From: "Linus Björnstam" <linus.bjornstam@veryfast.biz>
To: "Damien Mattei" <damien.mattei@gmail.com>,
guile-user <guile-user@gnu.org>,
guile-devel <guile-devel@gnu.org>
Subject: Re: expression and definition context in Scheme
Date: Tue, 30 Aug 2022 12:47:06 +0200 [thread overview]
Message-ID: <a1f66de8-15fe-4166-8ef5-5864272aeedb@www.fastmail.com> (raw)
In-Reply-To: <CADEOadf5pS917wVJhrNtMU7N_-3=g71CEeRKai+-NZMXsVE1BQ@mail.gmail.com>
I am working on a patch to guile to add definitions to just about every body except for (begin ...) outside definition context.
The patch is trivial, but I have to document it and a patch to r6rs that makes the r6rs cons work according to spec.
I had a kid recently so it might take some time before I have any computer time, so if anyone has some time this is a really simple thing. You can find the first patch somewhere in this mailing list, it only changes the (begin ...)s in the derived forms in (ice-9 boot-9) to (let () ...). Then i was going to copy the cond and case from the r6rs appendix and add some error reporting.
The most difficult part is documenting it :)
Andy have the idea hos blessing, and will mean guile gets define in expression context in when, unless, cond, case, while, and do as well as in derived forms.
--
Linus Björnstam
On Sat, 27 Aug 2022, at 18:48, Damien Mattei wrote:
> Hello,
>
> i'm facing sometimes recursively the problem to have definitions in
> expression context, which i manage every time by adding an upper empty
> (let () my definitions goes here )
> the last case i was facing this probleme is defining a 'for macro:
>
> ;; scheme@(guile-user)> (for ({i <+ 0} {i < 5} {i <- {i + 1}}) (display
> i) (newline))
> ;; 0
> ;; 1
> ;; 2
> ;; 3
> ;; 4
>
>
> (define-syntax for
>
> (syntax-rules ()
>
> ((_ (init test incrmt) b1 ...)
>
> (let ()
> init
> (let loop ()
> (when test
> b1 ...
> incrmt
> (loop)))))))
>
> this one fails in my Scheme+ code below:
> (define (compute-carries n)
>
> (for ( {k <+ 0} {k <= n} {k <- {k + 1}} )
>
> { Ckp1 <+ (compute-Ck-plus1 k) }
> (display-nl Ckp1)))
>
> because { Ckp1 <+ (compute-Ck-plus1 k) } expands in :
> (define Ckp1 (compute-Ck-plus1 k))
> and i get a compilation error:
> ;;; Syntax error:
> ;;; logic-syracuse+.scm:15:7: definition in expression context, where
> definitions are not allowed, in form (define Ckp1 (compute-Ck-plus1 k))
>
> so i replace my 'for macro definition with:
>
> (define-syntax for
>
> (syntax-rules ()
>
> ((_ (init test incrmt) b1 ...)
>
> (let ()
> init
> (let loop ()
> (when test
> (let ()
> b1 ...
> incrmt
> (loop))))))))
>
> and it works, but you will notice an abusive use of empty (let () ...)
> in the code to avoid the restrictions of definitions not allowed in
> expression context.
>
> My ideas is as it is so easy to cheat the compiler from seeing the
> expressio context why does the compiler restrict this? expression and
> defintion context, i'm not sure they are in scheme standarts, are they
> really usefull?
> why not remove this from Scheme at all?
>
> Regards,
>
> Damien
next prev parent reply other threads:[~2022-08-30 10:47 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-27 16:48 expression and definition context in Scheme Damien Mattei
2022-08-27 17:00 ` Maxime Devos
2022-08-27 19:02 ` Damien Mattei
2022-08-27 19:14 ` Maxime Devos
2022-08-27 19:20 ` Maxime Devos
[not found] ` <CADEOadfCCLaqEvzLYUzEYhg94pqN3r9=c6LT=FmCivJsPudUzg@mail.gmail.com>
[not found] ` <b95df274-b727-5bb9-4657-f263a837e0f1@telenet.be>
[not found] ` <CADEOadfKR_u3qS5N5MwRuCRTxPjKi_wyWzgv-xJJ96e56X2meg@mail.gmail.com>
2022-08-29 10:10 ` Fwd: " Damien Mattei
2022-08-30 10:47 ` Linus Björnstam [this message]
2022-08-31 7:01 ` Damien Mattei
2022-08-31 15:29 ` Maxime Devos
2022-09-01 20:21 ` Damien Mattei
2022-09-04 8:08 ` Damien Mattei
2022-09-07 23:12 ` Aleix Conchillo Flaqué
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