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* expression and definition context in Scheme
@ 2022-08-27 16:48 Damien Mattei
  2022-08-27 17:00 ` Maxime Devos
  2022-08-30 10:47 ` Linus Björnstam
  0 siblings, 2 replies; 12+ messages in thread
From: Damien Mattei @ 2022-08-27 16:48 UTC (permalink / raw)
  To: guile-user, guile-devel

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


^ permalink raw reply	[flat|nested] 12+ messages in thread

end of thread, other threads:[~2022-09-07 23:12 UTC | newest]

Thread overview: 12+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
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
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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