14-Jul-15 Recognizers

2 Parsers and recognizers Given a grammar (say, in BNF) and a string, A recognizer will tell whether the string belongs to the language defined by the grammar A parser will try to build a tree corresponding to the string, according to the rules of the grammar Input stringRecognizer resultParser result * 4true *falseError

3 Building a recognizer One way of building a recognizer from a grammar is called recursive descent Recursive descent is pretty easy to implement, once you figure out the basic ideas Recursive descent is a great way to build a “quick and dirty” recognizer or parser Production-quality parsers use much more sophisticated and efficient techniques In the following slides, I’ll talk about how to do recursive descent, and give some examples in Java

4 Recognizing simple alternatives, I Consider the following BNF rule: ::= “+” | “-” That is, an add operator is a plus sign or a minus sign To recognize an add operator, we need to get the next token, and test whether it is one of these characters If it is a plus or a minus, we simply return true But what if it isn’t? We not only need to return false, but we also need to put the token back because it doesn’t belong to us, and some other grammar rule probably wants it We need a tokenizer that can take back characters We will make do with putting back only one token at a time

5 Creating a decorator class A Decorator class is a class that extends another class and add functionality In this case, StringTokenizer may do what we need, except be able to take back tokens We can decorate StringTokenizer and add the ability to take back tokens For simplicity, we’ll allow only a single token to be returned Our decorator class will need a little extra storage, and will need to override and extend some methods

6 PushbackTokenizer I public class PushbackTokenizer extends StringTokenizer { String pushedToken = null; // to hold returned token public PushbackTokenizer(String s) { super(s); // superclass has no default constructor } public void pushBack(String token) { // added method pushedToken = token; }

7 PushbackTokenizer II public boolean hasMoreTokens() { if (pushedToken != null) return true; else return super.hasMoreTokens(); } Notice how we not only overrode this method, but if we didn’t have anything special to do, we just let the superclass’s method handle it public String nextToken() { if (pushedToken == null) return super.nextToken(); // We only return a pushedToken once String result = pushedToken; pushedToken = null; return result; } Again, we just use the superclass’s method, we don’t reinvent it

8 Sample use of PushbackTokenizer public static void main(String[ ] args) { PushbackTokenizer pb = new PushbackTokenizer("This is too cool"); String token; System.out.print(pb.nextToken( ) + " "); // “This” System.out.print(pb.nextToken( ) + " "); // “is” System.out.print (token = pb.nextToken() + " "); // “too” pb.pushBack(token); // return “too” System.out.print(pb.nextToken( ) + " "); // get “too” again System.out.print(pb.nextToken( ) + " "); // “cool” } Output: This is too too cool Question: Why the extra space?

9 Recognizing simple alternatives, II Our rule is ::= “+” | “-” Our method for recognizing an (which we will simply call addOperator ) looks like this: public boolean addOperator() { Get the next token, call it t If t is a “+”, return true If t is a “-”, return true If t is anything else, put the token back return false }

10 Helper methods We could turn the preceding pseudocode directly into Java code But we will be writing a lot of very similar code......and it won’t be very readable code We should write some auxiliary or “helper” methods to hide some of the details for us First helper method: private boolean symbol(String expectedSymbol) Gets the next token and tests whether it matches the expectedSymbol If it matches, return true If it doesn’t match, put the symbol back and return false We’ll look more closely at this method in a moment

11 Recognizing simple alternatives, III Our rule is ::= “+” | “-” Our pseudocode is: public boolean addOperator() { Get the next token, call it t If t is a “+”, return true If t is a “-”, return true If t is anything else, put the token back return false } Thanks to our helper method, our actual Java code is: public boolean addOperator() { return symbol("+") || symbol("-"); }

12 Categories of tokens Tokens are always strings, but they come in a variety of kinds: Names: "limit", "y", "maxValue" Keywords: "if", "while", "instanceof" Numbers: "25", "3" Symbols: "+", "=", ";" Special: "\n", end_of_input Instead of treating tokens as simple strings, it’s convenient to create a Token class that holds both its string value and a constant telling what kind of token it is class Token { String value; int type;...and this class should define some constants to represent the various types: public static final int NAME = 1; public static final int SYMBOL = 2; etc. If you are using Java 5.0, this is what enum s were invented for!

