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Crystal / Generics and Macros

case type narrowing

Learn case type narrowing in Crystal with a construct-specific explanation, unique example, engineering boundary, and mastery check.

10 min read • Intermediate

case type narrowing is a focused Crystal topic inside Generics and Macros. This lesson explains the construct itself, the state or value it operates on, and the observable result it produces.

Read the example as a small executable experiment. Identify the exact syntax that belongs to case type narrowing, then change one input and predict the result before running it.

Topic-specific example

crystal-06-05-case-type-narrowing.cr
crystal Try It Live 
case value
when Int32 then puts value + 1
when String then puts value.upcase
end
Edge case / engineering note

case type narrowing has rules that are easy to miss when the example is copied mechanically. Check the feature-specific boundary shown by this construct: scope, mutability, evaluation order, type constraints, lifetime, error behavior, compile-time restrictions, or resource cleanup where applicable.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

Lesson quizzes

case type narrowing Mastery Check →
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is_a? narrowing
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