E0121: skeleton_lock

E0121 skeleton_lock — a skeleton differs from the one skeleton.lock holds (frozen with soma skeleton --freeze).

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0121).

The rule

soma skeleton app.cell --freeze writes skeleton.lock, and from then on soma check refuses with E0121 any program in that directory whose skeletons differ from the frozen ones, printing the changed lines. Here a body wrote log outside its frame, and the frame was widened (writes log added to signal pay) to make that error go away. Restore the frozen skeleton and fix the body instead; when the design itself must change, accept it on purpose with soma skeleton app.cell --amend.

A program that triggers it

skeleton Orders {
    memory {
        log: Map<String, String> [persistent]
    }
    state flow {
        initial: placed
        placed -> paid
        paid -> shipped
    }
    face {
        signal pay(id: String) -> String     moves placed -> paid   writes log
        signal ship(id: String) -> String    moves paid -> shipped   writes log
    }
}

flesh Orders {
    on pay(id) {
        transition(id, "placed", "paid")
        log.set(id, "paid")
        return "paid"
    }
    on ship(id) {
        transition(id, "paid", "shipped")
        log.set(id, "shipped")
        return "shipped"
    }
}

What soma check prints:

$ soma check wrong.cell
error: skeleton `Orders` differs from skeleton.lock:
    - signal pay(id: String) -> String  moves placed -> paid
    + signal pay(id: String) -> String  writes log  moves placed -> paid
    (in `Orders`: signal pay)
  the skeleton is frozen: restore it, or accept the change on purpose with `soma skeleton right.cell --amend`
  = code: E0121
✗ 1 error(s), 0 warning(s).

The fix field of soma check --json: skeleton `Orders` differs from skeleton.lock: - signal pay(id: String) -> String moves placed -> paid + signal pay(id: String) -> String writes log moves placed -> paid (in `Orders`: signal pay) the skeleton is frozen: restore it, or accept the change on purpose with `soma skeleton right.cell --amend`

The fix

On line 11, signal pay(id: String) -> String moves placed -> paid writes log becomes signal pay(id: String) -> String moves placed -> paid; line 19 is removed.

--- wrong.cell
+++ right.cell
@@ -9,5 +9,5 @@
     }
     face {
-        signal pay(id: String) -> String     moves placed -> paid   writes log
+        signal pay(id: String) -> String     moves placed -> paid
         signal ship(id: String) -> String    moves paid -> shipped   writes log
     }
@@ -17,5 +17,4 @@
     on pay(id) {
         transition(id, "placed", "paid")
-        log.set(id, "paid")
         return "paid"
     }

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.