To add to that a little bit, this is what I found:
#lang multi-file
#file param.rkt
#lang racket
(provide my-param)
(define my-param (make-parameter 'default))
#file param-test.rkt
#lang racket
(require "param.rkt")
(define-namespace-anchor a)
(parameterize ([current-namespace (make-base-namespace)]
[error-print-width 4242]
[my-param 'custom])
;(namespace-attach-module (namespace-anchor->namespace a) "param.rkt"
(current-namespace))
(namespace-require "param.rkt")
(eval '(begin (printf "(error-print-width) = ~v\n" (error-print-width)) ;;
4242, not 250
(printf "(my-param) = ~v\n" (my-param)) ;;
'default
)))
With the namespace-attach-module call commented out, this prints
#file param.rkt
#file param-test.rkt
(error-print-width) = 4242
(my-param) = 'default
But if I uncomment that line, I get:
#file param.rkt
#file param-test.rkt
(error-print-width) = 4242
(my-param) = 'custom
And if I move the namespace-attach-module call to after the (namespace-require
"param.rkt"), I get this:
#file param.rkt
#file param-test.rkt
. namespace-attach-module: a different module with the same name is already in
the destination namespace
module name: "/Users/Alex/Documents/DrRacket/param-thing/param.rkt"
I hope that helps!
Alex Knauth
> On Aug 30, 2015, at 3:36 PM, Matthew Flatt <mfl...@cs.utah.edu> wrote:
>
> The `make-base-namespace` function creates a new namespace, but it
> attaches the `racket/base` instance of the current namespace to the new
> one. The `error-print-width` parameter comes from `racket/base`, so
> it's still the same parameter.
>
> You can use `namespace-attach-module` to attach "param.rkt" to the new
> namespace, too, if you want its instance and parameter to be shared
> across the namespaces.
>
> At Sun, 30 Aug 2015 12:04:30 -0700, Matthew Butterick wrote:
>> In the example below, I'm curious about the reason for the difference
>> between
>> the behavior of the system parameter `error-print-width` and the custom
>> parameter `my-param`.
>>
>> I understand why `my-param` doesn't take the parameterized value 'custom (=
>> because `namespace-require` causes a fresh instantiation of "param.rkt" in
>> the
>> new namespace, which binds `my-param` with its default value).
>>
>> But if that's the case, then why does `error-print-width` accept the
>> parameterized value? Why doesn't it get instantiated at its default value of
>> 250 when the new namespace is made?
>>
>>
>> ;; param.rkt
>> #lang racket
>> (provide my-param)
>> (define my-param (make-parameter 'default))
>>
>>
>> ;; param-test.rkt
>> #lang racket
>> (require "param.rkt")
>> (parameterize ([current-namespace (make-base-namespace)]
>> [error-print-width 4242]
>> [my-param 'custom])
>> (namespace-require "param.rkt")
>> (namespace-require 'rackunit)
>> (eval '(begin (check-equal? (error-print-width) 4242) ;; not 250
>> (check-equal? (my-param) 'default))))
>>
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