can defmacro quota parameter in clojure?

Question

I have a string which will evaluate to true or false, can I use macro and pass the string as parameter? I write the following, but the result is string of (= 0 0) instead of true. How to get true result?

(def a "(= 0 0)")

(defmacro test [code-string]  code-string)

(test a)

update: The purpose is replace dynamic SQL. Currently we store code like 'column_a > 1' in database, and then we will get the code, and assemble a sql like

select case when column_a>1 then 0 else 1 end as result from table 

There are many such code, and I hope to use clojure run in parallel to speed it up. To use clojure I could store '(> row["column_a"] 1)' in database, and then in jdbc looping, call (> row["column_a"] 1) to do my logic, like storing some code section in database and need to run it.

Answer 1

As TaylanUB already said, Clojure provides eval to evaluate some expression at run-time. However, using eval is frowned upon unless you have very good reasons to use it. It's not clear what you're really intending to do, so it would be helpful to provide a more real world example. If you don't have one, you don't need eval. Similarly, macros are used to transform code and are not run at run-time, instead the code to which the macro evaluates gets run. The typical approach would be to try to solve a problem with a mere function, only if a macro would buy you something in terms of applicability to a wider range of code, consider turning the code into a macro. Edit: take a look at some introduction to macros in Clojure, e.g. this part from Clojure from the ground up

Answer 2

No, you cannot directly use a string as code. Defmacro takes s-expressions not strings. Clojure might have something like read which can parse a string and make an s-expression out of it which you might then be able to execute as code via something like eval.

There is usually no good reason to put code in strings or other data structures which will exist during program execution anyway, try to just work with first-class functions instead. Or mention the precise problem you're trying to solve and people might be able to give better answers. This might be an instance of the XY problem.

Note: I don't know Clojure, but all of this is pretty Lisp-generic.

Answer 3

(defn eval-code [code-string]
  (eval (read-string code-string)))

(eval-code "(= 0 0)")

;; you don't need macro here.