Class BooleanTruthGuard

java.lang.Object

org.ek9lang.compiler.support.BooleanTruthGuard

public final class BooleanTruthGuard
extends Object

THE Boolean truth guard rule (Steve, 2026-10-07, decision D9.2) - use this, do not re-derive it.

An if or else if head whose pre-flow is a declaration guard (<-) or an assign-if-unset (:=?), and whose target's DECLARED type is exactly Boolean, enters only when the target is SET AND TRUE. Every other guard enters when its target is set. So if ok <- check() skips the body when check() returns false, exactly as a reader expects; before this rule a set false entered it.

Deliberately narrow:

• Only if / else if (and their when spelling). A switch keeps enter-if-set, or its case false could never be reached; while, do, for and try keep enter-if-set too.
• Only <- and :=?. A := head is blind (no gate at all).
• Only a target whose type IS Boolean (by symbol identity, isExactSameType). A type that merely promotes to Boolean (#^) keeps enter-if-set; so does a generic's T, whatever it is parameterised with, because the declared type is T.

Phase 7 adds the _true() test to the guard's gate (GuardedConditionEvaluator); phase 5 narrows the target to true inside the body and does not narrow it to unset on the skip path (it may be false).

Method Summary

Modifier and Type	Method	Description
static boolean	isTruthGuardPosition(EK9Parser.PreFlowStatementContext preFlow)	Is this pre-flow a <- or :=? (to a plain identifier) in the head of an if or else if? Says nothing about the type - see targetOf(EK9Parser.PreFlowStatementContext, Function, ISymbol).
static Optional<ISymbol>	targetOf(EK9Parser.PreFlowStatementContext preFlow, Function<org.antlr.v4.runtime.tree.ParseTree, ISymbol> recordedSymbol, ISymbol ek9Boolean)	The target of a Boolean truth guard, if this pre-flow is one.

Methods inherited from class Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Details

targetOf

public static Optional<ISymbol> targetOf(EK9Parser.PreFlowStatementContext preFlow,
 Function<org.antlr.v4.runtime.tree.ParseTree, ISymbol> recordedSymbol,
 ISymbol ek9Boolean)

The target of a Boolean truth guard, if this pre-flow is one.

Parameters:

preFlow - the pre-flow statement (may be null)

recordedSymbol - how the caller reads the symbol recorded against a parse tree node

ek9Boolean - the built-in Boolean type

Returns:

the guard's target when the truth guard applies, otherwise empty

isTruthGuardPosition

public static boolean isTruthGuardPosition(EK9Parser.PreFlowStatementContext preFlow)

Is this pre-flow a <- or :=? (to a plain identifier) in the head of an if or else if? Says nothing about the type - see targetOf(EK9Parser.PreFlowStatementContext, Function, ISymbol).