E0045: unused_value

E0045 unused_value — an expression statement whose value is dropped.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0045).

The rule

An expression that stands alone on its own line, and is not the last statement of its block, is computed and thrown away. This usually means a missing operator: a newline ends a statement, so let total = a + b followed by tax on the next line is two statements. Put the operator at the end of the first line (a + b +) so the expression continues, or bind or use the value.

A program that triggers it

cell Invoice {
    face {
        signal total(subtotal_cents: Int, shipping_cents: Int, tax_cents: Int) -> Int
    }
    on total(subtotal_cents: Int, shipping_cents: Int, tax_cents: Int) {
        let total = subtotal_cents + shipping_cents
            tax_cents
        return total
    }
}

What soma check prints:

$ soma check wrong.cell
error: `tax_cents` on its own does nothing — a value that is not the last statement of its block is thrown away (a missing operator between two values? `let x = a b` is two statements)
  --> wrong.cell:7:13
  |
7 |             tax_cents
  |             ^
  = code: E0045
✗ 1 error(s), 0 warning(s).

The fix field of soma check --json: a value that is not the last statement of its block is thrown away (a missing operator between two values? `let x = a b` is two statements)

The fix

On line 6, let total = subtotal_cents + shipping_cents becomes let total = subtotal_cents + shipping_cents +.

--- wrong.cell
+++ right.cell
@@ -4,5 +4,5 @@
     }
     on total(subtotal_cents: Int, shipping_cents: Int, tax_cents: Int) {
-        let total = subtotal_cents + shipping_cents
+        let total = subtotal_cents + shipping_cents +
             tax_cents
         return total

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.