progv in scheme

progv in scheme

[Panicz Maciej Godek]

Hello,
Is there any clever way of binding values to the list of unknown
symbols in scheme?

In common lisp there is a form "progv" that takes the list of symbols
and their corresponding values and binds them within the body of
progv.

It is possible to do it using eval, like this:
(define (bind-and-eval symbols values body)
(eval `((lambda ,symbols ,body) . ,values)
      (interaction-environment)))
(define-syntax let-symbols
(syntax-rules ()
  ((_ symbols values (body ...))
   (bind-and-eval symbols values (quote (body ...))))))

but using eval for this just seems too heavy. Is there any way of
doing it that would be more legal?

Best regards,
M.

Re: progv in scheme

[Andrew Gwozdziewycz]

On Tue, Sep 13, 2011 at 9:54 AM, Panicz Maciej Godek
<godek.maciek@gmail.com> wrote:
> Hello,
> Is there any clever way of binding values to the list of unknown
> symbols in scheme?
>
> In common lisp there is a form "progv" that takes the list of symbols
> and their corresponding values and binds them within the body of
> progv.
>
> It is possible to do it using eval, like this:
> (define (bind-and-eval symbols values body)
> (eval `((lambda ,symbols ,body) . ,values)
>      (interaction-environment)))
> (define-syntax let-symbols
> (syntax-rules ()
>  ((_ symbols values (body ...))
>   (bind-and-eval symbols values (quote (body ...))))))
>
> but using eval for this just seems too heavy. Is there any way of
> doing it that would be more legal?
>
> Best regards,
> M.

Seems likely that you could use `syntax-case' to create a `let` out of these:

(define-syntax progv
  (lambda (stx)
    (define (create-bindings syms vals)
      (datum->syntax stx (zip (syntax->datum syms) (syntax->datum vals))))
    (syntax-case stx ()
      ((_ symbols values body ...)
       (with-syntax ((bindings (create-bindings #'symbols #'values)))
          #'(let bindings
               (begin body ...)))))))



;; usage
(progv (foo bar baz) (1 2 3)
   (format (current-output-port) "Values: ~a ~a ~a\n" foo bar baz))
;; output: "Values: 1 2 3"

Hope this helps!

Andrew
-- 
http://www.apgwoz.com

Re: progv in scheme

[Andrew Gwozdziewycz]

On Tue, Sep 13, 2011 at 10:25 AM, Andrew Gwozdziewycz <apgwoz@gmail.com> wrote:
> On Tue, Sep 13, 2011 at 9:54 AM, Panicz Maciej Godek
> <godek.maciek@gmail.com> wrote:
>> Hello,
>> Is there any clever way of binding values to the list of unknown
>> symbols in scheme?
>>
>> In common lisp there is a form "progv" that takes the list of symbols
>> and their corresponding values and binds them within the body of
>> progv.
>>
>> It is possible to do it using eval, like this:
>> (define (bind-and-eval symbols values body)
>> (eval `((lambda ,symbols ,body) . ,values)
>>      (interaction-environment)))
>> (define-syntax let-symbols
>> (syntax-rules ()
>>  ((_ symbols values (body ...))
>>   (bind-and-eval symbols values (quote (body ...))))))
>>
>> but using eval for this just seems too heavy. Is there any way of
>> doing it that would be more legal?
>>
>> Best regards,
>> M.
>
> Seems likely that you could use `syntax-case' to create a `let` out of these:
>
> (define-syntax progv
>  (lambda (stx)
>    (define (create-bindings syms vals)
>      (datum->syntax stx (zip (syntax->datum syms) (syntax->datum vals))))
>    (syntax-case stx ()
>      ((_ symbols values body ...)
>       (with-syntax ((bindings (create-bindings #'symbols #'values)))
>          #'(let bindings
>               (begin body ...)))))))
>
>
>
> ;; usage
> (progv (foo bar baz) (1 2 3)
>   (format (current-output-port) "Values: ~a ~a ~a\n" foo bar baz))
> ;; output: "Values: 1 2 3"
>
> Hope this helps!
>
> Andrew

I should point out that I'm using zip in srfi-1 (use-modules (srfi srfi-1))



