assuming that one file is called "file1.extension" and it's content is:
function Add(a,b:integer):integer;
begin
result:=a+b;
end;
and another file called "main.extension" with content:
var
a,b,c:integer;
a:=1;
b:=2;
c:=Add(a,b);
println(inttoStr(c));
you need to add the following line at the beginning of "main.extension" file:
// note that file name is case sensitive
// file1.extension <> FILE1.EXTENSION
// include_once is to solve cycle-includes
// i.e. file1.extension includes main.extension and vice-versa
{$include_once 'file1.extension'}
// or include if file1.extension does not require functions/objects/variables/etc.
// from main.extension
{$include 'file1.extension'}
I suggest using {$include_once ...} rather than {$include ...}.