Is this a 'bug' with eval?
On 28 November 2010 14:09, Ken Wesson <kwess...@gmail.com> wrote:
> On Sat, Nov 27, 2010 at 10:42 PM, Andreas Kostler
> <andreas.koestler.le...@gmail.com> wrote:
> > Hi all,
> > Sorry for my noob question (again). I'm trying to understand clojures
> > binding model.
> > Typing:
> > (def state {:status "foo"})
> > (def rule '(if (= (:status sate) "foo") (println "foo") (println
> > ("bar")))
> >
> > (defn fn []
> > (let [state {:status "bar"}]
> > (eval rule)))
> >
> > This prints "foo". However, I would have expected the binding of state
> > created by let would shadow the global binding of state.
> > Now
> >
> > (defn fn1 []
> > (binding [state {:status "bar"}]
> > (eval rule)))
> > Does the right thing. Can someone explain what's going on here please?
>
> This is eval not seeing local bindings. There is a workaround:
>
> (defmacro eval-with-local-vars [vars sexp]
> (let [quoted-vars (vec (map #(list 'quote %) vars))]
> `(let [varvals# (vec (interleave ~quoted-vars ~vars))]
> (eval (list 'clojure.core/let varvals# ~sexp)))))
>
> user=> (let [a 1 b 2] (eval-with-local-vars [a b] '(+ a b)))
> 3
> user=> (def state {:status "foo"})
> #'user/state
> user=> (def rule
> '(if (= (:status state) "foo")
> (println "foo")
> (println "bar")))
> #'user/rule
> user=> (let [state {:status "bar"}]
> (eval rule))
> foo
> nil
> user=> (let [state {:status "bar"}]
> (eval-with-local-vars [state] rule))
> bar
> nil
>
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--
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Andreas Koestler, Software Engineer
Leica Geosystems Pty Ltd
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