E0035: slot_method_on_local

E0035 slot_method_on_local — a slot method (.set / .push / .delete) on a local value.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0035).

The rule

.set() and .delete() used as a value on a local map are refused: they are slot methods and, on a local, they do not return the updated map. Written as a statement (line.set(k, v)), the call does change the local and passes check; it is the expression form (let priced = line.set(…)) that is the error. Write the local with brackets (line["total"] = v) and use the variable itself, or rebuild it (line = without(line, k) to remove a key).

A program that triggers it

cell Pricing {
    face {
        signal quote(sku: String, qty: Int) -> Map
    }
    on quote(sku: String, qty: Int) {
        let line = map("sku", sku, "qty", qty)
        let priced = line.set("total", qty * 499)
        return priced
    }
}

What soma check prints:

$ soma check wrong.cell
error: `.set()` is a memory-slot method and 'line' is a local — a local map is written with brackets: `line[k] = v`
  --> wrong.cell:7:22
  |
7 |         let priced = line.set("total", qty * 499)
  |                      ^
  = code: E0035
✗ 1 error(s), 0 warning(s).

The fix field of soma check --json: a local map is written with brackets: `line[k] = v`

The fix

Lines 7–8 are rewritten.

--- wrong.cell
+++ right.cell
@@ -5,6 +5,6 @@
     on quote(sku: String, qty: Int) {
         let line = map("sku", sku, "qty", qty)
-        let priced = line.set("total", qty * 499)
-        return priced
+        line["total"] = qty * 499
+        return line
     }
 }

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.