3.20 Quasiquoting: quasiquote, unquote, and unquote-splicing
Quasiquoting: quasiquote and ‘ in The Racket Guide introduces quasiquote.
syntax
(quasiquote datum)
An unquote or unquote-splicing form is recognized in any of the following escaping positions within datum: in a pair, in a vector, in a box, in a prefab structure field after the name position, and in hash table value position (but not in a hash table key position). Such escaping positions can be nested to an arbitrary depth.
An unquote-splicing form must appear as the car of a quoted pair, as an element of a quoted vector, or as an element of a quoted prefab structure. In the case of a pair, if the cdr of the relevant quoted pair is empty, then expr need not produce a list, and its result is used directly in place of the quoted pair (in the same way that append accepts a non-list final argument).
If unquote or unquote-splicing appears within quasiquote in an escaping position but in a way other than as (unquote expr) or (unquote-splicing expr), a syntax error is reported.
> (quasiquote (0 1 2)) '(0 1 2)
> (quasiquote (0 (unquote (+ 1 2)) 4)) '(0 3 4)
> (quasiquote (0 (unquote-splicing (list 1 2)) 4)) '(0 1 2 4)
> (quasiquote (0 (unquote-splicing 1) 4)) unquote-splicing: contract violation
expected: list?
given: 1
> (quasiquote (0 (unquote-splicing 1))) '(0 . 1)
A quasiquote, unquote, or unquote-splicing form is typically abbreviated with `, ,, or ,@, respectively. See also Reading Quotes.
> `(0 1 2) '(0 1 2)
> `(1 ,(+ 1 2) 4) '(1 3 4)
> `#s(stuff 1 ,(+ 1 2) 4) '#s(stuff 1 3 4)
> `#hash(("a" . ,(+ 1 2))) '#hash(("a" . 3))
> `#hash((,(+ 1 2) . "a")) '#hash((,(+ 1 2) . "a"))
> `(1 ,@(list 1 2) 4) '(1 1 2 4)
> `#(1 ,@(list 1 2) 4) '#(1 1 2 4)
A quasiquote form within the original datum increments the level of quasiquotation: within the quasiquote form, each unquote or unquote-splicing is preserved, but a further nested unquote or unquote-splicing escapes. Multiple nestings of quasiquote require multiple nestings of unquote or unquote-splicing to escape.
The quasiquote form allocates only as many fresh cons cells, vectors, and boxes as are needed without analyzing unquote and unquote-splicing expressions. For example, in
`(,1 2 3)
a single tail '(2 3) is used for every evaluation of the quasiquote expression. When allocating fresh data, the quasiquote form allocates mutable vectors, mutable boxes and immutable hashes.
> (immutable? `#(,0)) #f
> (immutable? `#hash((a . ,0))) #t
syntax
syntax