Part I

How to Purchase

Paperback Copy (with the option to get 50% off the pdf version) $16.95

Get the eBook (pdf) $15.95

For more information contact: hsauro@gmail.com

This is part 1 of a series that will show you how to write an interactive interpreter in Object Pascal. Part 1 of the series will cover introductory material including a description of the language we’ll create, a full lexical analyzer for the language, how to use DUnitX for unit testing, and an introduction to the essential concepts in syntax analysis, recursive descent, grammar, and EBNF. Along the way, we’ll create a simple REPL, give a detailed discussion of how to parse expressions and build a simple interactive calculator to illustrate the theory. The book provides fully working code and explains in plain English how the code works and why certain decisions were made, including alternative designs. The book makes liberal use of code throughout the book chapters. Everything is done without the help of third-party tools such as Yacc, ANTLR or Flex. All you need is a standard installation of Free Pascal or Embarcaderos’s Delphi (including the free community edition).

The text is geared to hobbyists and midlevel developers who need an accessible introduction to lexical analysis and parsing. It’s also for students starting out in compiler and interpreter design and need something more digestible.

All source code is open source under Apache 2.0 and available from Github.

Available in paperback form in the first week of January 2019 from Amazon.

Price $16.95 (paper), $15.95 (eBook) 170 pages

ISBN: 978-1-7325486-0-2

Contents:

1. Introduction
2. a) Why Object Pascal
3. b) What is an interpreter
4. c) Parts of an interpreter
5. The Rhodus Language
6. Lexical Analysis
   1. Initial API
   2. Input streams
   3. Retrieving tokens
   4. First run
   5. Adding more tokens
7. Testing
   1. Introduction to testing
   2. Using DUnitX
8. An Interactive Console
9. Introduction to Syntax Analysis
   1. Grammar
   2. Production rules
   3. LL(k)
   4. Recursive descent
   5. Factoring, the dangling else
   6. Left recursion
   7. Ambiguous Grammars
   8. A simple calculator
   9. Syntax trees
   10. Adding exponentiation and the unary minus
10. Testing the Calculator
11. Adding Assignments and Variables
    1. Using a queue for token lookahead
    2. Updating the syntax analyzer
    3. Updating the lexical analyzer
12. Building a Recognizer
13. Appendix EBNF

A brief description of the Rhodus language Version One

The language for the interpreter is a mix of ideas from Pascal, Basic and Python. This is a toy language in the sense it is used to illustrate how one might go about building an interpreter. This doesn’t mean it couldn’t be used in a serious application but given the availability of existing mature scripting languages, it would seem less likely. It’s primary purpose is therefore educational. The easiest way to describe the language is via some examples. A couple of initial points worth noting:

1. Newlines are not part of the syntax
2. Semicolons separate statements

// Data Types

a = 3;

b = 3.1415; b = -1.234E-12;

c = True; c = False;

d = "String";

myList = {1,2,3,{5,6},{"xyz", True, {False}}};

/* Operators:

   +, -, *, /, ^, (), mod, div, and or, not, xor, ==, >, >=, <, <=, !=

*/

println ("Hello World")

// Conditionals

if a > 6 then

   b = 9

else

   if b < 3 then

      x = 7

   else

      x = 8

   end

end

// Loops

a = 10

while a > 5 do

     a = a - 1

end;

a = 10

repeat

   a = a - 1

until a < 5;

for i = 1 to 10 do

    println (i)

    if i == 7 then

       break;

end;

for i = 0 to 10 do

    a = 5;

    for j = 0 to 20 do

        b = 7;

        for k = 0 to 30 do

            c = 9

        end    

    end 

end

// User defined functions

function add (a, b)

   return a + b

end;

x = 6;

function sub (a)

  global x

  return a - b;

end;

println ("Add two numbers: ", add (4,4))

// Functions can be passed to other functions

function callfunc (func)

    return func (5,6)

end;

println ("Add two numbers: ", callfunc (add))

// Lists

function bubbleSort (a, length)

   for i = 0 to length - 1 do

       for j = i+1 to length - 1 do

           if (a[i] > a[j])

               temp = a[i]; a[i] = a[j]; a[j] = temp;

           end

       end

   end

end;

// Sort the following array

alist = {5,2,6,7,3,2,5,1,4,9};

lengthOfList = 9;

bubbleSort (alist, lengthOfList);

println (alist);

// Larger example

function hamming (limit)

   h[0] = 1;

   x2 = 2; x3 = 3; x5 = 5;

   i  = 0; j  = 0; k = 0;

   for n = 1 to limit do

        h[n] = min(x2, min(x3, x5));

        if x2 == h[n] then i = i + 1; x2 = 2*h[i] end;

        if x3 == h[n] then j = j + 1; x3 = 3*h[j] end;

        if x5 == h[n] then k = k + 1; x5 = 5*h[k] end;

    end;

    return h[limit -1];       

end;

// Currently no library support for lists until Part 2 so

// we have to declare a list of a specific size like this

h = {0,0,0,0,0,0,0,0,0,0,0,0,0};

for i = 1 to 20 do

     println (hamming [i]);

end