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Support multiple ELSE in IF statements — core_plus 12→11

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Forth 2012 allows multiple ELSEs: IF 1 ELSE 2 ELSE 3 ELSE 4 ELSE 5 THEN
produces (1 3 5) for true and (2 4) for false. Desugars by saving the
condition flag on the return stack with >R/R@ and building nested
If/Else pairs. The final THEN cleans up with R> DROP.
This commit is contained in:
Oleksandr Kozachuk
2026-04-08 12:12:27 +02:00
parent 57f5f66704
commit 533ef2d223
1 changed files with 34 additions and 1 deletions
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crates/core/src/outer.rs
+34 -1
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@@ -1075,6 +1075,27 @@ impl ForthVM {
});
// compiling_ir is now empty and will collect the else_body
}
Some(ControlEntry::IfElse {
then_body,
else_body: mut prefix,
}) => {
// Multiple ELSE: save the condition flag on the return stack
// so subsequent IFs can re-test it with R@.
let first_else = std::mem::take(&mut self.compiling_ir);
prefix.push(IrOp::ToR); // save flag to return stack
prefix.push(IrOp::RFetch); // copy for first If test
prefix.push(IrOp::If {
then_body,
else_body: Some(first_else),
});
// R-stack still holds the flag; push R@ for next If test
prefix.push(IrOp::RFetch);
// Push an If entry — the next code will be the "then" body
// of the next branch pair (e.g., code "3" in IF 1 ELSE 2 ELSE 3 ELSE 4)
self.control_stack
.push(ControlEntry::If { then_body: prefix });
// compiling_ir is empty, collects the next then-code
}
Some(ControlEntry::PostDoubleWhileRepeat {
outer_test,
inner_test,
@@ -1103,12 +1124,18 @@ impl ForthVM {
Some(ControlEntry::If { then_body: prefix }) => {
// compiling_ir has the then_body ops
let then_body = std::mem::take(&mut self.compiling_ir);
// Restore prefix and append the If node
// Check if this was created by a multi-ELSE desugaring
// (prefix ends with RFetch which pushed the flag for this If)
let multi_else = matches!(prefix.last(), Some(IrOp::RFetch));
self.compiling_ir = prefix;
self.compiling_ir.push(IrOp::If {
then_body,
else_body: None,
});
if multi_else {
self.compiling_ir.push(IrOp::FromR);
self.compiling_ir.push(IrOp::Drop);
}
}
Some(ControlEntry::IfElse {
then_body,
@@ -1116,11 +1143,17 @@ impl ForthVM {
}) => {
// compiling_ir has the else_body ops
let else_body = std::mem::take(&mut self.compiling_ir);
// Check if this was created by a multi-ELSE desugaring
let multi_else = matches!(prefix.last(), Some(IrOp::RFetch));
self.compiling_ir = prefix;
self.compiling_ir.push(IrOp::If {
then_body,
else_body: Some(else_body),
});
if multi_else {
self.compiling_ir.push(IrOp::FromR);
self.compiling_ir.push(IrOp::Drop);
}
}
Some(ControlEntry::PostDoubleWhileRepeat {
outer_test,