eval and load-string behavior

[Jonathan Fischer Friberg]

I got a very nasty bug from this behavior today

user=> (def a (fn [] "outside a"))
#'user/a
user=> (let [a (fn [] "inside a")] (load-string "(a)"))
"outside a"
user=> (let [a (fn [] "inside a")] (eval '(a)))
"outside a"

Is this really how these functions should behave?

Jonathan

[Sean Corfield]

On Mon, Jun 20, 2011 at 4:45 PM, Jonathan Fischer Friberg
<odys...@gmail.com> wrote:
> user=> (def a (fn [] "outside a"))
> #'user/a
> user=> (let [a (fn [] "inside a")] (load-string "(a)"))
> "outside a"
> user=> (let [a (fn [] "inside a")] (eval '(a)))
> "outside a"
>
> Is this really how these functions should behave?

Since let provides a lexical binding rather than a dynamic binding,
I'm not too surprised by the result.

Would the following do what you want?

(def ^{:dynamic true} *a* (fn [] "outside a"))
(binding [*a* (fn [] "inside a")] (load-string "(*a*)"))
(binding [*a* (fn [] "inside a")] (eval '(*a*)))

(FWIW, those yield "inside a" in both cases)
--
Sean A Corfield -- (904) 302-SEAN
An Architect's View -- http://corfield.org/
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"Perfection is the enemy of the good."
-- Gustave Flaubert, French realist novelist (1821-1880)

[David Nolen]

Relevant previous threads:

http://groups.google.com/group/clojure/browse_thread/thread/e1226810b6ac7bfc/8e0f53c141c26fcc

http://groups.google.com/group/clojure/browse_thread/thread/ead8a3a8ff009a8f/17bb00a546724435

David 

[Jonathan Fischer Friberg]

Thanks, but I think I'll simply input the let in the string, i.e

(load-string "(let [a (fn [] "inside fn")] (a))")

But I still think it's odd behavior. I've always thought of eval as equal to putting that code there.

(eval '(a)) is equal to (a)

Which I think would be much more intuitive.

Jonathan

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[Jonathan Fischer Friberg]

It seems like the following macro would work as well:

(defmacro load-string-here [string] (read-string string))

=> (let [a (fn [] "inside fn")] (load-string-here "(a)"))
"inside fn"

Jonathan

[David Nolen]

If Clojure was interpreted this behavior would be simple to support.
It seems like you'd have to complicate the compiler for little
benefit.

[Meikel Brandmeyer]

Hi,

note that this breaks down as soon as you don't provide a literal string. And if you provide a literal string, you can provide the form directly anyway...

Sincerely
Meikel

[Jonathan Fischer Friberg]

Yes, unless you wrap it in another macro.

(defmacro a [] (vec (map (fn [x] `(load-string-here ~x)) ["1" "2" "3" "4"])))

=> (a)
[1 2 3 4]

But it's still pretty useless, unless macros are to replace functions...

Jonathan