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?
~'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