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Variable Expected

Velox reports Variable Expected when a construct needs an addressable, writable storage location but receives a literal, constant, type, routine result or other value expression instead.

Example​

A var parameter can modify its caller's storage, so a literal cannot be supplied:

procedure SetPrepared(var Text: string);
begin
  Text := 'Prepared';
end;

procedure ScriptEvent(var Value: Variant);
begin
  SetPrepared('fixed text'); // Error: Variable Expected
end;

Pass a compatible variable:

procedure SetPrepared(var Text: string);
begin
  Text := 'Prepared';
end;

procedure ScriptEvent(var Value: Variant);
var
  Text: string;
begin
  SetPrepared(Text);
  Value := Text;
end;

Current emission families​

The modified compiler emits this diagnostic when:

  • an actual argument for a var, out or other non-input parameter is not a variable value;
  • a string or Variant element is assigned through an expression that is not backed by writable storage;
  • a record field is selected from a temporary value that cannot be addressed as a variable;
  • a variable-only identifier lookup finds a type, procedure/function or constant instead;
  • a for statement supplies something other than a writable counter variable; or
  • an identifier-led statement resolves to a value expression that is neither an assignment target nor a callable routine.

The check occurs before or alongside type compatibility. A literal passed to a var parameter produces Variable Expected; a real variable of the wrong type proceeds to the stricter check and produces Type mismatch.

Correction procedure​

  1. Identify why the construct needs storage: assignment target, counter, var/out parameter, address or mutable element/member.
  2. Declare a local variable of the required type.
  3. Assign any starting value to that variable separately.
  4. Pass or use the variable in the variable-only position.
  5. If a function/property returns a record or value, copy it to a local variable, modify the local value, then assign the complete value back through a supported setter when one exists.
  6. Recompile and resolve any subsequent Type mismatch separately.

Edge cases and quirks​

  • A constant has a value but no writable storage. Renaming a constant does not make it suitable for a var parameter.
  • A routine result or calculated expression can be assignment-compatible as an input parameter but still cannot be used as a non-input parameter.
  • Properties are implemented through getter/setter calls and do not necessarily have an address. Passing a property as a var argument can therefore fail even if reading and assigning that property separately are supported.
  • The formatter ignores the optional name supplied by some emission sites, so the public message is always just Variable Expected. Use the reported row/column to identify the expression.
  • A for counter must be writable storage and must also have a supported counter type. The two requirements produce different diagnostics.
  • An assignment to a read-only or read from a write-only registered property produces the more specific property diagnostic.
  • This is a compile-time error; the attempted write or call does not occur.
  • Type mismatch - storage exists, but its type is incompatible.
  • Assignment (':=') expected - a writable target was resolved but the assignment token is missing.
  • Read-only property - a property has no registered setter.
  • Write-only property - a property has no registered getter.

External references​

The Embarcadero E2036 page describes the address-of form of the same storage requirement. Velox uses one broader PascalScript diagnostic for the additional variable-only contexts listed above.