Worked out very quickly but it's headed in the direction you want:
(defmacro dynlet [bvec body]
`(let ~(if (vector? bvec)
bvec
(var-get (resolve bvec)))
~body))
(def bds '[a 42])
(dynlet bds (prn a))
(dynlet [a 42] (prn a))
On Sun, Dec 13, 2009 at 5:43 AM, mk <michak...@gmail.com> wrote:
> Hi,
>
> im just porting Chapter 19 to clojure (my objective is to learn
> macros). (For those that do not know the book: A database system is
> introduced that can hold and query painters). It goes nicely until the
> macro with-answer, which generate bindings on-the-fly so that one can
> say
>
> (with-answer (painter ?a ?b english) (prn ?a))
>
> and the system will print the names of the english painters. So with-
> answer has two arguments, the first is the query and the second is a
> block of code which is then evaluated using the bindings that are
> generated by the query. I thought it would be possible to write a
> macro that generates something like (let [?a name1] (prn ?a)), but I
> was not able to do so. Why is that? Is there a trick?
>
> The problem boils down to this: Write a macro dynlet that can be
> called like this
>
> (def bds '[a 42])
> (dynlet bds (prn a))
>
> and will print 42.
>
> I came up with
>
> (defmacro dynlet [bvec body]
> `(let ~bvec
> ~body))
>
> but this only works if the vector with the bindings is given directly
> to the macro like this:
> (dynlet [a 42] (prn a))
>
> How does one do this in clojure?
>
> I was able to solve the original problem by doing a string-replace of
> the variables in the body with the generated values like this:
>
> (defmacro with-answer [query body]
> `(do
> ~@(map
> (fn [x]
> (let [rbody# (walk/postwalk-replace x body)]
> rbody#)
> )
> (interpret-query query)))
> )
>
> but of cause this is not the same as the original thing. It has to be
> called like this
> (with-answer (painter ?a ?b english) (prn '?a))
> and the last quote is sadly necessary.
>
> I would greatly appreciate some help here!
>
> TIA,
>
> Michael
>
>
>
> --
> 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
> Note that posts from new members are moderated - please be patient with
> your first post.
> To unsubscribe from this group, send email to
> clojure+unsubscr...@googlegroups.com<clojure%2bunsubscr...@googlegroups.com>
> For more options, visit this group at
> http://groups.google.com/group/clojure?hl=en
--
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
Note that posts from new members are moderated - please be patient with your
first post.
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
