What are the best practices for defining constants in Clojure in terms of style, conventions, efficiency, etc.
For example, is this right?
(def *PI* 3.14)
Questions:
Should constants be capitalized in Clojure?
Stylistically, should they have the asterisk (*) character on one or both sides?
Any computational efficiency considerations I should be aware of?
I don't think there is any hard and fast rules. I usually don't give them any special treatment at all. In a functional language, there is less of a distinction between a constant and any other value, because things are more often pure.
The asterisks on both sides are called "ear muffs" in Clojure. They are usually used to indicate a "special" var, or a var that will be dynamically rebound using binding later. Stuff like out and in which are occasionally rebound to different streams by users and such are examples.
Personally, I would just name it pi. I don't think I've ever seen people give constants special names in Clojure.
EDIT: Mister Carper just pointed out that he himself capitalizes constants in his code because it's a convention in other languages. I guess this goes to show that there are at least some people who do that.
I did a quick glance through the coding standards but didn't find anything about it. This leads me to conclude that it's really up to you whether or not you capitalize them. I don't think anyone will slap you for it in the long run.
On the computational efficiency front you should know there is no such thing as a global constant in Clojure. What you have above is a var, and every time you reference it, it does a lookup. Even if you don't put earmuffs on it, vars can always be rebound, so the value could always change, so they are always looked up in a table. For performance critical loops this is most decidedly non-optimal.
There are some options like putting a let block around your critical loops and let the value of any "constant" vars so that they are not looked up. Or creating a no-arg macro so that the constant value is compiled into the code. Or you could create a Java class with a static member.
See this post, and the following discussion about constants for more info:
The earmuffs are a way of denoting that a given symbol will have its own thread-local binding at some point. As such, it does not make sense to apply the earmuffs to your Pi constant.
*clojure-version* is an example of a constant in Clojure, and it's entirely in lower-case.