2.20 Quasiquoting: quasiquote, unquote, and unquote-splicing

► |  | Reference: Racket

1 | Language Model
2 | Syntactic Forms
3 | Datatypes
4 | Structures
5 | Classes and Objects
6 | Units
7 | Contracts
8 | Pattern Matching
9 | Control Flow
10 | Concurrency
11 | Macros
12 | Input and Output
13 | Reflection and Security
14 | Operating System
15 | Memory Management
16 | Unsafe Operations
17 | Running Racket
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▼ | 2 | Syntactic Forms

2.1 | Modules: module, ...
2.2 | Importing and Exporting: require and provide
2.3 | Literals: quote and #%datum
2.4 | Expression Wrapper: #%expression
2.5 | Variable References and #%top
2.6 | Locations: #%variable-reference
2.7 | Procedure Applications and #%app
2.8 | Procedure Expressions: lambda and case-lambda
2.9 | Local Binding: let, let*, letrec, ...
2.10 | Local Definitions: local
2.11 | Constructing Graphs: shared
2.12 | Conditionals: if, cond, and, and or
2.13 | Dispatch: case
2.14 | Definitions: define, define-syntax, ...
2.15 | Sequencing: begin, begin0, and begin-for-syntax
2.16 | Guarded Evaluation: when and unless
2.17 | Assignment: set! and set!-values
2.18 | Iterations and Comprehensions: for, for/ list, ...
2.19 | Continuation Marks: with-continuation-mark
2.20 | Quasiquoting: quasiquote, unquote, and unquote-splicing
2.21 | Syntax Quoting: quote-syntax
2.22 | Interaction Wrapper: #%top-interaction
2.23 | Limiting Scope: define-package, open-package, ...
2.24 | Blocks: block
2.25 | Internal-Definition Limiting: #%stratified-body

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quasiquote
unquote
unquote-splicing

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#### 2.20`` Quasiquoting: quasiquote, unquote, and unquote-splicing

> > > Quasiquoting: quasiquote and ` in Guide: Racket introduces quasiquote.

( quasiquote datum)

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 the expr must produce a list, and its elements are spliced as multiple values place of the ( unquote-splicing expr), which must appear as the car or 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). In a quoted hash table, an ( unquote expr) or ( unquote-splicing expr) expression escapes only in the second element of an entry pair (i.e., the value), while entry keys are always implicitly quoted. If unquote or unquote-splicing appears within quasiquote in any other way 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: expected argument of type <proper 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.

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 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.

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 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.

unquote

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

unquote-splicing

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