3.16 Guarded Evaluation: when and unless

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▼ | 3 | Syntactic Forms

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

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#### 3.16`` Guarded Evaluation: when and unless🔗ℹ

> > > 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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