Re: expression and definition context in Scheme

[Linus Björnstam]

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