Why does this clojure macro need `'~?

Question

(Apologies if this is a duplicate of another question, my search for all those fancy special characters didn't yield anything.)

I'm reading Mastering Clojure Macros and have trouble understanding the following example:

(defmacro inspect-caller-locals []
  (->> (keys &env)
       (map (fn [k] [`'~k k]))
       (into {})))
=> #'user/inspect-caller-locals
(let [foo "bar" baz "quux"]
  (inspect-caller-locals))
=> {foo "bar", baz "quux"}

What is the difference between the following and the much simpler 'k?

`'~k

As far as I understand, the innermost unquote ~ should simply reverts the effect of the outermost syntax-quote `, but a short experiment reveals that there's more to it:

(defmacro inspect-caller-locals-simple []
  (->> (keys &env)
       (map (fn [k] ['k k]))
       (into {})))
=> #'user/inspect-caller-locals-simple
(let [foo "bar" baz "quux"]
  (inspect-caller-locals-simple))
CompilerException java.lang.RuntimeException: Unable to resolve symbol: k in this context, compiling:(/tmp/form-init4400591386630133028.clj:2:3)

Unfortunately, my usual investigation approach doesn't apply here:

(macroexpand '(let [foo "bar" baz "quux"]
                 (inspect-caller-locals)))
=> (let* [foo "bar" baz "quux"] (inspect-caller-locals))
(let [foo "bar" baz "quux"]
  (macroexpand '(inspect-caller-locals)))
=> {}

What am I missing here?

Instead of macroexpand, try just evaluating the part of the macro you care about, by running this in your repl: (let [k 'foo] [k 'k `'~k]) — amalloy

ClojureMostly ClojureMostly · Accepted Answer · 2016-06-23T19:39:07

Let's first establish what the k inside the macro is:

(defmacro inspect-caller-locals []
  (mapv (fn [k]
          (println (class k)))
        (keys &env))
  nil)
(let [x 1]
  (inspect-caller-locals))
;; Prints:
;; clojure.lang.Symbol

So you each k inside the function is a symbol. If you return a symbol from a macro (ie generate code from it), clojure will lookup the value that it refers to and print it. For instance you could do this:

(defmacro inspect-caller-locals []
  (mapv (fn [k]
          [(quote x) k]) ;; not the "hard coded" x
        (keys &env)))
(let [x 1]
  (inspect-caller-locals))
;; Prints:
;; [[1 1]]

What you want however is the actual symbol. The problem (as you noted) is that quote is a special form that DOES NOT EVALUTE whatever you pass it. Ie, the k will not obtain the function parameter but stay k which is not usually defined:

(defmacro inspect-caller-locals []
  (mapv (fn [k]
          [(quote k) k])
        (keys &env)))
(let [x 1]
  (inspect-caller-locals))
;; => Error
(let [k 1]
  (inspect-caller-locals))
;; Prints:
;; [[1 1]]

You somehow need to evaluate what you pass into quote, this is not however possible since that isn't what quote does. Other functions, such as str don't have that problem:

(defmacro inspect-caller-locals []
  (mapv (fn [k]
          [(str k) k])
        (keys &env)))
(let [x 1]
  (inspect-caller-locals))
;; Prints:
;; [["x" 1]]

The trick is to go one level deeper and quote the quote itself so you can pass the symbol to it:

(defmacro inspect-caller-locals []
  (mapv (fn [k]
          [;; This will evaluate k first but then generate code that 
           ;; wraps that symbol with a quote:
           (list (quote quote) k)
           ;; Or equivalently and maybe easier to understand:
           (list 'quote k)
           k])
        (keys &env)))
(let [x 1]
  (inspect-caller-locals))
;; Prints:
;; [[x x 1]]

Or by using the reader that can do this for you:

(defmacro inspect-caller-locals []
  (mapv (fn [k]
          [`(quote ~k)
           `'~k
           k])
        (keys &env)))
(let [x 1]
  (inspect-caller-locals))
;; Prints:
;; [[x x 1]]

Because after all:

(read-string "`'~k")
=> (clojure.core/seq (clojure.core/concat (clojure.core/list (quote quote)) (clojure.core/list k)))
(defmacro inspect-caller-locals []
  (mapv (fn [k]
          [(clojure.core/seq (clojure.core/concat (clojure.core/list (quote quote)) (clojure.core/list k)))
           k])
        (keys &env)))
(let [x 1]
  (inspect-caller-locals))
;; Prints:
;; [[x 1]]

Why does this clojure macro need `'~?

2 Answers