E0097: empty_match

E0097 empty_match — a match with no arm.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0097).

The rule

A match with no arms evaluates to () whatever the subject is, so a placeholder left as match zone { } would quietly give () to everything after it; soma check refuses it with E0097. Write the arms (pattern -> value), and end with a _ -> … arm when the listed patterns do not cover every value.

A program that triggers it

cell Shipping {
    face {
        signal fee(zone: String, weight_kg: Int) -> Int
    }
    on fee(zone: String, weight_kg: Int) {
        let base = match zone {
            // TODO: domestic, eu, world
        }
        return base + weight_kg * 50
    }
}

What soma check prints:

$ soma check wrong.cell
error: `match` with no arms always evaluates to `()` — add at least one arm (`_ -> …`)
  --> wrong.cell:6:20
  |
6 |         let base = match zone {
  |                    ^
  = code: E0097
✗ 1 error(s), 0 warning(s).

The fix field of soma check --json: add at least one arm (`_ -> …`)

The fix

Line 7 is rewritten.

--- wrong.cell
+++ right.cell
@@ -5,5 +5,7 @@
     on fee(zone: String, weight_kg: Int) {
         let base = match zone {
-            // TODO: domestic, eu, world
+            "domestic" -> 500
+            "eu" -> 900
+            _ -> 1500
         }
         return base + weight_kg * 50

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.