On Sun, Jun 16, 2019 at 07:29:59AM -0400, Mark H Weaver wrote:
> Hello again,
>
> Vladimir Zhbanov 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
