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Re: Self-evaluating function and closure

From: Vladimir Zhbanov
Subject: Re: Self-evaluating function and closure
Date: Mon, 17 Jun 2019 11:48:15 +0300
User-agent: Mutt/1.10.1 (2018-07-13)

On Sun, Jun 16, 2019 at 07:29:59AM -0400, Mark H Weaver wrote:
> Hello again,
> Vladimir Zhbanov <address@hidden> writes:
> > scheme@(guile-user)> (define (function-generator)
> >                        (let ((func #f))                         
> >                          (lambda () (set! func (let a () a)) func)))
> [...]
> > - Is there a way to work around this (either using the above 'let'
> >   construct or anything else)?
> Ideally, the code would be reworked to not expect equivalent procedures
> to be distinguishable.  However, I should probably offer a hacky but
> expedient workaround.  Here's one way to make otherwise equivalent
> procedures distinguishable:
> Allocate a fresh tag using (list #f), and arrange for the procedure to
> return that tag if it's called with a special input that's outside of
> the normal domain.  Note that for Scheme procedures, the "input" is in
> general a list of arguments of arbitrary length.  You could use
> 'case-lambda', which creates procedures that evaluate different body
> expressions depending on how many arguments are passed to it.  Just add
> a case for an arity that you will never use, which returns the unique
> tag.
> In the example you gave, (let a () a) is equivalent to:
>   ((letrec ((a (lambda () a)))
>      a))
> The procedure returned by (let a () a) expects 0 arguments.  It will
> raise an error otherwise.  We can repurpose the previously erroneous
> arity-1 case to return the unique tag, as follows:
>   (let ((unique-tag (list #f)))
>     ((letrec ((a (case-lambda
>                    (() a)
>                    ((x) unique-tag))))
>        a)))
> Every time the above expression is evaluated, it will necessarily return
> a unique procedure, which, if passed 0 arguments, behaves the same as
> the procedure returned by (let a () a).
>        Mark

Great, I've adapted your example to our code and it works
nice. Thank you very much!


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