13 Implementing symbol symbol gets a token, makes sure it’s a symbol, compares it to the desired value, possibly puts the token back, and returns true or false We will want to do something similar for numbers, names, end of lines, and maybe end of input It would be foolish to write and debug all of these separately Again, we should use an auxiliary method private boolean symbol(String expectedSymbol) { return nextTokenMatches(Token.SYMBOL, expectedSymbol); }

14 nextTokenMatches #1 The nextTokenMatches method should: Get a token Compare types and values Return true if the token is as expected Put the token back and return false if it doesn’t match private boolean nextTokenMatches(int type, String value) { Token t = tokenizer.next(); if (type == t.getType() && value.equals(t.getValue())) { return true; } else { tokenizer.pushBack(t); return false; } }

15 nextTokenMatches #2 The previous method is fine for symbols, but what if we only care about the type? For example, we want to get a number—any number We need to compare only type, not value private boolean nextTokenMatches(int type, String value) { Token t = tokenizer.next(); omit this parameter if (type == t.getType() && value.equals(t.getValue())) return true; else tokenizer.pushBack(t); omit this test return false; } The two versions of nextTokenMatches are difficult to combine and fairly small, so we won’t worry about the code duplication too much

16 addOperator reprise public boolean addOperator() { return symbol("+") || symbol("-"); } private boolean symbol(String expectedSymbol) { return nextTokenMatches(Token.SYMBOL, expectedSymbol); } private boolean nextTokenMatches(int type, String value) { Token t = tokenizer.next(); if (type == t.getType() && value.equals(t.getValue())) return true; else tokenizer.pushBack(t); return false; }

17 Sequences, I Suppose we want to recognize a grammar rule in which one thing follows another, for example, ::= “[” “]” The code for this would be fairly simple... public boolean emptyList() { return symbol("[") && symbol("]"); }...except for one thing... What happens if we get a “ [ ” and don’t get a “ ] ”? The above method won’t work—why not? Only the second call to symbol failed, and only one token gets pushed back

18 Sequences, II The grammar rule is ::= “[” “]” And the token string contains [ 5 ] Solution #1: Write a pushBack method that can keep track of more than one token at a time (say, in a Stack ) This will allow you to put the back both the “ [ ” and the “ 5 ” The code gets pretty messy You have to be very careful of the order in which you return tokens Solution #2: Call it an error You might be able to get away with this, depending on the grammar For example, for any reasonable grammar, ( ) is clearly an error Solution #3: Change the grammar Tricky, and may not be possible Solution #4: Combine rules See the next slide

19 Sequences, III Suppose the grammar really says ::= “[” “]” | “[” “]” Now your pseudocode should look something like this: public boolean list() { if first token is “[” { if second token is “]” return true else if second token is a number { if third token is “]” return true else error } else put back first token } Revised grammar: ::= “[” ::= “]” | “]”

20 Simple sequences in Java Suppose you have this rule: ::= “(” “)” A good way to do this is often to test whether the grammar rule is not met public boolean factor() { if (symbol("(")) { if (!expression()) error("Error in parenthesized expression"); if (!symbol(")")) error("Unclosed parenthetical expression"); return true; } return false; }

21 Sequences and alternatives Here’s the real grammar rule for : ::= | | “(” “)” And here’s the actual code: public boolean factor() { if (name()) return true; if (number()) return true; if (symbol("(")) { if (!expression()) error("Error in parenthesized expression"); if (!symbol(")")) error("Unclosed parenthetical expression"); return true; } return false; }

22 Recursion, I Here’s an unfortunate (but legal!) grammar rule: ::= “+” Here’s some code for it: public boolean expression() { if (!expression()) return false; if (!addOperator()) return true; if (!term()) error("Error in expression after '+' or '-'"); return true; } Do you see the problem? We aren’t recurring with a simpler case, therefore, we have an infinite recursion Our grammar rule is left recursive (the recursive part is the leftmost thing in the definition)

23 Recursion, II Here’s our unfortunate grammar rule again: ::= “+” Here’s an equivalent, right recursive rule: ::= “+” Here’s some (much happier!) code for it: public boolean expression() { if (!term()) return false; if (!addOperator()) return true; if (!expression()) error("Error in expression after '+' or '-'"); return true; }

24 Extended BNF—optional parts Extended BNF uses brackets to indicate optional parts of rules Example: ::= “if” [ “else” ] Pseudocode for this example: public boolean ifStatement() { if you don’t see “if”, return false if you don’t see a condition, return an error if you don’t see a statement, return an error if you see an “else” { if you see a “statement”, return true else return an error } else return true; }

25 Extended BNF—zero or more Extended BNF uses braces to indicate parts of a rule that can be repeated Example: ::= { “+” } Note that this is not a good definition for an expression Pseudocode for example: public boolean expression() { if you don’t see a term, return false while you see a “+” { if you don’t see a term, return an error } return true }

26 Back to parsers A parser is like a recognizer The difference is that, when a parser recognizes something, it does something about it Usually, what a parser does is build a tree If the thing that is being parsed is a program, then You can write another program that “walks” the tree and executes the statements and expressions as it finds them Such a program is called an interpreter You can write a similar program that “walks” the tree and produces code in some other language (usually assembly language) that does the same thing Such a program is called a compiler

27 Conclusions If you start with a BNF definition of a language, You can write a recursive descent recognizer to tell you whether an input string “belongs to” that language (is a valid program in that language) Writing such a recognizer is a “cookbook” exercise—you just follow the recipe and it works (hopefully) You can write a recursive descent parser to create a parse tree representing the program The parse tree can later be used to execute the program BNF is purely about syntax BNF tells you what is legal, and how things are put together BNF has nothing to say about what things actually mean

28 The End