W0001: unhandled_signal

W0001 unhandled_signal — an emitted signal no cell handles.

A warning of soma check, the checker of the Soma programming language (soma explain W0001).

The rule

Inside an interior block, soma check warns with W0001 when a cell's face declares a signal that no sibling cell handles and no cell awaits; the message says the signal "will be lost". In this compiler the warning comes together with error E0003, because a face signal must also have an on handler in its own cell. Add the missing handler (on cancel(id: String) { … }) or remove the signal from the face. An emit of an event nobody handles is a different diagnostic: it is reported as W0011.

A program that triggers it

cell Shop {
    interior {
        cell Orders {
            face {
                signal place(id: String, total: Int)
                signal cancel(id: String)
            }
            memory {
                orders: Map<String, Int> [persistent]
            }
            on place(id: String, total: Int) {
                orders.set(id, total)
                return id
            }
        }
    }
}

What soma check prints:

$ soma check wrong.cell
warning: signal 'cancel' emitted by 'Orders' has no handler (signal will be lost)
  --> wrong.cell:6:17
  |
6 |                 signal cancel(id: String)
  |                 ^
error: face contract: signal 'cancel' declared in cell 'Orders' has no handler
  --> wrong.cell:6:17
  |
6 |                 signal cancel(id: String)
  |                 ^
  = code: E0003
✗ 1 error(s), 1 warning(s).

The fix field of soma check --json: Add a handler 'on cancel(...)' to cell 'Orders', or remove the emit if it's unused.

The fix

4 lines are added after line 14.

--- wrong.cell
+++ right.cell
@@ -13,4 +13,8 @@
                 return id
             }
+            on cancel(id: String) {
+                orders.delete(id)
+                return id
+            }
         }
     }

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.