Hello again,
Vladimir Zhbanov <vzhba...@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
