mono/mono · warning

%s: %d rules never reduced

Error message

%s: %d rules never reduced

What it means

Emitted by unused_rules() in mcs/jay/mkpar.c:251 (plural branch) when two or more grammar rules are never selected as a reduction in any parser state. Same mechanism as the singular form: post-table-build scan of `rules_used` counts rules with no reduction action. It is a warning; the parser still generates but carries dead productions.

Source

Thrown at mcs/jay/mkpar.c:251

    for (i = 0; i < nstates; ++i)
    {
	for (p = parser[i]; p; p = p->next)
	{
	    if (p->action_code == REDUCE && p->suppressed == 0)
		rules_used[p->number] = 1;
	}
    }

    nunused = 0;
    for (i = 3; i < nrules; ++i)
	if (!rules_used[i]) ++nunused;

    if (nunused)
    {
	if (nunused == 1)
	    fprintf(stderr, "%s: 1 rule never reduced\n", myname);
	else
	    fprintf(stderr, "%s: %d rules never reduced\n", myname, nunused);
     }
}

static void
remove_conflicts (void)
{
    register int i;
    register int symbol;
    register action *p, *pref;

    SRtotal = 0;
    RRtotal = 0;
    SRconflicts = NEW2(nstates, short);
    RRconflicts = NEW2(nstates, short);
    for (i = 0; i < nstates; i++)
    {
	SRcount = 0;
	RRcount = 0;

View on GitHub (pinned to 0f53e9e151)

Solutions

1. Remove each unreachable rule, or wire its LHS nonterminal into a reachable production.
2. Audit the nonterminal graph from the start symbol and prune dead branches.
3. Re-run jay to confirm the count drops to zero.

Example fix

// before
start : a ;
a : 'x' ;
b : 'y' ;
c : 'z' ;   /* b and c unreachable -> 2 rules never reduced */
// after
start : a ;
a : 'x' ;

# Treat any 'rules never reduced' line as a build failure.
jay -v -c -o Parser.cs grammar.jay 2>jay.log
grep -qE '[0-9]+ rules never reduced' jay.log && { echo 'multiple unreachable rules'; cat jay.log; exit 1; } || true

When it happens

Trigger: Multiple productions (or whole nonterminals) unreachable from the start symbol.

Common situations: Large grammar after deleting a feature but leaving its rules; importing a grammar subset whose entry points are unused; superseded alternatives accumulating.

Related errors

• %s: 1 rule never reduced
• %s: e - line %d of "%s", unexpected end-of-file
• %s: e - line %d of "%s", syntax error
• %s: e - line %d of "%s", unmatched /*
• %s: e - line %d of "%s", unterminated string

AI-assisted analysis of mono/mono@0f53e9e151 (2026-08-13). Data as JSON: /api/errors/95619ca237d9d80d. Report an issue: GitHub.