Why try ... except block is not working here - try-catch

Question

I am trying to run following code with try.. except block:
program TriangleArea;
uses crt, sysutils;
type
num = real;
var
a, b, c, s, area : num;
begin
write('Enter lengths of 3 sides (separated by spaces): ');
try
readln (a, b, c);
s := (a + b + c)/2.0;
area := sqrt(s * (s - a)*(s-b)*(s-c));
writeln(area);
except
on E: Exception do
ShowMessage( 'Error: '+ E.ClassName + #13#10 + E.Message );
end;
end.
But it is giving following error:
$ fpc triangle_area.pas
Free Pascal Compiler version 3.0.0+dfsg-11+deb9u1 [2017/06/10] for x86_64
Copyright (c) 1993-2015 by Florian Klaempfl and others
Target OS: Linux for x86-64
Compiling triangle_area.pas
triangle_area.pas(14,2) Error: Identifier not found "try"
triangle_area.pas(15,3) Fatal: Syntax error, ";" expected but "identifier READLN" found
Fatal: Compilation aborted
Error: /usr/bin/ppcx64 returned an error exitcode
Why "try" identifier is not being found. I am using fpc version 3.0.0 on Debian stable Linux.
Where is the problem and how can it be solved? Thanks for your help.

Answer 1

Place {$MODE OBJFPC} or {$MODE DELPHI} beneath your program declaration.
The reason is that by default the compiler will compile in MODE FPC, which does not support exceptions.
Additional sources: One Two Three
On the other hand, the ShowMessage instruction will not compile with Free Pascal. The correct code is:
program TriangleArea;
{$mode delphi}
uses crt, sysutils;
type
num = real;
var
a, b, c, s, area : num;
begin
write('Enter lengths of 3 sides (separated by spaces): ');
try
readln (a, b, c);
s := (a + b + c)/2.0;
area := sqrt(s * (s - a)*(s-b)*(s-c));
writeln(area);
except
on E: Exception do
write( 'Error: '+ E.ClassName + #13#10 + E.Message );
end;
end.
Later edit: the declaration type num=real is correct, but I don't see any real use of it.