Why Pascal?

[metis]

@Handoko

No, I mean Vars, that are declared in a Routine and keep their Value until the next Call. 

 

[Handoko]

Have you read the link? Typed constant is not the constant that you thought.

Contrary to ordinary constants, a value can be assigned to them at run-time.
Source: https://freepascal.org/docs-html/ref/refse10.html

[Pascal]

@Handoko

No, I mean Vars, that are declared in a Routine and keep their Value until the next Call. 

 

That's what he meant! It's possible:

Code: Pascal  [Select][+][-]

1. procedure xyz;
2. const
3.   lLock: boolean = False;
4. begin
5.   if lLock then exit;
6.   lLock := True;
7.   try
8.     ...
9.   finally
10.     lLock := False;
11.   end;
12. end;

[Thaddy]

Indeed, the most use for a typed const is to maintain state over function calls, it behaves like a persistent (global) var, not a local var but is hidden from global access..
This is a slightly more revealing example that I have given before:

Code: Pascal  [Select][+][-]

1. program untitled;
2. {$ifdef fpc}{$mode delphi}{$H+}{$J+}{$endif}
3. function MaintainStateOverCall:integer;
4. const x:integer = 0;
5. begin
6.   Result := x;
7.   inc(x);
8. end;
9.  
10. var i:integer;
11. begin
12.   for i := 0 to 9 do
13.    writeln(MaintainStateOverCall);
14. end.

It is a very powerful tool once you know how to use it.
The alternative is writing a class, but this feature is much older and a precursor to Object Pascal.

[Pascal]

This is a slightly more revealing example

Sure! 

[Thaddy]

Sure! 

Note you need to have {$writeableconst on} or the equivalent {$J+} for this to work.

[Handoko]

It works on {$mode objfpc} too. Every times I start a new project using Lazarus, I get {$mode objfpc} and I can use typed constant without problem.

[Kays]

Indeed, the most use for a typed const is to maintain state over function calls, it behaves like a persistent (global) var, not a local var but is hidden from global access.. […]
The alternative is writing a class, but this feature is much older and a precursor to Object Pascal.

Or a unit where the var is allocated in the implementation section.

[…] {$writeableconst on} […]

One of the worst compiler switches, ever (besides {$COperators on}). I mean constants, hence their name are supposed to remain constant, i.e. you can't assign any value to them.

[Thaddy]

That would actually be a bug. Same goes for mode delphi. In delphi it should be off by default. Oh, minor issue.

[Thaddy]

One of the worst compiler switches, ever (besides {$COperators on}). I mean constants, hence their name are supposed to remain constant, i.e. you can't assign any value to them.

No it is not: it is a constant over calls, not a traditional constant without type (which many people will probably also do not understand: how to determine the type of a constant....).
Actually this is THE compiler feature (internally) that allowed OOP in pascal. And sometime later lead to Object Pascal and subsequently Delphi and FreePascal.
And the difference is actually quite well explained in the manuals.
Being able to hide from global scope is a feature that allows classes and objects.

That was a really silly remark, Kai. You can do much better.

[Zoran]

However, the syntax is quite unfortunate. I never understood why are these declared with const keyword.
Perhaps we should call these global variables with local scope (although accessible from local scope only, they are allocated in global memory region).

Of course, as we know what we are doing, there is nothing wrong with using this feature.

[Thaddy]

However, the syntax is quite unfortunate. I never understood why are these declared with const keyword.
Perhaps we should call these global variables with local scope (although accessible from local scope only, they are allocated in global memory region).

Of course, as we know what we are doing, there is nothing wrong with using this feature.

I somewhat agree with you, but note typed consts are typed. So although const necessarily  has a different meaning it is quite natural.
I would worry more about open array parameters vs dynamic array parameters. Now that's a clash that is hard to understand for most.

[Zoran]

but note typed consts are typed. So although const necessarily  has a different meaning it is quite natural.

Typed? Natural? What makes it typed?
Seriously, why do we call these typed?
A true const also have its type. Only you don't always have to type its type.
Is that what makes it not typed?

Not to mention that you can declare true constant's type explicitly, as well:

Code: Pascal  [Select][+][-]

1. const
2.   N = Int64(123); // a true const
3.  

[Zoran]

@Zoran

"Don't declare variable outside of exact block they belong to" is what I really miss in Pascal.

How would You declare global Vars then ?

Are you serious?

I'd do it this way:

Code: Pascal  [Select][+][-]

1. var
2.   N: Integer;
3.  

[lainz]

@Zoran

"Don't declare variable outside of exact block they belong to" is what I really miss in Pascal.

How would You declare global Vars then ?

Are you serious?

I'd do it this way:

Code: Pascal  [Select][+][-]

1. var
2.   N: Integer;
3.  

He means in each scope. With your code you are creating a variable for all function scope, that doesn't means is the local scope, for example a 'for scope' or an 'if scope', in other languages variables are local to the if or local to the for.

Code: Pascal  [Select][+][-]

1. function
2. begin // function scope
3.  
4. for ... do
5. begin // for scope
6.  
7. if ... then
8. begin // if scope
9.  
10. end // if scope
11.  
12. end // for scope
13.  
14. end // function scope