E0015: signal_type_mismatch

E0015 signal_type_mismatch — a handler's parameter types differ from its face signal.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0015).

The rule

When an interior cell handles or awaits a signal that a sibling declares, the parameter types must agree with the sibling's signal declaration. Here Checkout sends total: Int but Fulfillment's handler takes total: String, which soma check reports as a signal type mismatch. Make the receiving handler's parameter types match the emitter's signal. (A handler that disagrees with its own cell's face is a different code, E0092.)

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 {
            face {
                await order_placed(order_id: String, total: Int)
            }
            memory {
                queue: List<String> [persistent]
            }
            on order_placed(order_id: String, total: String) {
                queue.push(order_id)
            }
        }
    }
}

What soma check prints:

$ soma check wrong.cell
error: signal type mismatch: signal 'order_placed' has incompatible parameter types between emitter and handler
  --> wrong.cell:18:13
   |
18 |             on order_placed(order_id: String, total: String) {
   |             ^
  = code: E0015
✗ 1 error(s), 0 warning(s).

The fix field of soma check --json: signal type mismatch: signal 'order_placed' has incompatible parameter types between emitter and handler

The fix

On line 18, on order_placed(order_id: String, total: String) { becomes on order_placed(order_id: String, total: Int) {.

--- wrong.cell
+++ right.cell
@@ -16,5 +16,5 @@
                 queue: List<String> [persistent]
             }
-            on order_placed(order_id: String, total: String) {
+            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.