Should I use a function or a macro to validate arguments in Clojure?

Question

I have a group of numeric functions in Clojure that I want to validate the arguments for. There are numerous types of arguments expected by the functions, such as positive i

Answer 1

Clojure already has (undocumented, maybe subject-to-change) support for pre- and post-conditions on fns.

user> (defn divide [x y]
        {:pre [(not= y 0)]}
        (/ x y))
user> (divide 1 0)
Assert failed: (not= y 0)
   [Thrown class java.lang.Exception]

Kind of ugly though.

I'd probably write a macro just so I could report which tests failed in a succinct way (quote and print the test literally). The CL code you linked to looks pretty nasty with that enormous case statement. Multimethods would be better here in my opinion. You can throw something like this together pretty easily yourself.

(defmacro assert* [val test]
  `(let [result# ~test]              ;; SO`s syntax-highlighting is terrible
     (when (not result#)
       (throw (Exception.
               (str "Test failed: " (quote ~test)
                    " for " (quote ~val) " = " ~val))))))

(defmulti validate* (fn [val test] test))

(defmethod validate* :non-zero [x _]
  (assert* x (not= x 0)))

(defmethod validate* :even [x _]
  (assert* x (even? x)))

(defn validate [& tests]
  (doseq [test tests] (apply validate* test)))

(defn divide [x y]
  (validate [y :non-zero] [x :even])
  (/ x y))

Then:

user> (divide 1 0)
; Evaluation aborted.
; Test failed: (not= x 0) for x = 0
;   [Thrown class java.lang.Exception]

user> (divide 5 1)
; Evaluation aborted.
; Test failed: (even? x) for x = 5
;   [Thrown class java.lang.Exception]

user> (divide 6 2)
3

Answer 2

A case where you need a macro would be if you wanted to modify the language to automatically add the tests to any function defined within a block, like this:

(with-function-validators [test1 test2 test4]  
    (defn fun1 [arg1 arg2] 
        (do-stuff))
    (defn fun2 [arg1 arg2] 
        (do-stuff))
    (defn fun3 [arg1 arg2] 
        (do-stuff)))  

Answer 3

Just a few thoughts.

I have a feeling it depends on the complexity and number of validations, and the nature of the functions.

If you are doing very complex validations, you should break your validators out of your functions. The reasoning is that you can use simpler ones to build up more complex ones.

For example, you write:

1. a validator to make sure a list is not empty,
2. a validator to make sure a value is greater than zero,
3. use 1 and 2 to make sure a value is a non empty list of values greater than zero.

If you're just doing a huge amount of simple validations, and your issue is verbosity, (e.g. you have 50 functions that all require non-zero integers), then a macro probably makes more sense.

Another thing to consider is that function evaluation is Clojure is eager. You could get a performance boost in some cases by not evaluating some parameters if you know the function will fail, or if some parameters are not needed based on values of other parameters. E.g. the every? predicate doesn't need to evaluate every value in a collection.

Finally, to address "others you haven't thought of". Clojure supports a generic dispatching pbased on a dispatch functon. That function could dispatch to appropriate code, or error messages based on any number of factors.