Package sleep.parser

The parser package.


Class Summary

Package sleep.parser Description

The parser package. Nothing really here for people integrating sleep into there application. If you want to know how this package works then read on:

Parser Pipeline Architecture

Define "code"

Your code is input as one large string containing all of the code in the sleep file to be parsed. We'll call this code, the "code".

           |   -- (String)
.---> StringIterator
|          |
|          |   -- (StringIterator), contains a way to iterate through each char and grab an
|          |       associated line number, also contains some other nice functions
|         \|/
|     LexicalAnalyzer
|          |
|          |   -- (TokenList), static methods that return a TokenList which is a structure holding
|          |      some tokens (a token is a String and a line number coupled together).  Comments are
|          |      also stripped out here and handed back to the Parser object itself.
|          |
|         \|/
|     TokenParser
|          |
|          |   -- (LinkedList of Statement objects), outputs a linked list of statement objects primarily.
|          |      A statement is little more than a TokenList with a type associated with it.  TokenParser
|          |      takes the TokenList from the lexical analyzer and assigns types to a stream of tokens. 
|          |
|         \|/
`--  CodeGenerator
           |   -- (Block), Code Generator takes the Statements output by the TokenParser looks at the type
           |      and generates the appropriate atomic steps for that type.
*Voila a runnable Block object*

The parser is a hand-coded recursive descent (aka top down) parser. I thought about using a parser generator but opted against it as this parser is now in its 3rd or 4th overhaul and well it works and the other data structures that go along with it work as well. So we're stuck with the hand coded stuff.

A kind of editorialized description of how this works:

Overall here is what I do. I take a raw stream of characters (StringIterator) and turn that raw stream of characters into tokens. Tokens are kind of like units of the program that stand alone and should not be picked apart (at least not yet).

Tokens in general are things like quoted string literals, blocks of code, and just plain single words. The stream is put into tokens so later on the tokens can be identified and analyzed looking for certain patterns to say "ah, I have a word if, followed by something in parenthesis, followed by something enclosed in { }'s.. it must be an if statement.

Once a first pass has occured, the tokens are fed into the TokenParser. The Token Parser uses a series of what are called "checkers". Checkers are functions that determine if a series of n tokens fall within a certain pattern or not. For example a checker for an if statement would take 3 tokens as its parameters, it would then look at the first token to see if it is the keyword "if", it would then analyze the second token and see if it is an expression enclosed in parentheses, finally it would look at the third token and determine if it is a block of code enclosed in { }'s. If these three things hold true then these 3 tokens comprise one statement, a statement that is labeled as an IF statement.

All the tokens are analyzed in this manner. Once this is done then they are fed into the code generator. The code generator loops through all of the statements and looks at the type of each statement.

From this type the code generator does a straight generation of the atomic steps that the interpreter executes. Although by this point the code hasn't all been 100% processed yet. In our if statement example the second token and third token which contain the predicate statement and the block of code need to be analyzed. So these items are then turned into a StringIterator, which is turned into tokens, which is parsed by the token parser, which is then fed into the code generator. Once all this has happened the code generator goes back to processing its statements.

So that is how the parser works.

Why did I do it this way? I was taking a lisp class a couple of years ago. Our main assignment for the term consisted of implementing a lisp-like language in lisp. We had a series of checkers that looked at the form of each statement and based upon that performed whatever action the statement was supposed to perform. Writing this lisp like language in lisp was incredibly easy as the structure of the program was so easy to pick at, it was just a list after all. My lisp may be rusty so excuse me if this all seems jacked up:

(def 'program (if (equals 3 3) (print 'good)))

For my if statement checker I just had to look at the first parameter (equals 'if (first program)), and make sure the second and third parameters are parameters (and (listp? (second program)) (listp? (third program)))

One day I was sitting in class kind of bored and realized that if I had an easy way to analyze the structure of a program at a high level then I could easy write a language with some more c-like syntax using this methodology. So I sat to work and wrote my scripting language sleep in a weekend. None of these ideas I used are original or new or anything. Just fun. This is by far one of my favorite programming adventures ever.

Dr. John Lowther did an excellent job teaching the programming languages class at Michigan Tech. Dr. Lowther had an incredible amount of enthusiasm and a fun approach to teaching CS students about programming languages (make them implement one... good idea!). Anyways his teaching style made the topic click very well and this project is a product of that knowledge gained.