Clojure - difference between quote and syntax quote

Question

(def x 1)
user=> '`~x
x
user=> `'~x
(quote 1)

Can anyone explain please how it is evaluated step by step?

Answer 1

The single-quote operator returns the expression or symbol that you are quoting without evaluating it. The syntax-quote operator returns the expression or symbol that you are quoting (with namespaces added), without evaluating it. The syntax-unquote operator "cancels out" the syntax-quote operator, so to speak, but not the single-quote. You can nest syntax-quotes and syntax-unquotes to perform weird and wonderful feats. My favorite analogy I read for understanding these is to consider syntax-quoting and syntax-unquoting as moving up and down rungs of a ladder (possible source).

In the form `x, x is syntax-quoted, so it isn't evaluated; instead, you get a namespaced symbol (like user/x). But in the form `~x, x is syntax-unquoted again, so it is evaluated:

user=> `~x
1

On to your examples:

Example 1

' is just sugar for (quote ...).

So '`~x becomes (quote `~x). This in turn becomes (quote x) (remember that `~ doesn't really do anything), which is why the whole expression evaluates to the symbol x.

Example 2

In `'~x, let's first replace the ' with quote: `(quote ~x). The expression is syntax-quoted, so it won't be evaluated.

So you can think of the expression (quote ~x) as an "intermediate step." But we're not done. x inside the syntax-quote is syntax-unquoted, just as in my example up above. So even though this expression as a whole won't be evaluated, x will be, and its value is 1. In the end, you get the expression: (quote 1).

Blog post on the topic.