E0092: face_mismatch

E0092 face_mismatch — a handler and its face signal disagree.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0092).

The rule

When a cell declares a signal in its face, the on handler for it must take the same parameter types in the same order; soma check reports E0092 at the face signal when a position differs (here the face says cents: Int and the handler says cents: Float). Callers, soma describe and HTTP clients follow the face, while the handler's body runs with its own declaration, so the two must agree. Fix whichever side is wrong; a mismatch in a parameter's name alone is only a warning.

A program that triggers it

cell Wallet {
    face {
        signal deposit(owner: String, cents: Int) -> Int
    }
    memory {
        balances: Map<String, Int> [persistent]
    }
    on deposit(owner: String, cents: Float) {
        let total = (balances.get(owner) ?? 0) + cents
        balances.set(owner, total)
        return total
    }
}

What soma check prints:

$ soma check wrong.cell
warning: handler 'deposit' returns a Float (`total`) but its face declares `-> Int` — fix the value or the face (an error in edition 2027)
  --> wrong.cell:11:16
   |
11 |         return total
   |                ^
error: parameter 2 of `deposit`: the face declares `cents: Int` but the handler takes `cents: Float` — callers follow the face (describe publishes it); make them agree
  --> wrong.cell:3:9
  |
3 |         signal deposit(owner: String, cents: Int) -> Int
  |         ^
  = code: E0092
✗ 1 error(s), 1 warning(s).

The fix field of soma check --json: callers follow the face (describe publishes it); make them agree

The fix

On line 8, on deposit(owner: String, cents: Float) { becomes on deposit(owner: String, cents: Int) {.

--- wrong.cell
+++ right.cell
@@ -6,5 +6,5 @@
         balances: Map<String, Int> [persistent]
     }
-    on deposit(owner: String, cents: Float) {
+    on deposit(owner: String, cents: Int) {
         let total = (balances.get(owner) ?? 0) + cents
         balances.set(owner, total)

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.