% Here are some sample code in Postscript

% Note: any text after percent symbol and the end of the line
% is a comment

% How to use:
% 1. Cut and paste each sample code into a separate file.
%    Let's take the second example and save it to a file called "t2.mps".

% 2. Load support code into SML: use "hw2support.sml";
%    Call the parser: parse "t2.mps";
%    Study the output to understand how a piece of postscript code is parsed.
% 3. If you write your "interp", then you can test whether it interprets 
%    correctly: interp "t2.mps";
%    you should see a question mark ?, which is to prompt you for input.
%    Say, you type 5 and return. You should see >>> 15
 
%example code 1:

/sub {neg add} def 7 4 sub

%example code 2:
% sum is a recursive procedure that takes a natural number n
% and return the summation of all natural numbers from 1 to n

/sum {
  dup 1 eq {} {dup -1 add sum add} ifelse
} def

% A number is taken from the keyboard and saved onto the stack.
% The procedure "sum" is executed on that number and the result
% is written to the screen

read sum write

%example code 3:
% A procedure "mul" is defined, which takes two integers
% and multiply them and return the product. 

/sum {
  dup 1 eq {} {dup -1 add sum add} ifelse
} def

/mul {
  begin
  /y exch def
  /x exch def
  /sum 0 def
  {
    y 0 eq {exit} {} ifelse
    /sum sum x add def
    /y y -1 add def
  } loop
  sum
  end
} def

% Take two integers from the keyboard, call "mul" and write the result
% to the screen.
read read mul write

%example code 4:
% A recursive procedure for calculating a factorial.

/sum {
  dup 1 eq {} {dup -1 add sum add} ifelse
} def

/mul {
  begin
  /y exch def
  /x exch def
  /sum 0 def
  {
    y 0 eq {exit} {} ifelse
    /sum sum x add def
    /y y -1 add def
  } loop
  sum
  end
} def

/fact {
  begin
    /n exch def
    n 0 eq {1} {n n -1 add fact mul} ifelse
  end
} def 

read fact write