What is the purpose of ~' or '~ in Clojure?

Question

I am learning about Clojure macros, and the code examples will sometimes have the constructs '~symbol or alternately ~'symbol. I know that (quote and ' prevent a form from being evaluated, and that the backquote additionally adds namespace qualification, and that ~ causes a quoted form to be evaluated. My question is: why is it useful to stop then start evaluation? I also assume that ~'symbol and '~symbol are different, but how so?

Answer 1

~'symbol is used to produce an unqualified symbol. Clojure's macros capture namespace by default, so a symbol in a macro would normally be resolved to (your-namespace/symbol). The unquote-quote idiom directly results in the simple, unqualified symbol name - (symbol) - by evaluating to a quoted symbol. From The Joy Of Clojure:

(defmacro awhen [expr & body]   `(let [~'it ~expr] ; refer to the expression as "it" inside the body     (when ~'it       (do ~@body))))  (awhen [:a :b :c] (second it)) ; :b 

'~symbol is likely used to insert a name in the macro or something similar. Here, symbol will be bound to a value - let [symbol 'my-symbol]. This value is then inserted into the code the macro produces by evaluating symbol.

(defmacro def-symbol-print [sym]   `(defn ~(symbol (str "print-" sym)) []     (println '~sym))) ; print the symbol name passed to the macro  (def-symbol-print foo) (print-foo) ; foo