clojure way on flow control statement

Question

Suppose I have a python function like this:

def required(value):
    if value is None:
        throw Exception
    if isintance(value, str) and not value:
        throw Exception
    return value

Basically I want to check if the value is null or not. If the value is a String, also check if it's empty or not.

What is the clojure way of doing something like this?

Answer 1

The Clojure way of doing something like this is not to throw exceptions or so. The idiomatic way would be something that returns nil and nothing else.

So my advise: Do this without exceptions.

Your function will then look like this:

(defn required [value]
  (when (string? value)
    value))

It checks for the type of value and returns nil if it is not a String. Otherwise return your value.

Or if you want an error message in your terminal:

(defn required [value]
  (if (string? value)
    value
    (println "Value must be a String.")))

Note that println prints the String and then returns again nil.

Answer 2

Preconditions would do the trick nicely in this instance. Otherwise use Clojure's control flow special forms/macros, e.g. if, cond with throw.

user=> (defn required 
         [value] 
         {:pre [(string? value) (not-empty value)]} 
         value)
#'user/required

user=> (required nil)
AssertionError Assert failed: (string? value)  user/required ...

user=> (required "")
AssertionError Assert failed: (not-empty value) ...

user=> (required "foo")
"foo"