Clojure - Quoting Confusion

Question

Sorry for the terribly vague title :)

I am new to macros and am having trouble understanding the difference between these two statements:

`(+ 1 2 ~(+ 2 3)) ; => (clojure.core/+ 1 2 5)
'(+ 1 2 ~(+ 2 3)) ; => (+ 1 2 (clojure.core/unquote (+ 2 3)))

When I run them without the unquote, they seem rather identical other than qualifying?

`(+ 1 2 (+ 2 3)) ; => (clojure.core/+ 1 2 (clojure.core/+ 2 3))
'(+ 1 2 (+ 2 3)) ; => (+ 1 2 (+ 2 3))

So basically I'm confused by ` vs '. My understanding is that they both quote everything in the list, which is why I'm not sure why unquoting behaves differently. Basically ` behaves the way I would expect both ` and ' to behave.

Thanks!

Answer 1

The short answer is, that unquoting only works inside a backquote. In a normal quoted expression, everything – included ~ and everything inside/behind – is just passed as-is, while inside a backquoted expression, everything inside/behind ~ is evaluated (but everything else stays unevaluated). So, no, not everything inside a backquoted expression remains unevaluated – you can use ~ inside it, to use it as a kind of template, where you "fill in the blanks" with ~.

Edit: To quote (pun intended) the documentation relevant to your question:

Quote:

Quote (')
'form => (quote form)

and (from the special forms section):

(quote form) Yields the unevaluated form.

user=> '(a b c)
(a b c)

Note there is no attempt made to call the function a. The return value is a list of 3 symbols.

Syntax-quote (also called quasiquote, backquote):

For Lists/Vectors/Sets/Maps, syntax-quote establishes a template of the corresponding data structure. Within the template, unqualified forms behave as if recursively syntax-quoted, but forms can be exempted from such recursive quoting by qualifying them with unquote or unquote-splicing, in which case they will be treated as expressions and be replaced in the template by their value, or sequence of values, respectively.

Answer 2

Try running eval on the results of your first two expressions. The first one, with `, "expands" to (+ 1 2 3), which nicely evaluates to 6. The second, with ', "expands" to (+ 1 2 (unquote (+ 1 2))), and unquote is not valid in that context since you're no longer inside of a quote! So this completely fails to evaluate at all.

Basically there are two differences between ' and `:

• ` namespace-qualifies everything
• ` allows unquoting