E0014: unmatched_handler

E0014 unmatched_handler — a handler answers a signal the face does not declare.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0014).

The rule

In an interior block, a cell's on handler is expected to answer a signal that a sibling declares (or that the parent emits). A handler for order_created when the only sibling signal is order_placed would never be called, so soma check reports it as an unmatched handler. Rename the handler to the signal a sibling sends, or add the signal to the sibling's face.

A program that triggers it

cell Shop {
    interior {
        cell Checkout {
            face {
                signal order_placed(order_id: String, total: Int)
            }
            on order_placed(order_id: String, total: Int) {
                return order_id
            }
        }
        cell Fulfillment {
            memory {
                queue: List<String> [persistent]
            }
            on order_created(order_id: String, total: Int) {
                queue.push(order_id)
            }
        }
    }
}

What soma check prints:

$ soma check wrong.cell
error: unmatched handler: cell 'Fulfillment' handles signal 'order_created' but no sibling emits it
  --> wrong.cell:15:13
   |
15 |             on order_created(order_id: String, total: Int) {
   |             ^
  = code: E0014
✗ 1 error(s), 0 warning(s).

The fix field of soma check --json: unmatched handler: cell 'Fulfillment' handles signal 'order_created' but no sibling emits it

The fix

On line 15, on order_created(order_id: String, total: Int) { becomes on order_placed(order_id: String, total: Int) {.

--- wrong.cell
+++ right.cell
@@ -13,5 +13,5 @@
                 queue: List<String> [persistent]
             }
-            on order_created(order_id: String, total: Int) {
+            on order_placed(order_id: String, total: Int) {
                 queue.push(order_id)
             }

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.