3.20 Quasiquoting: quasiquote, unquote, and unquote-splicing

Quasiquoting: quasiquote and ‘ in The Racket Guide introduces quasiquote.

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.

A quasiquote, unquote, or unquote-splicing form is typically abbreviated with `, ,, or ,@, respectively. See also Reading Quotes.

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.