2.16 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	Syntactic Forms
3	Datatypes
4	Structures
5	Classes and Objects
6	Units
7	Contracts
8	Pattern Matching
9	Control Flow
10	Concurrency and Parallelism
11	Macros
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13	Reflection and Security
14	Operating System
15	Memory Management
16	Unsafe Operations
17	Running Racket
	Bibliography
	Index

2.1	Modules: module, 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	Blocks: block
2.24	Internal-Definition Limiting: #%stratified-body
2.25	Performance Hints: begin-encourage-inline