Clojure: The subtle difference between quote (‘) and backtick (`)

The code presented here is available on github .

The obvious parts

You’ll be aware of these if you’ve ever bothered writing macros:

• Both prevent evaluation of the subsequent expression:

  `(1 2 3) ;; => (1 2 3)
  '(1 2 3) ;; => (1 2 3)
  

• Backtick allows you to unquote parts of the expression:

  (def x 3)
  `(1 2 ~x) ;; => (1 2 3)
  '(1 2 ~x) ;; => (1 2 (clojure.core/unquote x))
  

  As you can see, quote does not interpret the tilde character (which is just a reader macro for clojure.core/unquote).

• …or even additionally splice a sequence:

  (def y (list 3 4))
  `(1 2 ~@y) ;; => (1 2 3 4)
  '(1 2 ~@y) ;; => (1 2 (clojure.core/unquote-splicing y))
  

  Same thing here – quote does not do anything with the “~@” reader macro, as it is just an alias for clojure.core/unquote-splicing.

The subtle part

• However, quote and backtick show different behavior in how they handle namespace resolution for the symbols they quote:

  (def z 111)
  `(1 2 z) ;; => (1 2 user/z)
  '(1 2 z) ;; => (1 2 z)
  

  So why is this relevant?

  Well, consider the following code. The essence of this is that we have a namespace macro-ns in which macros get defined. Those macros are being used in another namespace (user here):

  (in-ns 'macro-ns)
  (clojure.core/use 'clojure.core)
  
  (def z 222)
  
  (defmacro m-quote []
    'z)
  (defmacro m-backtick []
    `z)
  
  ;; Now, we'll want to use these macros in the "user" namespace:
  
  (in-ns 'user)
  
  (def z 111)
  
  (macroexpand-1 '(macro-ns/m-quote))    ;; => z
  (macroexpand-1 '(macro-ns/m-backtick)) ;; => macro-ns/z
  
  (macro-ns/m-quote)    ;; => 111
  (macro-ns/m-backtick) ;; => 222
  

  So obviously it does matter whether you use quote or backtick in a macro, even if you don’t intend to use any unquoting within. As you can see above, m-quote refers to user/z, while m-backtick results in an expansion that uses macro-ns/z.