The Idea

As a real, practical example, we will now write a simple, interpreted programming language called XPL (eXample Programming Language) using JS/CC. XPL is a C-styled script language interpreter, providing simple user input/output operations, loops, and conditional execution as well as variables. As variable type, only numbers are supported (integer and floating point both use the same, internal JavaScript type "Number"). The definition of user-defined functions is not possible in this scripting language, but it would be simple to add this feature.

For text output, XPL provides a special "say" command which allows to output constant texts.

To get familiar with XPL's syntax, look at the following example scripts.

              // This is a simple Hello World script, written in XPL.
              say 'Hello World';
              // The wonderful "99 bottles of beer" program
              bottles = 99;
                  // The output will not be the prettiest, but that is limited
                  // by the implementation (you can change it if you want)
                  write bottles;
                  if bottles == 1 say 'bottle of beer on the wall,';
                  else say 'bottles of beer on the wall,';
                  write bottles;
                  if bottles == 1
                      say 'bottle of beer';
                      say 'bottles of beer';
                  say 'Take one down, pass it around,';
                  bottles = bottles - 1;
                  write bottles;
                  if bottles == 0 say 'no more bottles of beer on the wall';
                  else if bottles == 1 say 'bottle of beer on the wall';
                  else say 'bottles of beer on the wall';
                  say ''; //Empty line
              while bottles > 0;
              say 'That''s it!';
              // A rocketry launch countdown
              say '--- The final countdown program ---';
                  say 'Enter your starting number (it must be greater than or equal to 10):';
                  read count;
                  if count < 10 say 'The number is lower than 10!';
              while count < 10;
              say 'Starting sequence...';
              while count >= 0 do
                  write count;
                  //Ignition at 3 loops before lift-off...
                  if count == 3 say 'Ignition...';
                  else if count == 0 say '...and lift-off!';
                  count = count - 1;