Thanks much.
On Mon, Jan 19, 2009 at 11:03 AM, Michael Reid <kid.me...@gmail.com> wrote:
>
> On Fri, Jan 16, 2009 at 9:57 PM, David Nolen <dnolen.li...@gmail.com>
> wrote:
> > Just made sense to me today as well.
> > #^Class
> > is short form for saying set the metadata for the symbol being defined
> (in
> > this case list) to the map {:tag Class}.
> > #^ is a reader macro for setting metadata for the compiler. That code is
> > simple tagging the symbol clojure/list.
> > (meta #'list)
> > will give you the map representing the metadata. :tag is used by the
> > compiler, but it's not clear to me in what cases...
> >
>
> The most common case where :tag is used by the compiler is to generate
> direct method calls into Java code rather than using reflection to do
> the call, which is much slower, e.g:
>
> (defn index-of [s substr]
> (.indexOf s substr))
>
> The call to String.indexOf will likely be compiled as a reflective
> call because in the general case, the compiler doesn't know that 's'
> and 'substr' are meant to be instances of java.lang.String. If however
> you add a couple of type hints:
>
> (defn index-of [#^String s #^String substr]
> (.indexOf s substr))
>
> Then the compiler will generate an optimized code path that directly
> invokes String.indexOf(String,String), and the other which will fall
> back to the reflective invocation. The reason that two code paths are
> needed is because Clojure is dynamically typed. Therefore it is
> perfectly legal to pass in any type of objects as the parameter in
> which case you want the call to .indexOf to succeed if the passed in
> instance does in fact have a method named indexOf that can accept
> 'substr' as its argument.
>
> To see where the compiler needs to help you can use the
> *warn-on-reflection* flag to ask for warnings where the compiler can't
> resolve the calls. See:
>
> http://clojure.org/java_interop#toc35
>
> for example usage.
>
> /mike.
>
> >
>
--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups
"Clojure" group.
To post to this group, send email to clojure@googlegroups.com
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
-~----------~----~----~----~------~----~------~--~---