-- 
http://www.apgwoz.com

Re: progv in scheme

[Panicz Maciej Godek]

2011/9/13, Andrew Gwozdziewycz <apgwoz@gmail.com>:
>
> Seems likely that you could use `syntax-case' to create a `let` out of
> these:
>
> (define-syntax progv
>   (lambda (stx)
>     (define (create-bindings syms vals)
>       (datum->syntax stx (zip (syntax->datum syms) (syntax->datum vals))))
>     (syntax-case stx ()
>       ((_ symbols values body ...)
>        (with-syntax ((bindings (create-bindings #'symbols #'values)))
>           #'(let bindings
>                (begin body ...)))))))
>
>
>
> ;; usage
> (progv (foo bar baz) (1 2 3)
>    (format (current-output-port) "Values: ~a ~a ~a\n" foo bar baz))
> ;; output: "Values: 1 2 3"

Hey,
thanks for this code, maybe one day I will be able to comprehend it :)
for now, the problem is that I want to bind the variables that are
contained within a list, and that I don't know what's in that list.

So, the following code should work as well:
(let ((s '(foo bar baz))
      (v '(1 2 3)))
  (progv s v
     (+ foo bar baz)))
===> 6

regards
M.

Re: progv in scheme

[Andrew Gwozdziewycz]

On Tue, Sep 13, 2011 at 12:04 PM, Panicz Maciej Godek
<godek.maciek@gmail.com> wrote:
> 2011/9/13, Andrew Gwozdziewycz <apgwoz@gmail.com>:
>>
>> Seems likely that you could use `syntax-case' to create a `let` out of
>> these:
>>
>> (define-syntax progv
>>   (lambda (stx)
>>     (define (create-bindings syms vals)
>>       (datum->syntax stx (zip (syntax->datum syms) (syntax->datum vals))))
>>     (syntax-case stx ()
>>       ((_ symbols values body ...)
>>        (with-syntax ((bindings (create-bindings #'symbols #'values)))
>>           #'(let bindings
>>                (begin body ...)))))))
>>
>>
>>
>> ;; usage
>> (progv (foo bar baz) (1 2 3)
>>    (format (current-output-port) "Values: ~a ~a ~a\n" foo bar baz))
>> ;; output: "Values: 1 2 3"
>
> Hey,
> thanks for this code, maybe one day I will be able to comprehend it :)
> for now, the problem is that I want to bind the variables that are
> contained within a list, and that I don't know what's in that list.
>
> So, the following code should work as well:
> (let ((s '(foo bar baz))
>      (v '(1 2 3)))
>  (progv s v
>     (+ foo bar baz)))
> ===> 6

Ah, yes. I believe there's no way around using eval here. The
difficulty of course is compile time vs. runtime.


-- 
http://www.apgwoz.com

Re: progv in scheme

[Bill Schottstaedt]

I think this works in Guile 1.8.7 (I don't have a later version):

(define-macro (progv vars vals . body)
  `(let (,@(map (lambda (var val)
		  (list var val))
		(cadr vars) 
		(cadr vals)))
     ,@body))

(progv '(one two) '(1 2) (+ one two))
3


Maybe prettier syntax:

(define-macro (progv vars vals . body)
  `(let (,@(map (lambda (var val)
		  (list var val))
		vars vals))
     ,@body))

(progv (one two) (1 2) (+ one two))
3

Re: progv in scheme

[Bill Schottstaedt]

> and that I don't know what's in that list. 

oops, I missed that requirement.  hmmm -- there must be a way.

Re: progv in scheme

[Ludovic Courtès]

Hi,

Panicz Maciej Godek <godek.maciek@gmail.com> skribis:

> for now, the problem is that I want to bind the variables that are
> contained within a list, and that I don't know what's in that list.

I would recommend using (ice-9 match):

  (match '(1 2 3)
    ((a b c) `((a . ,a) (b . ,b) (c . ,c))))

  => ((a . 1) (b . 2) (c . 3))

HTH,
Ludo’.

