clojure quote symbols

Well, quote and its related symbols is too confusing for a clojure newbie.
Thanks to this excellent article.

1. ‘

The symbol ', the single quote symbol near the Enter button in your keyboard. Can turn off evaluation functionality for the following expression, its functionality is totally equivalent to (quote).
Functionality of quote is simple and dedicate, which means the expression after this symbol will be treated just as literal, nothing more.

Let’s take some examples:

user=> '()                   ;()
user=> '[]                   ;[]
user=> '(1 2 3)              ;(1 2 3)
user=> '(list 1 2 3)         ;(list 1 2 3)
user=> (eval '(list 1 2 3))  ;(1 2 3)

As you can see the (eval) can evaluate the quoted or unevaluated(literal) expression and unveil its original ability.

2. `

This is a confusing symbol just because it looks like '. But actually this symbol called back-quote, located above the Tab button in your keyboard.
Clojure call it syntax-quote.
Very similar to quote, this syntax-quote , will left expression behind it unevaluated, but it has some more functionalities.

1. syntax-quote will try to find out the corresponding namespace of following symbols.
2. syntax-quote could combine use with some other special characters such like ~, whereas quote could not.

let us have some examples:

user=> `[name]      ;[clojure.core/name]
user=> `[`name]     ;[(quote clojure.core/name)]
user=> `['name]     ;[(quote clojure.core/name)]

You may notice the (syntax-quote) and (quote) have identical name in clojure, called quote.
You could use ' as short for quote, but there have no short hand for syntax-quote.

3. ~

This is a magic symbol to unquote expression within the effect area of syntax-quote, you could only use it in the scope of a syntax-quote, it named unquote.
By unquoting, the expression affected by ~ now can be evaluated again even if it in domain of a syntax-quote.

Examples below:

user=> `[~(quote name)]   ;[name], same as `[~'name]
user=> `[~(quote (name))] ;[(name)], equivalent to `[ ~'(name)]
user=> `[~'name]     ;[name], same as ['name]
user=> `[`~name]     ;[clojure.core/name], same as `[name]
user=> `['~name]     ;[(quote #<core$name clojure.core$name@d75415>)]

4. ~@

If you have seen my post of state management, you will know the @ is about to derefer the ref/atom/Agent.
But here this symbol again have another behavior that is ~ alike.
@ symbol must combined use with ~, which finally becomes ~@, called unquote-splicing symbol.
It could extract the inner elementns from the quoted list.

sample code:

user=> `(max @(shuffle (range 10)))  ; have no effect without ~ as prefix
;(clojure.core/max (clojure.core/deref (clojure.core/shuffle (clojure.core/range 10))))

user=> `(max ~(shuffle (range 10)))  ; just behaves as what syntax-quote could do
;(clojure.core/max [8 2 6 7 3 9 5 1 0 4]) ;There is an square parenthesis there

user=> `(max ~@(shuffle (range 10))) ; ~@ extract list without parenthesis.
;(clojure.core/max 0 4 7 5 1 2 6 3 9 8)

5. let’s confusing

You will graduately get accustom to it.
syntax-quote will affect items one by one in it follow scope, while encounter ~ then temporary disabled syntax-quote’s ability in this the scope of ~.

Try to learn more by yourself.

user=> `{:a 1 :b '~(+ 1 2)} ;{:a 1, :b (quote 3)}
user=> `[:a ~(+ 1 1) c]     ;[:a 2 user/c]
user=> `[:a ~(+ 1 1) ~`c]   ;[:a 2 user/c]
user=> `[:a ~(+ 1 1) ~'c]   ;[:a 2 c]
user=> `[:a ~(+ 1 1) 'c]    ;[:a 2 (quote user/c)]
user=> `[:a ~(+ 1 1) '~'c]  ;[:a 2 (quote c)]

user=> `{:a 1 :b '~@(list 1 2)}  ;{:a 1, :b (quote 1 2)}

user=> `(1 `(2 3) 4)
;(1 (clojure.core/seq (clojure.core/concat (clojure.core/list 2) (clojure.core/list 3))) 4)
user=> `(list 1 `(2 ~(- 9 6)) 4)
;(clojure.core/list 1 (clojure.core/seq (clojure.core/concat (clojure.core/list 2) (clojure.core/list (clojure.core/- 9 6)))) 4)
user=> `(list 1 `(2 ~~(- 9 6)) 4)
;(clojure.core/list 1 (clojure.core/seq (clojure.core/concat (clojure.core/list 2) (clojure.core/list 3))) 4)
user=> (eval `(list 1 `(2 ~~(- 9 6)) 4))   ;(1 (2 3) 4)

Congratulation

You finally complete this article, what have you learnt about the quote and unquote symbol in clojure?