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