Re: progv in scheme

[Ian Price]

Panicz Maciej Godek <godek.maciek@gmail.com> writes:

> Hello,
> Is there any clever way of binding values to the list of unknown
> symbols in scheme?
I have to admit, I don't understand why you would want to bind values
to a list of _unknown_ symbols; changing random bindings is just asking
for trouble. :-)

> In common lisp there is a form "progv" that takes the list of symbols
> and their corresponding values and binds them within the body of
> progv.
Strictly, this isn't what 'progv' does. 'progv' creates new _dynamic_
bindings, not lexical ones. e.g.

(setq *x* 1)
(defun foo () (write *x*))
(foo)
prints 1
(progv '(*x*) '(4) (foo))
prints 4

> It is possible to do it using eval, like this:
> (define (bind-and-eval symbols values body)
> (eval `((lambda ,symbols ,body) . ,values)
>       (interaction-environment)))
> (define-syntax let-symbols
> (syntax-rules ()
>   ((_ symbols values (body ...))
>    (bind-and-eval symbols values (quote (body ...))))))
>
> but using eval for this just seems too heavy. Is there any way of
> doing it that would be more legal?
Not really, doing so would break lexical scope. In a lexically
scoped language, the symbol isn't what matters; the "location" is. An
implementation is free to rename your identifiers, and once you reach
run-time, all the names have been forgotten anyway. Of course, there are
'unnatural' ways of expressing this, as you did with 'eval'.

If you know the list before hand, you can use match.

(use-modules (ice-9 match)

(match (list 1 2 3)
  ((a b c) (list 'success b c a))
  (else 'fail))
; => (success 2 3 1)

If you want 'dynamic variables', then Scheme does not provide them, but
there are 'fluids' or 'parameters' which behave similarly.

(define x (make-fluid))

(define (print)
  (write (fluid-ref x)))

(cons (fluid-ref x)
      (with-fluids* (list x) '(rebound)
        (lambda () (print) (fluid-ref x))))
;; => (#f . rebound)
;; prints 'rebound        

Hope that helps

-- 
Ian Price

"Programming is like pinball. The reward for doing it well is
the opportunity to do it again" - from "The Wizardy Compiled"

Re: progv in scheme

[Panicz Maciej Godek]

2011/9/13, Ian Price <ianprice90@googlemail.com>:
>> Hello,
>> Is there any clever way of binding values to the list of unknown
>> symbols in scheme?
> I have to admit, I don't understand why you would want to bind values
> to a list of _unknown_ symbols; changing random bindings is just asking
> for trouble. :-)

The existence is asking for trouble :)

Actually I've been porting a pattern matcher described by
Peter Norvig (in his great "Paradigms of AI Programming")
from common lisp to scheme.

The pattern matcher differs in many ways from the one
provided with guile -- it is not implemented with reader
macros and therefore it does not syntactically bind the
variables that appear in the pattern forms; instead,
it returns an assoc list containing symbols and values.

Norvig allows to place arbitrary code within the pattern
forms, so a call to eval is inevitable in the long run;
I thought, however, that maybe there could be some other
way to introduce a variable to a scope.

I do like the pattern matcher designed by Andrew K. Wright
and written by Alex Shinn, but it lacks some capabilities,
and the pattern language is still too difficult for me to
apprehend, and so it is hard for me to modify it. (The
author didn't allow to use multiple ellipses at one
level, because he "didn't want to make it easy to construct
very expensive operations", and I don't know how to bypass
this limitation)

Thanks,
M.

