LDC-E0207 — An underscore outside its one lawful slot

What it means

The underscore means “there is some value here, and its identity does not matter”. The language grants it exactly one slot: a positive rule-body literal argument. Everywhere else — the head of a rule, under negation, in a status test, inside a term, in a case assertion, in a question — the underscore is not a term, and the expression is refused at its position.

The fix depends on what the underscore stood for. If the value truly does not matter but the position needs a term, bind a fresh variable with for and use it. If any value is acceptable because the literal only checks presence, keep the body literal — the lawful slot — and move the reasoning there.

Example

Arxo Law

language "law.core" version "0.2";
package demo.diagnostics version "0.1.0";
namespace "urn:law:demo:diagnostics";

entity A;
relation p(a: A) kind institutional;
relation q(a: A) kind institutional;
rule R strict {
    for a: A;
    when q(a);
    then p(_);
}

Fix

Arxo Law

language "law.core" version "0.2";
package demo.diagnostics version "0.1.0";
namespace "urn:law:demo:diagnostics";

entity A;
relation p(a: A) kind institutional;
relation q(a: A) kind institutional;
rule R strict {
    for a: A;
    when q(a);
    then p(a);
}

Engine message

The engine reports this in its own wording:

Output

example.law:11:12: error LDC-E0207: `_` is allowed only as a positive rule-body literal argument (§98, E-0165): «есть какое-либо значение». In a head, under `not`, in a status test, inside a term, in a case assertion and in a question an underscore is not a term (§14)

Related

• LDC-E0204 — a let without a written type.