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Common Lisp / Bindings & Control

when

Learn when in Common Lisp through a focused explanation, a dedicated construct-specific example, an engineering boundary, and a mastery check.

10 min read • Beginner

when is an atomic Common Lisp topic inside Bindings & Control. This lesson focuses on how the when construct is written, what it changes, and when it is the right tool.

Trace the example line by line. Identify the exact syntax for when, the input state before it runs, and the resulting value, control flow, database state, hardware behavior, or runtime effect afterward. Neighboring lessons use different constructs rather than renamed copies of this one.

Topic-specific example

common-lisp-02-04-when.lisp
common-lisp Try It Live
(let ((ready t)) (when ready (format t "running~%")))
Edge case / engineering note

For when, test the boundary that belongs to this exact feature rather than a generic failure case. Check invalid or empty input, NULL/unknown state, numeric limits, transaction or exception behavior, synthesis/runtime constraints, ownership/permission rules, or backtracking/control-flow effects where they apply.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

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