Tutorial

Languages can be split into two components, their syntax and their semantics. It’s your understanding of English syntax that tells you the stream of words “Sleep furiously green ideas colorless” is not a valid sentence. Semantics is deeper. Even if we rearrange the above sentence to be “Colorless green ideas sleep furiously”, which is syntactically correct, it remains nonsensical on a semantic level. With Treetop, you’ll be dealing with languages that are much simpler than English, but these basic concepts apply. Your programs will need to address both the syntax and the semantics of the languages they interpret.

Treetop equips you with powerful tools for each of these two aspects of interpreter writing. You’ll describe the syntax of your language with a *parsing expression grammar*. From this description, Treetop will generate a Ruby parser that transforms streams of characters written into your language into *abstract syntax trees* representing their structure. You’ll then describe the semantics of your language in Ruby by defining methods on the syntax trees the parser generates.

Parsing Expression Grammars, The Basics

The first step in using Treetop is defining a grammar in a file with the ‘.treetop` extension. Here’s a grammar that’s useless because it’s empty:

# my_grammar.treetop
grammar MyGrammar
end

Next, you start filling your grammar with rules. Each rule associates a name with a parsing expression, like the following:

# my_grammar.treetop
grammar MyGrammar
  rule hello
    'hello chomsky'
  end
end

The first rule becomes the root of the grammar, causing its expression to be matched when a parser for the grammar is fed a string. The above grammar can now be used in a Ruby program. Notice how a string matching the first rule parses successfully, but a second nonmatching string does not.

# use_grammar.rb
require 'rubygems'
require 'treetop'
Treetop.load 'my_grammar'

parser = MyGrammarParser.new
puts parser.parse('hello chomsky').success?         # => true
puts parser.parse('silly generativists!').success?  # => false

Users of *regular expressions* will find parsing expressions familiar. They share the same basic purpose, matching strings against patterns. However, parsing expressions can recognize a broader category of languages than their less expressive brethren. Before we get into demonstrating that, lets cover some basics. At first parsing expressions won’t seem much different. Trust that they are.

Terminal Symbols

The expression in the grammar above is a terminal symbol. It will only match a string that matches it exactly. There are two other kinds of terminal symbols, which we’ll revisit later. Terminals are called *atomic expressions* because they aren’t composed of smaller expressions.

Ordered Choices

Ordered choices are *composite expressions*, which allow for any of several subexpressions to be matched. These should be familiar from regular expressions, but in parsing expressions, they are delimited by the ‘/` character. Its important to note that the choices are prioritized in the order they appear. If an earlier expression is matched, no subsequent expressions are tried. Here’s an example:

# my_grammar.treetop
grammar MyGrammar
  rule hello
    'hello chomsky' / 'hello lambek'
  end
end

# fragment of use_grammar.rb
puts parser.parse('hello chomsky').success?         # => true
puts parser.parse('hello lambek').success?          # => true
puts parser.parse('silly generativists!').success?  # => false

Sequences

Sequences are composed of other parsing expressions separated by spaces. Using sequences, we can tighten up the above grammar.

# my_grammar.treetop
grammar MyGrammar
  rule hello
    'hello ' ('chomsky' / 'lambek')
  end
end

Node the use of parentheses to override the default precedence rules, which bind sequences more tightly than choices.

Nonterminal Symbols

Here we leave regular expressions behind. Nonterminals allow expressions to refer to other expressions by name. A trivial use of this facility would allow us to make the above grammar more readable should the list of names grow longer.

# my_grammar.treetop
grammar MyGrammar
  rule hello
    'hello ' linguist
  end

  rule linguist
    'chomsky' / 'lambek' / 'jacobsen' / 'frege'
  end
end

The true power of this facility, however, is unleashed when writing *recursive expressions*. Here is a self-referential expression that can match any number of open parentheses followed by any number of closed parentheses. This is theoretically impossible with regular expressions due to the *pumping lemma*.

# parentheses.treetop
grammar Parentheses
  rule parens
    '(' parens ')' / ''
  end
end

The ‘parens` expression simply states that a `parens` is a set of parentheses surrounding another `parens` expression or, if that doesn’t match, the empty string. If you are uncomfortable with recursion, its time to get comfortable, because it is the basis of language. Here’s a tip: Don’t try and imagine the parser circling round and round through the same rule. Instead, imagine the rule is already defined while you are defining it. If you imagine that ‘parens` already matches a string of matching parentheses, then its easy to think of `parens` as an open and closing parentheses around another set of matching parentheses, which conveniently, you happen to be defining. You know that `parens` is supposed to represent a string of matched parentheses, so trust in that meaning, even if you haven’t fully implemented it yet.

Features to cover in the talk

  • Treetop files

  • Grammar definition

  • Rules

  • Loading a grammar

  • Compiling a grammar with the ‘tt` command

  • Accessing a parser for the grammar from Ruby

  • Parsing Expressions of all kinds

? Left recursion and factorization

- Here I can talk about function application, discussing how the operator
  could be an arbitrary expression

  • Inline node class eval blocks

  • Node class declarations

  • Labels

  • Use of super within within labels

  • Grammar composition with include

  • Use of super with grammar composition