E0040: face_return_type

E0040 face_return_type — a handler returns a value its face signal does not declare.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0040).

The rule

A handler returns a literal whose type differs from the return type its face signal declares, for example a String where the face says -> Int. The face is the cell's contract with its callers, so Soma refuses a body that contradicts it. Return a value of the declared type, or change the declared type in the face if the contract was wrong.

A program that triggers it

cell Wallet {
    face {
        signal balance(owner: String) -> Int
    }
    memory {
        balances: Map<String, Int> [persistent]
    }
    on balance(owner: String) {
        let cents = balances.get(owner) ?? 0
        return "{owner} has {cents} cents"
    }
}

What soma check prints:

$ soma check wrong.cell
error: handler 'balance' returns a String literal but its face declares `-> Int` — fix the value or the face
  --> wrong.cell:10:16
   |
10 |         return "{owner} has {cents} cents"
   |                ^
  = code: E0040
✗ 1 error(s), 0 warning(s).

The fix field of soma check --json: fix the value or the face

The fix

On line 10, return "{owner} has {cents} cents" becomes return cents.

--- wrong.cell
+++ right.cell
@@ -8,5 +8,5 @@
     on balance(owner: String) {
         let cents = balances.get(owner) ?? 0
-        return "{owner} has {cents} cents"
+        return cents
     }
 }

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.