How to get Inno Setup to check if a file doesn't exist

Question

When I compile the Inno Setup script below, it gives me an error (below). I borrowed the code from here so I'm not sure why it's not working properly.

Line 136:
Column 10:
Invalid prototype for 'FileDoesNotExist'

Line 136 is the function FileDoesNotExist(file: string): Boolean;

[Run]
Filename: "{sys}\regsvr32.exe"; Parameters: "msstdfmt.dll"; WorkingDir: {app}\Pronto\Programs\; BeforeInstall: FileDoesNotExist(ExpandConstant('{sys}\msstdfmt.dll')); StatusMsg: "Registering Controls..."

[Code]
function FileDoesNotExist(file: string): Boolean;
begin
  if (FileExists(file)) then
    begin
      Result := False;
    end
  else
    begin
      Result := True;
    end;
end;

Answer 1

From the Inno Setup documentation:

All BeforeInstall and AfterInstall functions must not have a return value.

In other words, it can't be a function, because it can't return anything; it's a procedure instead. (You can see from the examples in the linked page that they're all declared as procedure, and none of them contains a Result in the code.)

(The question you linked is also wrong, BTW. It shows a procedure that has a return type of Boolean, which of course isn't possible. I'm suspecting that if it worked it's because the Pascal parser used didn't notice the return value because of the procedure in the declaration.)

It appears you're trying to do it wrong, anyway. If msstdfmt.dll is being distributed with your setup, you should be adding it to the [Files] section with the onlyifdoesntexist and regserver flags set. If it's already installed on the user's system, it should already be registered.

[Files]
Source: "msstdfmt.dll"; DestDir: "{sys}"; Flags: onlyifdoesntexist regserver