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 <stch...@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
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