E0003: missing_handler

E0003 missing_handler — a face signal has no on handler.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0003).

The rule

Every signal declared in a cell's face needs an on handler with the same name in that cell. The face is the contract other cells and HTTP clients rely on, so a declared signal that nothing answers is an error, not a warning. Add the handler with the same parameters as the signal, or remove the signal from the face.

A program that triggers it

cell Inventory {
    face {
        signal add_stock(sku: String, qty: Int) -> Int
        signal stock_of(sku: String) -> Int
    }
    memory {
        stock: Map<String, Int> [persistent]
    }
    on add_stock(sku: String, qty: Int) {
        let total = (stock.get(sku) ?? 0) + qty
        stock.set(sku, total)
        return total
    }
}

What soma check prints:

$ soma check wrong.cell
error: face contract: signal 'stock_of' declared in cell 'Inventory' has no handler
  --> wrong.cell:4:9
  |
4 |         signal stock_of(sku: String) -> Int
  |         ^
  = code: E0003
✗ 1 error(s), 0 warning(s).

The fix field of soma check --json: Add a handler to cell 'Inventory': on stock_of() { // TODO: implement return map("status", "ok") }

The fix

3 lines are added after line 13.

--- wrong.cell
+++ right.cell
@@ -12,3 +12,6 @@
         return total
     }
+    on stock_of(sku: String) {
+        return stock.get(sku) ?? 0
+    }
 }

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.