwing an exception on
access is also an option.

>
>>
>> 3) Both fields will return string? when accessed, as that makes it
>> easier to insert into the DB when the time comes.
>
>
> As you may know, the reference warns that "some paths may not be
> representable as strings" and that "decoding and re-encoding the path’s byte
> string [to a string] may lose information". You may be able to ignore this
> in your particular circumstances, but you might consider using the
> byte-string representation of paths (which doesn't have this problem), or
> this might be an argument in favor of multiple accessor functions/methods
> which convert to different formats.
>
> More broadly, what is the meaning of mutating one of the fields of your
> struct? Is there a compelling reason not to use an immutable struct and
> functional update?

Good question, I should have provided the context originally.  Sorry for that.

Mutating the fields would usually happen when a chunk gets copied to
the scratchdir to be worked with, deleted from the scratchdir when
you're done with it, or moved to the chunksdir if it's finished being
assembled.  There's no reason not to use functional update, but I
haven't seen a straightforward way to do that since there is no
'set-db-chunk-foo' as opposed to 'set-db-chunk-foo!'.   I guess I need
to use 'struct-copy'?  At that point it seems more sensible to have a
small module pertaining solely to managing chunk information that uses
a hash for a backing store, since hashes are easier to work with.  Or
go full OO and create an object that tracks it for me.


>> (let ([old-mut set-db-chunk-scratchdir-path!])
>>   (set! set-db-chunk-scratchdir-path!
>>   (lambda (chnk val)
>> (old-mut chnk (if (path? val) (path->string val) val))
>
>
> Regardless of anything else, I would definitely not do this. To me, this
> would "work against the idea of functional programming" far more than using
> a class. (Also, I have the impression that mutating a module-level variable
> can have performance implications.)

Oh, absolutely.  This is krunky as heck and when I found myself
considering it is when I realized I needed to back away from the
keyboard and ask the list for the right way to do it.


>
> If I wanted to use a mutable struct with the sorts of wrappers you describe,
> I would write a module with the struct definition and the wrapper functions
> and export only the interface with guards, perhaps using rename-out (or the
> rename option with contract-out) to give the wrappers nice external names. I
> will sometimes even use a submodule for the purpose of packaging up an
> abstraction and giving only a high-level interface to surrounding code.
>
> Alternatively, using a class could be a valid choice, but I would probably
> only consider that if you have other code that uses racket/class: it seems
> like a fairly large set of concepts to add just for this. I have come to
> appreciate racket/class in the right circumstances, especially for
> representing stateful objects or lots of variants with slight variations in
> method behavior. I don't think there's anything inherently un-functional
> about classes (and I'd say an immutable object is more functional than a
> mutable struct).


Good advice, thanks.

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Re: [racket-users] structs vs classes

[Matthias Felleisen]

> On Dec 6, 2017, at 10:45 PM, David Storrs  wrote:
> 
> I have a struct that looks like this (simplified for concision):
> 
> (struct db-chunk (scratchdir-path chunkdir-path) #:mutable)
> 
> I'd like to ensure that:
> 
> 1) At least one of the fields must be set at creation
> 2) Both fields will accept only a path-string? value
> 3) Both fields will return string? when accessed, as that makes it
> easier to insert into the DB when the time comes.
> 
> I can put a #:guard parameter on db-chunk that will check #1 and #2
> and (if necessary) transform the input into a string in order to
> satisfy #3, but that only works at initialization.  From there I would
> need to redefine the mutator:
> 
> (struct db-chunk (scratchdir-path chunkdir-path) #:mutable)
> 
> (let ([old-mut set-db-chunk-scratchdir-path!])
>  (set! set-db-chunk-scratchdir-path!
>  (lambda (chnk val)
>(old-mut chnk (if (path? val) (path->string val) val))
> 
> Alternatively, I could define my struct and then make a bunch of
> custom manipulation functions, but then (a) I've got these unguarded
> functions floating around and (b) I've given up a major advantage of
> structs, which is their concision.
> 

Consider the introduction of a syntax transformer that refines struct 
according to your needs. The sketch below is a bit brittle but if you 
need to strengthen it, I am sure our virtual macrologists can help. 

— Matthias




#lang racket

;; ---
;; in some module/file:

#;
(provide
 ;; SYNTAX(struct/ccc Id [Id ...] MINUS Id ...)
 ;; SEMANTICS (struct/ccc a [b ...] MINUS c ...) is like
 ;;  (struct a [b ...]) but hides the struct-defined functions
 ;;  c ...
 struct/ccc)

(define-syntax (struct/ccc stx)
  (syntax-case stx (MINUS PLUS)
[(struct/ccc s [a ...] (MINUS x ...) (PLUS [f ex] ...))
 ;; ==>
 (let ((my-require (datum->syntax stx '(require 'server
   #`(begin
   (module server #,(datum->syntax stx 'racket)
 (provide 
  (except-out (struct-out s) x ...))
 (struct s [a ...])
 (set! f ex) ...)
   #,my-require))]))

;; ---
;; require the module here

(struct/ccc s [a b c]
(MINUS s-a)
(PLUS [s-b (let ((old s-b)) (λ (x) (define b (old x)) (* 2 b)))]))
(define s0 (s 1 2 3))
(s-b s0)

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