[racket-dev] patch for make-base-eval

Ok thanks for the explanations. I'll try doing one of the last two suggestions.

On Wed, Oct 2, 2013 at 4:09 PM, Ryan Culpepper <ryanc at ccs.neu.edu> wrote:
> No, the 'racket/pretty' module might be declared even if the symbol isn't
> defined (or "mapped") in the namespace:
>
>   > (define ns (make-base-namespace))
>   > (define repl-ns (current-namespace))
>   > (parameterize ((current-namespace ns))
>       (eval '(require (only-in racket/pretty))))
>   > (parameterize ((current-namespace ns))
>       (namespace-attach-module repl-ns 'racket/pretty))
>   namespace-attach-module: a different module with the same name is
>   already in the destination namespace
>     module name: "/opt/racket-5.3.6/collects/racket/pretty.rkt"
>     context...:
>      /opt/racket-5.3.6/collects/racket/private/misc.rkt:87:7
>
> And the symbol can be defined in the namespace even if the module is not
> declared:
>
>   > (define ns (make-base-namespace))
>   > (define repl-ns (current-namespace))
>   > (parameterize ((current-namespace ns))
>       (eval '(define pretty-print-handler #t)))
>   > (parameterize ((current-namespace ns))
>       (namespace-variable-value 'pretty-print-handler))
>   #t
>   ;; but racket/pretty is not declared,
>   ;; and #t is not a good print handler
>
> Ryan
>
>
>
> On 10/02/2013 03:58 PM, Stephen Chang wrote:
>>>
>>> A namespace is a mapping from top-level identifiers to whatever they are,
>>> as
>>> well as a separate mapping from module names to modules (roughly). What
>>> you
>>> care about here is the second mapping, but you're checking the first with
>>> the patch.
>>
>>
>> Thanks for the explanation. That helps a lot. So the danger with my
>> check is when someone has another definition of pretty-print handler
>> but racket/pretty has not been attached?
>>
>> But given the context, ie the dynamic require on the next line, it
>> seems like there's already an assumption about what the identifier I'm
>> checking is, so in this specific situation, isnt my check sufficient?
>>
>>
>>
>>>
>>> Robby
>>>
>>>
>>> On Wed, Oct 2, 2013 at 2:45 PM, Stephen Chang <stchang at ccs.neu.edu>
>>> wrote:
>>>>
>>>>
>>>>> Whether that identifier exists in the namespace has nothing to do with
>>>>> whether racket/pretty can be attached.
>>>>
>>>>
>>>> Can you explain this a little more because it's a little unintuitive to
>>>> me?
>>>>
>>>>
>>>>>
>>>>> One option would be for install-pretty-printer! to just catch and
>>>>> discard
>>>>> the error. Evaluators for some languages would mysteriously not have
>>>>> pretty-printing turned on by default.
>>>>>
>>>>> Another option would be to attach racket/pretty before requiring the
>>>>> initial
>>>>> language for the namespace.
>>>>>
>>>>> Another option is use #:pretty-print? #f when attaching racket/pretty
>>>>> would
>>>>> fail.
>>>>>
>>>>> Ryan
>>>>>
>>>> _________________________
>>>>    Racket Developers list:
>>>>    http://lists.racket-lang.org/dev
>>>
>>>
>>>
>