Re: expression and definition context in Scheme

[Aleix Conchillo Flaqué]

Just came here to say: Congratulations Linus! That was definitely the most
important part of your message :-).

Aleix

On Tue, Aug 30, 2022 at 3:48 AM Linus Björnstam <
linus.bjornstam@veryfast.biz> wrote:

> 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
>
>