From: Vladimir Zhbanov <vzhbanov@gmail.com>
To: guile-user@gnu.org
Subject: Re: Self-evaluating function and closure
Date: Mon, 17 Jun 2019 11:48:15 +0300 [thread overview]
Message-ID: <20190617084815.GB1403@newvzh.lokolhoz> (raw)
In-Reply-To: <878su13gz1.fsf@netris.org>
On Sun, Jun 16, 2019 at 07:29:59AM -0400, Mark H Weaver wrote:
> Hello again,
>
> Vladimir Zhbanov <vzhbanov@gmail.com> 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!
--
Vladimir
(λ)επτόν EDA — https://github.com/lepton-eda
next prev parent reply other threads:[~2019-06-17 8:48 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-12 20:29 Self-evaluating function and closure Vladimir Zhbanov
2019-06-15 19:35 ` John Cowan
2019-06-16 0:39 ` Mark H Weaver
2019-06-16 0:36 ` Mark H Weaver
2019-06-16 9:02 ` Vladimir Zhbanov
2019-06-16 10:09 ` Mark H Weaver
2019-06-17 8:03 ` Vladimir Zhbanov
2019-06-16 9:47 ` Mark H Weaver
2019-06-16 10:21 ` Thomas Morley
2019-06-16 10:32 ` Mark H Weaver
2019-06-16 10:42 ` Thomas Morley
2019-06-16 11:29 ` Mark H Weaver
2019-06-17 8:48 ` Vladimir Zhbanov [this message]
2019-06-17 14:21 ` Mark H Weaver
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