3.16 Guarded Evaluation: when and unless

Effects If...: when and unless in The Racket Guide introduces when and unless.

syntax
(when test-expr body ...+)

Evaluates test-expr. If the result is #f, then the result of the when expression is #<void>. Otherwise, the bodys are evaluated, and the last body is in tail position with respect to the when form.

Examples:

> (when (positive? -5)
    (display "hi"))
> (when (positive? 5)
    (display "hi")
    (display " there"))
hi there

syntax
(unless test-expr body ...+)

Equivalent to (when (not test-expr) body ...+).

Examples:

> (unless (positive? 5)
    (display "hi"))
> (unless (positive? -5)
    (display "hi")
    (display " there"))
hi there

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2	Notation for Documentation
3	Syntactic Forms
4	Datatypes
5	Structures
6	Classes and Objects
7	Units
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9	Pattern Matching
10	Control Flow
11	Concurrency and Parallelism
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14	Reflection and Security
15	Operating System
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17	Unsafe Operations
18	Running Racket
	Bibliography
	Index

3.1	Modules: module, module*, ...
3.2	Importing and Exporting: require and provide
3.3	Literals: quote and #%datum
3.4	Expression Wrapper: #%expression
3.5	Variable References and #%top
3.6	Locations: #%variable-reference
3.7	Procedure Applications and #%app
3.8	Procedure Expressions: lambda and case-lambda
3.9	Local Binding: let, let*, letrec, ...
3.10	Local Definitions: local
3.11	Constructing Graphs: shared
3.12	Conditionals: if, cond, and, and or
3.13	Dispatch: case
3.14	Definitions: define, define-syntax, ...
3.15	Sequencing: begin, begin0, and begin-for-syntax
3.16	Guarded Evaluation: when and unless
3.17	Assignment: set! and set!-values
3.18	Iterations and Comprehensions: for, for/ list, ...
3.19	Continuation Marks: with-continuation-mark
3.20	Quasiquoting: quasiquote, unquote, and unquote-splicing
3.21	Syntax Quoting: quote-syntax
3.22	Interaction Wrapper: #%top-interaction
3.23	Blocks: block
3.24	Internal-Definition Limiting: #%stratified-body
3.25	Performance Hints: begin-encourage-inline
3.26	Importing Modules Lazily: lazy-require