E0042: write_to_undefined

E0042 write_to_undefined — an assignment to a name that was never bound.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0042).

The rule

An indexed or field write such as counts[k] = v changes an existing List, Map or record, so the name must already be bound. Writing to a name that was never declared would fail at run time with "undefined variable", so soma check refuses it. Declare the value first, for example let counts = map() or let rows = [], or fix the name if it is a typo.

A program that triggers it

cell Orders {
    face {
        signal units_by_sku(lines: List<Map>) -> Map
    }
    on units_by_sku(lines: List<Map>) {
        for line in lines {
            counts[line.sku] = (counts[line.sku] ?? 0) + line.qty
        }
        return counts
    }
}

What soma check prints:

$ soma check wrong.cell
error: undefined variable 'counts' — an indexed or field write changes an existing List, Map or record; declare it first (`let counts = map()` / `let counts = []`), or fix the name
  --> wrong.cell:7:13
  |
7 |             counts[line.sku] = (counts[line.sku] ?? 0) + line.qty
  |             ^
  = code: E0042
✗ 1 error(s), 0 warning(s).

The fix field of soma check --json: an indexed or field write changes an existing List, Map or record; declare it first (`let counts = map()` / `let counts = []`), or fix the name

The fix

1 line is added after line 5.

--- wrong.cell
+++ right.cell
@@ -4,4 +4,5 @@
     }
     on units_by_sku(lines: List<Map>) {
+        let counts = map()
         for line in lines {
             counts[line.sku] = (counts[line.sku] ?? 0) + line.qty

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.