E0017: sum_type_issue

E0017 sum_type_issue — a match over a sum type is not exhaustive, or names an unknown variant.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0017).

The rule

A match over a value of a cell type with variants must have an arm for every variant (or a _ arm), and may not name a variant the type does not have. soma check names the missing variant, so adding Refunded to the type cannot leave a match that silently falls through. Add the missing arm.

A program that triggers it

cell type Payment {
    variants {
        Charged { tx: String }
        Declined { reason: String }
        Refunded { tx: String }
    }
}

cell Receipts {
    on describe(p: Payment) {
        return match p {
            Charged { tx } -> "paid ({tx})"
            Declined { reason } -> "declined: {reason}"
        }
    }
}

What soma check prints:

$ soma check wrong.cell
error: non-exhaustive match on 'Payment': missing variant `Refunded`
  --> wrong.cell:11:16
   |
11 |         return match p {
   |                ^
  = code: E0017
✗ 1 error(s), 0 warning(s).

The fix field of soma check --json: non-exhaustive match on 'Payment': missing variant `Refunded`

The fix

1 line is added after line 13.

--- wrong.cell
+++ right.cell
@@ -12,4 +12,5 @@
             Charged { tx } -> "paid ({tx})"
             Declined { reason } -> "declined: {reason}"
+            Refunded { tx } -> "refunded ({tx})"
         }
     }

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.