LDC-E1337 — Instance breaks the expansion contract

What it means

An instance fills every slot its template declares: each binder is bound to a binder parameter, each argument resolves to a value of the declared shape, and no binder is bound twice. A missing argument, a binder pointing at a non-binder, or a value that resolves nowhere — any of these breaks the contract, and the compiler stops at the offending line.

The repair is to complete the instance: add the missing line, point the binder at a real binder, or spell a value the package declares. A complete instance expands exactly as written.

Example

Arxo Law

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

entity Person;
entity Choice;
entity SpecialChoice : Choice;
entity Other;
const Selected: SpecialChoice = entity_ref("urn:choice:selected");
const Wrong: Other = entity_ref("urn:choice:wrong");
relation eligible(p: Person, choice: Choice) kind empirical;
relation applicable(p: Person, choice: Choice) kind institutional;
expansion fixed {
    params { subject: binder; choice: value; ground: relation(subject, choice); result: relation(subject, choice); }
    emit rule self/result defeasible {
        for subject;
        when ground(subject, choice);
        then result(subject, choice);
        scope from self; effective from self; labels from self; source from self;
    }
}
expand fixed Result {
    label en official "Fixed option";
    bind subject = person: Person;
    choice = entity_ref("urn:choice:raw");
    ground = eligible;
    result = applicable;
    effective [@2026-01-01, @2027-01-01);
}

Fix

Arxo Law

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

entity Person;
entity Choice;
entity SpecialChoice : Choice;
entity Other;
const Selected: SpecialChoice = entity_ref("urn:choice:selected");
const Wrong: Other = entity_ref("urn:choice:wrong");
relation eligible(p: Person, choice: Choice) kind empirical;
relation applicable(p: Person, choice: Choice) kind institutional;
expansion fixed {
    params { subject: binder; choice: value; ground: relation(subject, choice); result: relation(subject, choice); }
    emit rule self/result defeasible {
        for subject;
        when ground(subject, choice);
        then result(subject, choice);
        scope from self; effective from self; labels from self; source from self;
    }
}
expand fixed Result {
    label en official "Fixed option";
    bind subject = person: Person;
    choice = Selected;
    ground = eligible;
    result = applicable;
    effective [@2026-01-01, @2027-01-01);
}

Engine message

The engine reports this in its own wording:

Output

example.law:25:5: error LDC-E1337: expand Result: "choice": value is unresolved or incompatible with value<T>

Related

• LDC-E1336 — an instance must resolve to exactly one definition.
• LDC-E1339 — an instance must carry a label.