Why does Clojure have “keywords” in addition to “symbols”?

Question

I have a passing knowledge of other Lisps (particularly Scheme) from way back. Recently I\'ve been reading about Clojure. I see that it has both \"symbols\" and \"keywords\"

Answer 1

Keywords are global, symbols are not.

This example is written in JavaScript, but I hope it helps to carry the point across.

const foo = Symbol.for(":foo") // this will create a keyword
const foo2 = Symbol.for(":foo") // this will return the same keyword
const foo3 = Symbol(":foo") // this will create a new symbol
foo === foo2 // true
foo2 === foo3 // false

When you construct a symbol using the Symbol function you get a distinct/private symbol every time. When you ask for a symbol via the Symbol.for function, you will get back the same symbol every time.

(println :foo) ; Clojure

System.out.println(RT.keyword(null, "foo")) // Java

console.log(System.for(":foo")) // JavaScript

These are all the same.

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Function argument names are local. i.e. not keywords.

(def foo (fn [x] (println x))) ; x is a symbol
(def bar (fn [x] (println x))) ; not the same x (different symbol)