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AGR: Detect epsilon-reachable cycles in grammar compiler #1955

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Merged
curtisman merged 1 commit into from
Feb 28, 2026
Merged

AGR: Detect epsilon-reachable cycles in grammar compiler #1955

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128 changes: 112 additions & 16 deletions ts/packages/actionGrammar/src/grammarCompiler.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@ import {
Grammar,
GrammarPart,
GrammarRule,
RulesPart,
StringPart,
} from "./grammarTypes.js";
import {
Expand All @@ -29,6 +30,8 @@ type DefinitionRecord = {
pos: number | undefined;
grammarRules?: GrammarRule[];
hasValue: boolean;
compiling: boolean; // true while grammarRules is being populated
nullable?: boolean; // set after compilation; true if any alternative matches ε
};

type CompletedDefinitionRecord = DefinitionRecord & {
Expand Down Expand Up @@ -136,6 +139,7 @@ function createCompileContext(
pos: def.pos,
// Set this to true to allow recursion to assume that it has value.
hasValue: true,
compiling: false,
});
} else {
existing.rules.push(...def.rules);
Expand Down Expand Up @@ -300,6 +304,8 @@ const emptyRecord = {
pos: undefined,
grammarRules: [],
hasValue: true, // Pretend to have value to avoid cascading errors
compiling: false,
nullable: false,
};

/**
Expand Down Expand Up @@ -360,11 +366,32 @@ function validateVariableReferences(
break;
}
}
// ε-reachable cycle detection
//
// A grammar rule causes an infinite loop at match time when a named rule can
// recurse back to itself without ever consuming a mandatory input token — i.e.
// when the cycle is reachable via ε-transitions (optional parts, rule expansions
// that themselves match ε).
//
// Detection: `epsilonReachable` carries the set of rule names entered since the
// last mandatory input was consumed. When a back-reference is found
// (record.compiling === true) and the rule name is still in that set, the full
// path back to the entry point was traversed without consuming any input, so an
// error is reported.
//
// Nullability note — two asymmetric checks appear at each rule-reference site:
// record.nullable === false → clear currentEpr (only when *definitely*
// non-nullable; undefined/back-ref leaves it
// intact to avoid masking a cycle further along)
// record.nullable ?? false → propagate ruleNullable (treat back-refs
// conservatively as non-nullable)

function createNamedGrammarRules(
context: CompileContext,
name: string,
referencePosition?: number,
referenceVariable?: string,
epsilonReachable: Set<string> = new Set(),
referenceContext: CompileContext = context,
): CompletedDefinitionRecord {
const record = context.ruleDefMap.get(name);
Expand All @@ -385,18 +412,43 @@ function createNamedGrammarRules(
name,
referencePosition,
referenceVariable,
epsilonReachable,
referenceContext,
);
}
if (record.grammarRules === undefined) {
if (record.compiling) {
if (epsilonReachable.has(name)) {
// Back-reference reachable without consuming any input: infinite loop at match time
referenceContext.errors.push({
message: `Rule '<${name}>' creates an epsilon-reachable cycle that would cause an infinite loop at match time`,
definition: referenceContext.currentDefinition,
pos: referencePosition,
});
}
// else: non-epsilon back-reference (mandatory input consumed before the
// back-ref); just return the incomplete record — the grammar is valid.
} else if (record.grammarRules === undefined) {
const eprWithSelf = new Set(epsilonReachable).add(name);
const prev = context.currentDefinition;
context.currentDefinition = name;
// Assign an empty sentinel array before setting compiling=true so that
// any non-epsilon re-entrant call (record.compiling=true, name not in
// epsilonReachable) returns [] rather than undefined for grammarRules.
record.grammarRules = [];
record.hasValue = createGrammarRules(
record.compiling = true;
// Pass the sentinel as the output array so createGrammarRules pushes
// directly into it. Any RulesPart.rules captured during a circular
// back-reference holds a reference to this same array object and will
// see the populated rules without a separate copy step.
const { hasValue, nullable } = createGrammarRules(
context,
record.rules,
eprWithSelf,
record.grammarRules,
);
record.hasValue = hasValue;
record.compiling = false;
record.nullable = nullable;
context.currentDefinition = prev;
}

