E0066: scalar_slot

E0066 scalar_slot — a memory slot that is not a Map or a List.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0066).

The rule

A memory slot must be a Map or a List; a slot declared as a single value such as balance: Int is refused. Slots are keyed stores, and the check makes the declaration match how they behave. Keep a single value as one entry of a Map, for example balance: Map<String, Int> read with balance.get("value").

A program that triggers it

cell Wallet {
    memory {
        balance: Int [persistent]
    }
    on deposit(amount: Int) {
        let current = balance.get("value") ?? 0
        balance.set("value", current + amount)
        return current + amount
    }
}

What soma check prints:

$ soma check wrong.cell
error: slot 'balance' is declared as Int — a memory slot is a Map<K, V> or a List<T>; keep a single value as one entry: `balance: Map<String, Int>` and `balance.get("value")`
  --> wrong.cell:3:9
  |
3 |         balance: Int [persistent]
  |         ^
  = code: E0066
✗ 1 error(s), 0 warning(s).

The fix field of soma check --json: a memory slot is a Map<K, V> or a List<T>; keep a single value as one entry: `balance: Map<String, Int>` and `balance.get("value")`

The fix

On line 3, balance: Int [persistent] becomes balance: Map<String, Int> [persistent].

--- wrong.cell
+++ right.cell
@@ -1,5 +1,5 @@
 cell Wallet {
     memory {
-        balance: Int [persistent]
+        balance: Map<String, Int> [persistent]
     }
     on deposit(amount: Int) {

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.