LDC-E0205 — A match arm the slice does not read

What it means

A match chooses a value by the shape of a subject: each arm names a shape and the value for it. The current slice reads two shapes — a variant name and the catch-all underscore. A literal number or string as an arm, a constructor with field bindings, a qualified name: all three are grammar the slice does not parse yet, so the arm is refused at the pattern.

The refusal is about the arm’s shape, not the subject or the arm’s value. Rewrite the arm with a variant or the underscore. If the decision truly depends on a number, lift the comparison out of the match into the rule body, where comparisons belong — or move the whole decision table into a function, where a match over the slice shapes is at home.

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(n: Integer) kind institutional;
rule R strict {
    for a: A;
    for n: Integer;
    when p(a) and q(n) and (let m: Integer = match n { 1 => 2; _ => 3 } in m) = 2;
    then p(a);
}

Fix

Arxo Law

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

function pick(n: Integer) -> Integer = match n { _ => 3; };

Engine message

The engine reports this in its own wording:

Output

example.law:11:56: error LDC-E0205: pattern literal §55.1 outside slice 0.1: only a variant and `_` are parsed

Related

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