Why we cannot declare local const variables in inno-setup [Code]? - constants

Question

Do you know why when declaring local const vars, the script cannot compile?
Sorry, I know very little pascal and cannot figure out why this is not working!
This example (see function CircleArea) shows that my syntax should be fine.
http://www.tutorialspoint.com/pascal/pascal_quick_guide.htm
This is what I am trying to do:
//---placed within [Code]
procedure MyLog(const _functionName, _msg: String);
begin
Log(_functionName + '(): ' + _msg);
end;
function MyExec(const _filename, _params, _dir: String): Boolean;
const // <--- compilation fails in this line!
MethodName = 'MyExec';
var
ResultCode: Integer;
begin
MyLog(MethodName, _filename);
// ... invoke Exec(), etc. ...
end;
//---

Answer 1

You were trying that right. If Inno Setup were using Pascal it would even work, but since it is based on a custom Pascal Script language with limitation on declaring local constants, you can't do it that way. You must define your constant globally instead:
[Code]
const
MethodName = 'MyExec';
function MyExec(const _filename, _params, _dir: String): Boolean;
var
ResultCode: Integer;
begin
MyLog(MethodName, _filename);
// ... invoke Exec(), etc. ...
end;

Answer 2

You can use #define to define "local" constants and then use ExpandConstant('{#ConstantName}') whenever you need the value of that constant.
function MyExec(const _filename, _params, _dir: String): Boolean;
var
ResultCode: Integer;
begin
#define MethodName 'MyExec'
MyLog(ExpandConstant('{#MethodName}'), _filename);
// ... invoke Exec(), etc. ...
end;
Note that the constant isn't actually local, because it is just a preprocessor definition. So the functions after MyExec could refer to the constant as well, which might be a potential cause of error (but isn't worse than defining a real global constant). You can redefine the same constant in another function or procedure without causing a compile error though.
function MyExec(const _filename, _params, _dir: String): Boolean;
begin
#define MethodName 'MyExec'
MyLog(ExpandConstant('{#MethodName}'), _filename);
end;
function MyExec2(const _filename, _params, _dir: String): Boolean;
begin
// if you forget to put the next line, it will log 'MyExec'!!!
#define MethodName 'MyExec2'
MyLog(ExpandConstant('{#MethodName}'), _filename);
end;
For added safety, you might want to #undef the "local" constant at the end of the function, in which case InnoSetup would cause a compile error if you'd try to reference it in the next function.
function MyExec(const _filename, _params, _dir: String): Boolean;
begin
#define MethodName 'MyExec'
MyLog(ExpandConstant('{#MethodName}'), _filename);
#undef MethodName
end;
function MyExec2(const _filename, _params, _dir: String): Boolean;
begin
// Forgot to define MethodName, but InnoSetup will cause an error
// "undeclared identifier: MethodName".
MyLog(ExpandConstant('{#MethodName}'), _filename);
end;