Sam Tobin-Hochstadt wrote at 09/28/2011 10:42 AM:
(require racket/mpair racket/splicing)
I guess the advantage of "let-splicing" here is that you can put the
names by the definitions, rather than like the following more
traditional way of doing this:
#lang racket/base
(require racket/mpair)
(define-values (variable-ref variable-set!)
(let ((value-store (mlist (mcons 'variable #f))))
(values (lambda (variable)
(massoc variable value-store))
(lambda (variable value)
(set-mcdr! (variable-ref variable) value)))))
(variable-ref 'variable)
Incidentally, Marijn, given that that's a module right there, I suspect
that the main reason you want to hide "value-store" like this is if this
code was actually the result of a macro expansion. If not as the result
of a macro expansion, then a perfectly normal practice is to use a
top-level binding and simply not export it from the module, like:
#lang racket/base
(require racket/mpair)
(define value-store (mlist (mcons 'variable #f)))
(define (variable-ref variable)
(massoc variable value-store))
(define (variable-set! variable value)
(set-mcdr! (variable-ref variable) value))
(provide variable-ref variable-set!)
If you were writing a reusable library rather than experimenting, you'd
probably not have a "value-store" binding that (presumably) will be
mutated, but instead use a Racket parameter, so you'd have something like:
#lang racket/base
(require racket/mpair)
(define current-value-store (make-parameter (mlist (mcons 'variable #f))))
(define (variable-ref variable)
(massoc variable (current-value-store)))
(define (variable-set! variable value)
(set-mcdr! (variable-ref variable) value))
(provide variable-ref variable-set! current-value-store)
The parameter lets users of the library have multiple instances of the
value store, rather than only one value store per program. You'd
probably also provide a "make-value-store".
There's also a good chance you'd also use something other than mutable
pairs, too, such as hashes or non-mutable pairs. You'll see examples in
old Scheme textbooks of rolling your own records or objects using pairs,
and that's good for learning, but for most real-world development,
Racket has a large library of often better ways to accomplish things.
--
http://www.neilvandyke.org/
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