E1008: statement_as_value

E1008 statement_as_value — a statement where a value is expected.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E1008).

The rule

A statement keyword such as return, let, require, for or while cannot stand where a value is expected: in a lambda body, as an operand or as an argument. A lambda's value is its expression, so id => return x is refused. Drop the return (id => (totals.get(id) ?? 0) >= min_total), or use a block lambda whose last expression is the value.

A program that triggers it

cell Orders {
    memory {
        totals: Map<String, Int> [persistent]
    }
    on large_orders(min_total: Int) {
        let ids = keys(totals)
        return filter(ids, id => return (totals.get(id) ?? 0) >= min_total)
    }
}

What soma check prints:

$ soma check wrong.cell
error: `return` starts a statement and cannot stand where a value is expected (a lambda `x => …`, an operand, an argument) — a lambda's value is its expression: drop the `return` (`x => x + 1`, or `x => { let y = x * 2  y + 1 }`); as a name it is reserved (`return_id`)
  --> wrong.cell:7:34
  |
7 |         return filter(ids, id => return (totals.get(id) ?? 0) >= min_total)
  |                                  ^
  = code: E1008

The fix

On line 7, return filter(ids, id => return (totals.get(id) ?? 0) >= min_total) becomes return filter(ids, id => (totals.get(id) ?? 0) >= min_total).

--- wrong.cell
+++ right.cell
@@ -5,5 +5,5 @@
     on large_orders(min_total: Int) {
         let ids = keys(totals)
-        return filter(ids, id => return (totals.get(id) ?? 0) >= min_total)
+        return filter(ids, id => (totals.get(id) ?? 0) >= min_total)
     }
 }

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.