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
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when
unless

Version: 4.0.2
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#### 2.16`` Guarded Evaluation: when and unless

Effects If...: when and unless in Guide: PLT Scheme introduces when and unless.

(when test-expr expr ...)

Evaluates the text-expr. If the result is any value other than #f, the exprs are evaluated, and the results are ignored. No expr 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

(unless test-expr expr ...)

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

Examples:

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

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

hi there

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