3.20 Quasiquoting: quasiquote, unquote, and unquote-splicing (original) (raw)

3.20 Quasiquoting: quasiquote, unquote, and unquote-splicing🔗ℹ

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

The same as 'datum if datum does not include(unquote expr) or (unquote-splicing expr). An(unquote expr) form escapes from the quote, however, and the result of the expr takes the place of the(unquote expr) form in the quasiquote result. An(unquote-splicing expr) similarly escapes, but theexpr produces a list whose elements are spliced as multiple values place of the (unquote-splicing expr).

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 thecdr of the relevant quoted pair is empty, then exprneed 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 withinquasiquote in an escaping position but in a way other than as(unquote expr) or (unquote-splicing expr), a syntax error is reported.

Examples:

> (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-splicingform is typically abbreviated with `, ,, or,@, respectively. See also Reading Quotes.

Examples:

> `(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 datumincrements the level of quasiquotation: within the quasiquoteform, each unquote or unquote-splicing is preserved, but a further nested unquote or unquote-splicingescapes. Multiple nestings of quasiquote require multiple nestings of unquote or unquote-splicing to escape.

Examples:

> `(1 `,(+ 1 ,(+ 2 3)) 4)
'(1 `,(+ 1 5) 4)
> `(1 ```,,@,,@(list (+ 1 2)) 4)
'(1 ```,,@,3 4)

The quasiquote form allocates only as many fresh cons cells, vectors, and boxes as are needed without analyzing unquoteand unquote-splicing expressions. For example, in

`(,1 2 3)

a single tail '(2 3) is used for every evaluation of thequasiquote expression. When allocating fresh data, the quasiquote form allocates mutable vectors, mutable boxes and immutable hashes.

Examples:

See quasiquote, where unquote is recognized as an escape. An unquote form as an expression is a syntax error.

See quasiquote, where unquote-splicing is recognized as an escape. An unquote-splicing form as an expression is a syntax error.