This is part of a series I started in March 2008 - you may want to go back and look at older parts if you're new to this series.
start ::= "%s" sexp sexp ::= "(" ws* exp* ")" exp ::= (atom | int | quoted | sexp) ws* atom ::= [a-zA-Z][a-zA-Z0-9_]* int ::= "-"?[0-9]+ quoted ::= "\"" escaped* "\"" escaped ::= "\" . | [~"] ws ::= [ \t\r\n]
class Scanner def initialize io @io = io @buf = "" end def fill if @buf.empty? c = @io.getc c = c.chr if c @buf = c ? c.to_s : "" end end def peek fill return @buf[-1] end def get fill return @buf.slice!(-1,1) end def unget(c) c = c.reverse if c.is_a?(String) @buf += c end
This is pretty much the basic of most of the scanners I do. #initialize takes any object that supports #getc, which means any IO object (STDIN, File's etc.), but also anything you might decide to write that can implement a #getc method. Writing an adapter for practically anything that can be represented like a sequence of characters is in other words trivial.
#fill takes care of ensuring we always have at least one character waiting before #peek and #get does their jobs.
#peek gives us a single character lookahead without doing any additional buffering, and you'll see how that simplifies things later. #get returns a single character. Notice a difference in how they act that may be a bit annoying - I'm not sure whether to keep it that way or not: #peek returns the integer value that represents a specific character but #get returns a single character string. It makes some things simpler in the parser (through the savings are pretty trivial), but you need to keep an eye on it or risk confusion.
#unget puts a string or character back onto the buffer. As the name says, it "ungets" something you've previously used #get on. #unget combined with #get provides longer lookahead for the cases where that is convenient. Many languages are fine with just one character lookahead, but if you want to write that parser in a "scanner less" (i.e. without tokenization) style, it's often easier to use more lookahead even if the grammar otherwise can be parsed with one character lookahead. An example:
If the parser expects "defun" but the user has written "define", a tokenizing parser will see the "d", decide that this is a token of type "word" (or whatever the compiler writer has called it), parse the whole thing, and return a "Token" object or somesuch with the type "word" and the value "define", and the parser will raise an error at that level. This works because the grammar is designed so that when "d" occurs, it's always a word, and then the parser above it relies on a single token of lookahead, not a character. Without the tokenizer, you're stuck either writing the parser so that it will explicitly look for common prefixes first, and then the rest of the words, OR you use more lookahead.
Imagine if an alternative grammar rule for the parser above that expects "defun" is that "dsomethingelse" is also allowed at the same spot. Now the parser writer can either look for "d", and then look for "e" or "s", or he can use a scanner like the above that can use more than one character lookahead, and look directly for "defun", and if that fails look for "dsomethingelse". For handwritten parsers without a tokenizer the latter is simpler, and a lot easier to read, and it only results in more lookahead actually being used in cases where there are multiple rules that are valid at the same point, and they have common prefixes, which isn't too bad as it's something we'll generally want to avoid.
Note that I won't avoid more generic tokenization everywhere in the parser. Let's move on:
def expect(str) return true if str == "" return str.expect(self) if str.is_a?(Class) buf = "" str.each_byte do |s| c = peek if !c || c.to_i != s unget(buf) return false end buf += get end return true end end
This allows us to do things like myScanner.expect("defun"), and it will obediently recognize the string, and then unget the whole thing if it doesn't get to the end. So we can do myScanner.expect("defun"), and then myScanner.expect("dsomethingelse"), and it will handle the lookahead for us.
To make it easier to get it self hosted, it doesn't support regexps like the StringScanner class does. However it does have one concession: "return str.expects(self) if str.respond_to?(:expect)". If you pass it something that responds to "expect", it'll call expect and pass itself, and let that object handle the recognition itself instead. That'll let us do things like myScanner.expect(Token::Word) in the future, so we can happily mix a tokenizer style with a character-by-character style.
Now that we have the scanner, lets move on and go through the important bits:
First thing we do is implement a function to skip whitespace where it is allowed. I won't dwell on it, as it should be simple enough.
def ws while (c = @s.peek) && [9,10,13,32].member?(c) do @s.get; end end
Then we start at the top, literally, with our "start" rule:
def parse ws return nil if [email protected]("%s") return parse_sexp || raise("Expected s-expression") end
The one thing worth noting here is that we return nil if we can't find "%s" but we raise an exception (and we will switch to using a custom exception class rather than just passing a string to raise later on, don't worry) if parse_sexp doesn't return something. This is a general pattern: Return nil for as long as you leave the stream unchanged. When you are sure that you have satisfied the start of the rule (and you know that no other rule that is valid at this point starts the same way), you know that failure to satisfy the rest of the rule is an error.
next up, #parse_sexp needs to handle a list of expressions:
def parse_sexp return nil if [email protected]("(") ws exprs =  while exp = parse_exp; do exprs << exp; end raise "Expected ')'" if [email protected](")") return exprs end
#parse_exp is mostly a much of alternatives, and here you'll see the custom classes passed to expect:
def parse_exp (ret = @s.expect(Atom) || @s.expect(Int) || @s.expect(Quoted) || parse_sexp) && ws return ret end
... and, that's the whole s-expression parser. Of course, it won't really be complete without the custom tokenization classes, so let's quickly take a look at one of them - the most complex one:
class Atom def self.expect s tmp = "" if (c = s.peek) && ((?a .. ?z).member?(c) || (?A .. ?Z).member?(c)) tmp += s.get while (c = s.peek) && ((?a .. ?z).member?(c) || (?A .. ?Z).member?(c) || (?0 .. ?9).member?(c) || ?_ == c) tmp += s.get end end return nil if tmp == "" return tmp.to_sym end end
As you can see it's really just a container for a single function, and really nothing is stopping you from doing Tokens::Atom.expect(s) instead of @s.expect(Token::Atom), but I feel the latter reads better.