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https://issues.apache.org/jira/browse/GROOVY-12255?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18104587#comment-18104587
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ASF GitHub Bot commented on GROOVY-12255:
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daniellansun opened a new pull request, #2784:
URL: https://github.com/apache/groovy/pull/2784
https://issues.apache.org/jira/browse/GROOVY-12255
> Compile switch expressions as first-class AST (no closure desugar)
> ------------------------------------------------------------------
>
> Key: GROOVY-12255
> URL: https://issues.apache.org/jira/browse/GROOVY-12255
> Project: Groovy
> Issue Type: Dependency upgrade
> Reporter: Daniel Sun
> Priority: Major
>
> h3. Problem
> GROOVY-9272 added switch expressions. The 4.0 implementation rewrites them in
> {{AstBuilder}} to an immediately-called closure around a switch {{statement}}:
> {code:groovy}
> // source
> def r = switch (x) {
> case 0, 1 -> 'a'
> default -> 'z'
> }
> // compiled as
> def r = { ->
> switch (x) {
> case 0:
> case 1: return 'a'
> default: return 'z'
> }
> }.call()
> {code}
> That is a simulation, not a JEP 361 switch expression:
> * every evaluation allocates a closure and an extra call frame
> * an unmatched selector completes with {{null}} instead of throwing
> * {{return}} / {{break}} / {{continue}} are interpreted against the synthetic
> closure, not the enclosing method
> * locals assigned in an arm are closure-shared, not method locals
> * {{@CompileStatic}} cannot emit {{tableswitch}} / {{lookupswitch}} the way
> javac does
> h3. Goal
> Compile a switch expression as a first-class {{SwitchExpression}} whose arms
> {{yield}} (or throw). Emit the result on the operand stack. Keep Groovy
> {{isCase}} matching (Class, regex, Collection, Closure). Align control flow
> and exhaustiveness with [JEP 361|https://openjdk.org/jeps/361] for both
> dynamic Groovy and {{@TypeChecked}} / {{@CompileStatic}}.
> h3. Proposed shape
> * Parser builds {{SwitchExpression}} / {{YieldStatement}}; arrow expressions
> become implicit {{yield}}. No closure wrapper.
> * Codegen: join all completing arms at one label with the value on the stack.
> When the selector and labels allow it, emit {{tableswitch}} /
> {{lookupswitch}}, the Java string-switch (hash + {{equals}} + second switch),
> or {{Enum.ordinal()}}; otherwise sequential {{isCase}}.
> * Exhaustiveness: unmatched dynamic selector throws
> {{IllegalStateException}}; a complete enum may omit {{default}} (synthetic
> {{IncompatibleClassChangeError}} if a new constant appears at runtime).
> {{@TypeChecked}} / {{@CompileStatic}} reject a provably non-exhaustive
> expression at compile time.
> * Control flow: {{return}} must not leave the enclosing method through a
> switch expression; {{yield}} must not jump through a nested closure/lambda.
> An arrow arm must {{yield}} or throw on every path.
> {code:groovy}
> int n = switch (day) {
> case MONDAY, FRIDAY -> 6
> case TUESDAY -> 7
> default -> {
> int len = day.toString().length()
> yield len
> }
> }
> {code}
> h3. Compatibility
> || topic || 4.0-6.x (closure rewrite) || after this change ||
> | unmatched selector (dynamic) | {{null}} | {{IllegalStateException}} |
> | non-exhaustive under STC / CS | often accepted | compile error (unless a
> complete enum) |
> | arrow block with no {{yield}} | last expression is the closure result |
> compile error unless every path yields or throws |
> | Groovy {{isCase}} cases | works | still works (fast path only when labels
> are int / String / enum constants) |
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