CODEWITHSUNDEEP
clojure
Harsh Jain
Harsh Jain

Reputation: 135

~ 'unquote' vs normal reference to a variable in clojure?

I have been playing around with clojure for some time.But not able to figure out the difference between ~ vs normal reference.

For eg:

(defn f [a b] (+ a b))
(f 1 2)

outputs:

3

and on the other hand: 

(defn g [a b] `(+ ~a ~b))
(g 1 2)

outputs:

(clojure.core/+ 1 2)

So my question is what's need for different syntax ?

Upvotes: 1

Views: 198

Answers (2)

Arthur Ulfeldt
Arthur Ulfeldt

Reputation: 91577

There is a language feature called "syntax-quote" that provides some syntactic shortcuts around forming lists that look like clojure expressions. You don't have to use it to build lists that are clojure s-expressions, you can build what you want with it, though it's almost always used in code that is part of a macro. Where that macro needs to build a Clojure s-expression and return it.

so your example 

(defn g [a b] `(+ ~a ~b))

when it's read by the Clojure reader would run the syntax-quote reader macro (which is named `)
and that syntax-quote macro will take the list 

(+ ~a ~b)

as it's argument and return the list 

(+ 1 2)

because it interprets symbol ~ to mean "include in the list we are building, the result of evaluating this next thing".

Upvotes: 4

Alan Thompson
Alan Thompson

Reputation: 29966

The backquote symbol ` and the tilde ~ are normally only used when writing macros. You shouldn't normally use them when writing normal functions using defn etc.

You can find more information here and in other books.

Upvotes: 0

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