On Sat, Nov 27, 2010 at 4:31 PM, Ken Wesson <kwess...@gmail.com> wrote:
> On Sat, Nov 27, 2010 at 10:45 AM, Eduardo Julian <eduardo...@gmail.com> wrote:
>> user=> (let [a 'b] (str a))
>> "b"
>> user=> (let [b 5 a 'b] (eval a))
>> java.lang.Exception: Unable to resolve symbol: b in this context
>> (repl-1:7)
>> user=> (let [a 'b b 5] (eval a))
>> java.lang.Exception: Unable to resolve symbol: b in this context
>> (repl-1:9)
>>
>> user=> (def b 5)
>> #'user/b
>> user=> (def a 'b)
>> #'user/a
>> user=> a
>> b
>> userr=> (eval a)
>> 5
>>
>> How come this problem happens inside the let but not with def?
>
> If you
>
> (do
> (binding [*ns* (find-ns 'foo)]
> (eval bar)))
>
> the evaluation of bar sees the global bindings in the namespace foo,
> and can get at other global bindings and Java classes with
> fully-qualified names, but cannot see the lexical environment around
> the eval call. Even when called inside a let or a function body,
> eval's contents only see what they would see if executed at the top
> level of the source file for the namespace that is *current when eval
> is called* (not necessarily the same file the eval itself is in).
Adding to this, you can work around it with
(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=> (let [b 5 a 'b] (eval-with-local-vars [b a] a))
5
Note that the vars have to be in the correct order in the vector:
user=> (let [b 5 a 'b] (eval-with-local-vars [a b] a))
#<CompilerException java.lang.Exception: Unable to resolve symbol: b
in this context (NO_SOURCE_FILE:161)>
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