Daniel Sun created GROOVY-12255:
-----------------------------------
Summary: 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
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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