E0029: function_as_value

E0029 function_as_value — a handler or builtin used as a value instead of being called.

A compile-time error of soma check, the checker of the Soma programming language (soma explain E0029).

The rule

A handler or builtin name is not a value in Soma: map(with_tax) passes a bare name where a function value is needed, and soma check refuses it. Functions passed as arguments are lambdas. Wrap the call in a lambda, map(c => with_tax(c)).

A program that triggers it

cell Prices {
    on with_tax(cents: Int) {
        return cents + idiv(cents * 20, 100)
    }
    on all_with_tax(items: List<Int>) {
        return items |> map(with_tax)
    }
}

What soma check prints:

$ soma check wrong.cell
error: `with_tax` is a function, not a value — pass a lambda (`x => with_tax(x)`) or call it (`with_tax(…)`)
  --> wrong.cell:6:29
  |
6 |         return items |> map(with_tax)
  |                             ^
  = code: E0029
✗ 1 error(s), 0 warning(s).

The fix field of soma check --json: pass a lambda (`x => with_tax(x)`) or call it (`with_tax(…)`)

The fix

On line 6, return items |> map(with_tax) becomes return items |> map(c => with_tax(c)).

--- wrong.cell
+++ right.cell
@@ -4,5 +4,5 @@
     }
     on all_with_tax(items: List<Int>) {
-        return items |> map(with_tax)
+        return items |> map(c => with_tax(c))
     }
 }

With the fix, soma check accepts the program:

$ soma check right.cell
✓ All checks passed.