I'm writing some Cl开发者_如何学编程ojure code that depends upon a number of constants.

They will be used within tight inner loops, so it's important that they will be used and optimised as efficiently as possible by the Clojure compiler+JVM combination. I would normally used a "public static final" constant in Java for the same purpose.

What is the best way to declare these?

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I think def-ing things in the global namespace is about as close as you can come.

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I believe Clojure 1.3 (or maybe 1.4) allows you to put a ^:constant tag on a def, signifying to the compiler that this should be a constant and all references should be resolved at compile-time.

Edit

Apparently it's Clojure 1.3, and it's ^:const, not ^:constant. See How does Clojure ^:const work? for a summary.

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There's no defconst, so just using a global def is idiomatic; as far as optimisation is concerned, the JIT will make things fast.

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If really, really, really want the constant in place (I believe, the JIT will notice the value being constant and do the right thing, though), you can use a macro.

(defmacro my-constant [] 5)

This is rather ugly, but performance critical code will always be ugly, I guess.

(do-stuff (my-constant) in-place)

Pay care what you put into the macro, though! I wouldn't this for more than some literal constants. In particular not objects.

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If just using def is not fast enough, you could try creating a let bound alias before entering your tight loop to avoid going through a var each time.

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as said above use def or atom, remember, data is immutable, so if you declare some constants in a list, they don't change.