Expand All @@ -413,26 +465,39 @@ function createNamedGrammarRules(
function createGrammarRules(
context: CompileContext,
rules: Rule[],
grammarRules: GrammarRule[],
) {
epsilonReachable: Set<string>,
out: GrammarRule[] = [],
): { grammarRules: GrammarRule[]; hasValue: boolean; nullable: boolean } {
const grammarRules = out;
let hasValue = true;
let nullable = false; // nullable if ANY alternative is nullable
for (const r of rules) {
const result = createGrammarRule(context, r);
const result = createGrammarRule(context, r, epsilonReachable);
grammarRules.push(result.grammarRule);
hasValue = hasValue && result.hasValue;
nullable = nullable || result.nullable;
}
return hasValue;
return { grammarRules, hasValue, nullable };
}

function createGrammarRule(
context: CompileContext,
rule: Rule,
): { grammarRule: GrammarRule; hasValue: boolean } {
epsilonReachable: Set<string>,
): { grammarRule: GrammarRule; hasValue: boolean; nullable: boolean } {
const { expressions, value } = rule;
const parts: GrammarPart[] = [];
const availableVariables = new Set<string>();
let variableCount = 0;
let defaultValue = false;
// A rule alternative is nullable if ALL of its parts can match ε.
let ruleNullable = true;
let currentEpr = epsilonReachable;
// Call after any part that guarantees consuming ≥1 input token.
const consumedInput = () => {
currentEpr = new Set<string>();
ruleNullable = false;
};
for (const expr of expressions) {
switch (expr.type) {
case "string": {
Expand All @@ -444,6 +509,7 @@ function createGrammarRule(
parts.push(part);
// default value of the string
defaultValue = true;
consumedInput(); // string literals always consume mandatory input
break;
}
case "variable": {
Expand All @@ -459,25 +525,38 @@ function createGrammarRule(
}
availableVariables.add(name);
if (ruleReference) {
const { grammarRules } = createNamedGrammarRules(
const record = createNamedGrammarRules(
context,
refName,
refPos,
name,
currentEpr,
);
parts.push({
type: "rules",
rules: grammarRules,
rules: record.grammarRules,
variable: name,
name: refName,
optional: expr.optional,
});
if (!expr.optional) {
// === false: only clear when *definitely* non-nullable.
// undefined (back-ref still compiling) leaves epr intact
// to avoid masking an ε-cycle further along this path.
if (record.nullable === false) currentEpr = new Set();
// ?? false: treat undefined (back-ref) as non-nullable —
// conservative for nullability propagation, consistent with
// how cycles are broken (they require mandatory input).
ruleNullable =
ruleNullable && (record.nullable ?? false);
}
} else if (refName === "number") {
parts.push({
type: "number",
variable: name,
optional: expr.optional,
});
if (!expr.optional) consumedInput();
} else {
// Validate type name references
// All non-built-in types must be explicitly imported
Expand Down Expand Up @@ -513,6 +592,7 @@ function createGrammarRule(
optional: expr.optional,
typeName: refName,
});
if (!expr.optional) consumedInput();
}
break;
}
Expand All @@ -536,35 +616,50 @@ function createGrammarRule(
// Use defaultValue=true so single-part rules using a phrase set
// don't trip the "Start rule does not produce a value" check.
defaultValue = true;
consumedInput(); // phrase sets always consume input
break;
}
const { grammarRules, hasValue } = createNamedGrammarRules(
const record = createNamedGrammarRules(
context,
expr.name,
expr.pos,
undefined,
currentEpr,
);
// default value of the rule reference
defaultValue = hasValue;
defaultValue = record.hasValue;
parts.push({
type: "rules",
rules: grammarRules,
rules: record.grammarRules,
name: expr.name,
});
// RuleRefExpr has no optional modifier; it is always non-optional.
// === false: only clear when *definitely* non-nullable (same
// asymmetry as the variable ruleRef case above).
if (record.nullable === false) {
currentEpr = new Set();
}
// ?? false: treat undefined (back-ref) as non-nullable.
ruleNullable = ruleNullable && (record.nullable ?? false);
break;
}
case "rules": {
const { rules, optional, repeat } = expr;
const grammarRules: GrammarRule[] = [];
// default value of the nested rules
defaultValue = createGrammarRules(context, rules, grammarRules);
const rulesPart: import("./grammarTypes.js").RulesPart = {
const {
grammarRules,
hasValue: groupHasValue,
nullable: groupNullable,
} = createGrammarRules(context, rules, currentEpr);
defaultValue = groupHasValue;
const rulesPart: RulesPart = {
type: "rules",
rules: grammarRules,
optional,
};
if (repeat) rulesPart.repeat = true;
parts.push(rulesPart);

if (!optional && !groupNullable) consumedInput();
break;
}
default:
Expand Down Expand Up @@ -594,5 +689,6 @@ function createGrammarRule(
value !== undefined ||
variableCount === 1 ||
(variableCount === 0 && parts.length === 1 && defaultValue),
nullable: ruleNullable,
};
}
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