On Wed, 9 Feb 2011 20:34:56 -0800 (PST)
CuppoJava <patrickli_2...@hotmail.com> wrote:
> Description: (bind-later bindings & body) and (do-binding & body)
> (bind-later) is used like a let-form, except that it doesn't
> *immediately* make the bindings available.
> The bindings become available only within (do-binding).
> (do-binding) is always used within a (bind-later).
>
> Here's some sample code showing how it works.
>
> (def a "outer a")
> (bind-later (a "inner a")
> (println a) <-- this prints out "outer a" still.
> (do-binding
> (println a))) <-- this will print out "inner a"
>
> Is such a macro possible?
Yes. It's just - as you say - tricky. You need to create a
"do-binding" macro for each bind-later macro, meaning it should be
part of the macros lexical space. First trick - use
clojure.contrib.macro-utils/macrolet to do that sanely. Then comes the
really tricky part - figuring out the correct unquoting for everything.
But this does it:
(defmacro bind-later [bindings & body]
`(macrolet [(~'do-binding [& body#]
`(let ~'~bindings
~@body#))]
~@body))
Oh, one thing - since let requires a vector for binding, your example
has to be written as (bind-later [a "inner a"] .... But that's the
standard clojure idiom for bindings, so you should do it that way
anyway ;-).
<mike
--
Mike Meyer <m...@mired.org> http://www.mired.org/consulting.html
Independent Network/Unix/SCM consultant, email for more information.
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