Skip to content

Immutability

How do I make const variables constant?

Add a global directive {$J-} to your main pascal file.

Why is const section writeable in by default?

In Turbo Pascal 7.0, you could only assign values in the const section, not in var section.

As the official docs mentions, by default const is writeable (that is {$J+}) to keep Turbo Pascal compatibility.

It's a relic of old Delphi and Object Pascal and thus {$J+} is the default to allow old programs to compile, but it isn't required anymore

chsh -s /usr/bin/pwsh — 13/12/2023 12:09

Consider the following program. By the default {$J+}, you can change the gravity of the Earth, with the gravity of the Moon!

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
program Immutability;

{$mode objfpc}{$H+}

const
  gravityEarth:Real = 9.81;

begin
  gravityEarth := 1.625; // Look, we are on the moon.
  WriteLn('The gravity on the Earth is ', gravityEarth:0:2);
end.

To make const variables behaving like const variables in modern programming languages, use {$J-}

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
program Immutability;

{$mode objfpc}{$H+}{$J-}

const
  gravityEarth:Real = 9.81;

begin
  // With {$J-}, the following line will not compile
  // gravityEarth := 1.625;
  WriteLn('The gravity on the Earth is ', gravityEarth:0:2);
end.