>> In common lisp there is a form "progv" that takes the list of symbols
>> and their corresponding values and binds them within the body of
>> progv.
> Strictly, this isn't what 'progv' does. 'progv' creates new _dynamic_
> bindings, not lexical ones. e.g.
>
> (setq *x* 1)
> (defun foo () (write *x*))
> (foo)
> prints 1
> (progv '(*x*) '(4) (foo))
> prints 4
>
>> It is possible to do it using eval, like this:
>> (define (bind-and-eval symbols values body)
>> (eval `((lambda ,symbols ,body) . ,values)
>>       (interaction-environment)))
>> (define-syntax let-symbols
>> (syntax-rules ()
>>   ((_ symbols values (body ...))
>>    (bind-and-eval symbols values (quote (body ...))))))
>>
>> but using eval for this just seems too heavy. Is there any way of
>> doing it that would be more legal?
> Not really, doing so would break lexical scope. In a lexically
> scoped language, the symbol isn't what matters; the "location" is. An
> implementation is free to rename your identifiers, and once you reach
> run-time, all the names have been forgotten anyway. Of course, there are
> 'unnatural' ways of expressing this, as you did with 'eval'.
>
> If you know the list before hand, you can use match.
>
> (use-modules (ice-9 match)
>
> (match (list 1 2 3)
>   ((a b c) (list 'success b c a))
>   (else 'fail))
> ; => (success 2 3 1)
>
> If you want 'dynamic variables', then Scheme does not provide them, but
> there are 'fluids' or 'parameters' which behave similarly.
>
> (define x (make-fluid))
>
> (define (print)
>   (write (fluid-ref x)))
>
> (cons (fluid-ref x)
>       (with-fluids* (list x) '(rebound)
>         (lambda () (print) (fluid-ref x))))
> ;; => (#f . rebound)
> ;; prints 'rebound
>
> Hope that helps
>
> --
> Ian Price
>
> "Programming is like pinball. The reward for doing it well is
> the opportunity to do it again" - from "The Wizardy Compiled"
>

Re: progv in scheme

[Bill Schottstaedt]

if lambda were applicable, this would work in both cases:

(define-macro (progv vars vals . body)
  `(apply (apply lambda ,vars ',body) ,vals))

> (let ((s '(one two)) (v '(1 2))) (progv s v (+ one two)))
3
> (progv '(one two) '(1 2) (+ one two))
3

(running a mystery scheme...)

Re: progv in scheme

[Andrew Gwozdziewycz]

On Tue, Sep 13, 2011 at 3:09 PM, Bill Schottstaedt
<bil@ccrma.stanford.edu> wrote:
> if lambda were applicable, this would work in both cases:
>
> (define-macro (progv vars vals . body)
>  `(apply (apply lambda ,vars ',body) ,vals))
>
>> (let ((s '(one two)) (v '(1 2))) (progv s v (+ one two)))
> 3
>> (progv '(one two) '(1 2) (+ one two))
> 3
>
> (running a mystery scheme...)


Bill-

It seems as though this would work actually in many schemes that do
incremental expansion of macros as part of their eval. If eval was
written as such:

(define (eval form env)
  (cond
    ((self-evaluating? form) form)
    ((variable? form) (lookup env (car form)))
    (...
    ((macro? form env) (eval (expand-macro (car form) (cdr form)) env))
    ...))

Then it seems as though it'd work perfectly fine. I haven't been using
guile very long, but it seems as though it's doing bytecode
compilation, which makes the story a bit different.

(I could totally be wrong of course)



-- 
http://www.apgwoz.com

Re: progv in scheme

[Andy Wingo]

On Tue 13 Sep 2011 12:09, "Bill Schottstaedt" <bil@ccrma.Stanford.EDU> writes:

> if lambda were applicable, this would work in both cases:
>
> (define-macro (progv vars vals . body)
>   `(apply (apply lambda ,vars ',body) ,vals))

That's fun!  Makes me think of Kernel.  I think you'd enjoy Kernel,
Bill, if you haven't seen it already.

  http://fexpr.blogspot.com/2011/04/fexpr.html

Andy
-- 
http://wingolog.org/

Re: progv in scheme

[Andrew Gwozdziewycz]

That's exactly what I was thinking, Kernel that is. But, I didn't
wanna sound like an idiot saying, "if only we had FExprs"...

On Thu, Sep 15, 2011 at 2:11 PM, Andy Wingo <wingo@pobox.com> wrote:
> On Tue 13 Sep 2011 12:09, "Bill Schottstaedt" <bil@ccrma.Stanford.EDU> writes:
>
>> if lambda were applicable, this would work in both cases:
>>
>> (define-macro (progv vars vals . body)
>>   `(apply (apply lambda ,vars ',body) ,vals))
>
> That's fun!  Makes me think of Kernel.  I think you'd enjoy Kernel,
> Bill, if you haven't seen it already.
>
>  http://fexpr.blogspot.com/2011/04/fexpr.html
>
> Andy
> --
> http://wingolog.org/
>
>



-- 
http://www.apgwoz.com