Guide: PLT Scheme

1`` | Welcome to PLT Scheme
2`` | Scheme Essentials
3`` | Built-In Datatypes
4`` | Expressions and Definitions
5`` | Programmer-Defined Datatypes
6`` | Modules
7`` | Contracts
8`` | Input and Output
9`` | Regular Expressions
10`` | Exceptions and Control
11`` | Iterations and Comprehensions
12`` | Pattern Matching
13`` | Classes and Objects
14`` | Units (Components)
15`` | Reflection and Dynamic Evaluation
16`` | Macros
17`` | Performance
18`` | Running and Creating Executables
19`` | Compilation and Configuration
20`` | More Libraries
| Bibliography
| Index

Version: 4.0.2
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#### 4.11`` Quasiquoting: quasiquote and `

The quasiquote form is similar to quote:

> (quasiquote datum)

However, for each (unquote expr) that appears within the datum, the expr is evaluated to produce a value that takes the place of the unquote sub-form.

Examples:
> (quasiquote (1 2 (unquote (+ 1 2)) (unquote (- 5 1))))
(1 2 3 4)

The unquote-splicing form is similar to unquote, but its expr must produce a list, and the unquote-splicing form must appear in a context that produces either a list or a vector. As the name suggests, the resulting list is spliced into the context of its use.

Examples:
> (quasiquote (1 2 (unquote-splicing (list (+ 1 2) (- 5 1))) 5))
(1 2 3 4 5)

If a quasiquote form appears within an enclosing quasiquote form, then the inner quasiquote effectively cancels one layer of unquote and unquote-splicing forms, so that a second unquote or unquote-splicing is needed.

Examples:

> (quasiquote (1 2 (quasiquote (unquote (+ 1 2)
(unquote (unquote (- 5 1)))))))

(1 2 (quasiquote (unquote (+ 1 2)) (unquote 4)))

The evaluation above will not actually print as shown. Instead, the shorthand form of quasiquote and unquote will be used: ` (i.e., a backquote) and , (i.e., a comma). The same shorthands can be used in expressions:

Examples:
> `(1 2 `(,(+ 1 2) ,,(- 5 1)))
(1 2 `(,(+ 1 2) ,4))

The shorthand for of unquote-splicing is ,@:

Examples:
> `(1 2 ,@(list (+ 1 2) (- 5 1)))
(1 2 3 4)

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