I don't think there's currently a way to have a read-only parameter.
If you know when you're calling a function that might mutate a
parameter, then you can constrain mutation to the dynamic extent of the
call by wrapping the call with a `parameterize'. The `limit' wrapper
below does that.
----------------------------------------
#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)
(parameterize ([my-curr-out-port (open-output-nowhere)])
(hello)))
(define (mute)
(my-curr-out-port (open-output-nowhere))) ; I'd want an error here.
;; constrain any mutation of `my-cur-out-port' to the dynamic
;; extent of `thunk':
(define (limit thunk)
(parameterize ([my-curr-out-port (my-curr-out-port)])
(thunk)))
(hello) ;---> "Hello"
(limit quietly) ;---> ""
(hello) ;---> "Hello"
(limit mute)
(hello) ;---> "Hello" :)
____________________
Racket Users list:
http://lists.racket-lang.org/users
