2.16 Guarded Evaluation: when and unless

Version: 4.2.1

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 #f, then the result of the when expression is #<void>. Otherwise, the exprs are evaluated, and the last 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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2.1	Modules: 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	Limiting Scope: define-package, open-package, ...
2.24	Flattening Syntactic Sequences: nest