► |  | Reference: PLT Scheme

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 | Running PLT Scheme
| Bibliography
| Index

▼ | 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 | Flattening Syntactic Sequences: nest

On this page:

quasiquote
unquote
unquote-splicing

Version: 4.1.4
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#### 2.20`` Quasiquoting: quasiquote, unquote, and unquote-splicing

(quasiquote datum)

The same as 'datum if datum does not include ,expr or ,@expr. An ,expr expression escapes from the quote, however, and the result of the expr takes the place of the ,expr form in the quasiquote result. An ,@expr similarly escapes, but the expr must produce a list, and its elements are spliced as multiple values place of the ,@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). If unquote or unquote-splicing appears within quasiquote in any other way than as ,expr or ,@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)
> `(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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