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Re: question about values


From: Massimiliano Gubinelli
Subject: Re: question about values
Date: Wed, 18 Mar 2020 16:34:08 +0100

Yes. Sorry. My bad, I had coded by copy&paste my previous example and is 
obviously not going to work.

Indeed dynamic-wind works as it should: 

mgubi@Ulrike guile3-usr % bin/guile
GNU Guile 3.0.1
Copyright (C) 1995-2020 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> (call-with-values (lambda ()
 (dynamic-wind (lambda () (display "initialize\n"))
  (lambda () (values "a" "b" "c"))
  (lambda () (display "finalize\n"))))
  (lambda body body))
initialize
finalize
$2 = ("a" "b" "c")


It seems to me a very good solution to my problem. Thanks a lot to all!

Max



> On 18. Mar 2020, at 16:08, address@hidden wrote:
> 
> On Wed, Mar 18, 2020 at 03:48:03PM +0100, Massimiliano Gubinelli wrote:
>> 
>>> On 18. Mar 2020, at 15:40, Massimiliano Gubinelli <address@hidden> wrote:
>>> 
>>> Good point with dynamic-wind. Does it pass along also multiple values or 
>>> has the same problem?
>>> 
>> 
>> Does not work...
>> 
>> mgubi@Ulrike guile3-usr % bin/guile
>> GNU Guile 3.0.1
>> Copyright (C) 1995-2020 Free Software Foundation, Inc.
>> 
>> Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
>> This program is free software, and you are welcome to redistribute it
>> under certain conditions; type `,show c' for details.
>> 
>> Enter `,help' for help.
>> scheme@(guile-user)> (call-with-values (lambda ()
>> (dynamic-wind (lambda () (display "initialize\n"))
>>  (lambda () (let ((a (values "a" "b" "c"))) a))
>               ^^^^^^ HERE
>>  (lambda () (display "finalize\n"))))
>>  (lambda body body))
>> initialize
>> finalize
>> $1 = ("a")
>> scheme@(guile-user)> 
>> 
>> Isn't this a bug??? In my view it breaks composability of scheme code.
> 
> Above, marked with HERE is the crocodile eating
> your other values.
> 
> If you instead do, in that line
> 
>  (lambda () (values "a" "b" "c"))
> 
> then you get ("a" "b" "c").
> 
> The let binding construct only binds one value. There's a
> "let-values" for that.
> 
> Cheers
> -- tomás




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