On 02/11/2013 04:04 PM, Gustavo Massaccesi wrote:
I was trying to define a "read-only" parameter, i.e. a parameter that
can be "read" and "parameterized", but not "written". I'd want to call
functions without being worried that the any of them changes that
parameter, but I want to allow each function to parameterize the
parameter for internal use.
Perhaps an "example" is clearer:
;-----
#lang racket/base
(require racket/port)
(define my-curr-out-port (make-parameter (current-output-port)))
(define (hello)
(displayln "Hello" (my-curr-out-port)))
(define (quietly)>
Gustavo
____________________
Racket Users list:
http://lists.racket-lang.org/users
(parameterize ([my-curr-out-port (open-output-nowhere)])
(hello)))
(define (mute)
(my-curr-out-port (open-output-nowhere))) ; I'd want an error here.
(hello) ;---> "Hello"
(quietly) ;---> ""
(hello) ;---> "Hello"
(mute)
(hello) ;---> "" :(
;---
If you don't mind having a special form to parameterize
`my-curr-out-port', you can do it with a macro.
#lang racket/base
(module defs racket/base
(require racket/port)
(provide my-curr-out-port with-my-curr-out-port)
(define my-curr-out-port* (make-parameter (current-output-port)))
(define (my-curr-out-port) (my-curr-out-port*))
(define-syntax-rule (with-my-curr-out-port port . body)
(parameterize ([my-curr-out-port* port])
. body)))
(require (submod "." defs)
racket/port)
(define (hello)
(displayln "Hello" (my-curr-out-port)))
(define (quietly)
(with-my-curr-out-port (open-output-nowhere)
(hello)))
(define (mute)
(my-curr-out-port (open-output-nowhere)))
(hello) ;---> "Hello"
(quietly) ; prints nothing
(hello) ;---> "Hello"
(mute) ; error here
Neil ⊥
____________________
Racket Users list:
http://lists.racket-lang.org/users
