On Mar 31, 2014, at 1:27 PM, A. Webb <a.webb....@gmail.com> wrote:
>
> If you are working with quoted expressions, you'll have to eval in the macro
> and take the one-time hit there
>
> (defmacro functionalise2
> [ex var]
> (let [arg (gensym)
> body (clojure.walk/postwalk-replace {var arg} ex)]
> `(fn [~arg] ~(eval body))))
>
> (def foo2 (functionalise2 '(+ 2 x) x))
> (foo2 40) ;=> 42
>
Or if the reason that you're quoting is because that's a stand-in for an
expression that will be generated dynamically then maybe you don't want to do
it in a macro, but instead call eval on function-defining expression that
includes your expression and takes an argument for the variable.
Easier shown than said:
(defn functionalise
[ex var]
(eval (list 'fn (vector var) ex)))
(def ex '(+ 1 x))
(def exf (functionalise ex 'x))
(exf 3) ;; => 4
This calls eval only once when you call functionalise, and doesn't do any tree
walking.
FWIW this is the trick I use in the tiny genetic programming system at:
https://github.com/lspector/gp/blob/master/src/gp/evolvefn.clj
-Lee
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