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This page contains instructions on getting started with the Hapy library. The information contained and referenced on this page should be sufficient for experienced programmers to build a Hapy parser and interpret its output. This is not an in-depth tutorial.

Table of Contents

The big picture

We are going to build a Hapy parser for a simple grammar and use it to interpret program input. The complete corresponding C++ code is available as doc/calc.cc in your Hapy source code distribution.

Hapy parsing usually involves these major steps:

  1. locating an existing grammar or writing one from scratch (often in a BNF-like form),
  2. specifying a grammar for the parser generator,
  3. parsing input with a generated parser, and
  4. interpreting parsing results, including errors.

The first step is out of Hapy scope (let's assume that you have a ready-to-use grammar or can write one from scratch). Hapy is meant to make the second and third steps a snap. As for the last step (interpretation), Hapy parsers produce parsing trees that are easy to search, traverse, or extract values from, leaving interpretation logic to the programmer.

The sections below explain these steps in more detail.

Specifying a grammar

BNF conversion

Let's assume you have the following BNF-like grammar, involving recursion and multiple rules for a single non-terminal:

		expression = number | '(' expression ')' .
		expression = expression '+' expression .
		number     = DIGIT DIGIT* .

Convert the grammar to C++ code. First, for each unique non-terminal (the symbol on the left hand side of each rule), define a Hapy rule variable:

		Rule rExpression;
		Rule rNumber;

The Rule class constructor automatically assigns a unique identifier to each rule. That ID is often essential for interpreting parsing trees. It can be extracted using Rule's id() method. The order of Rule construction is not important.

Now, convert BNF rules to C++ rules using BNF-to-Hapy conversion table provided elsewhere. Merge multiple BNF rules with the same left-hand-side non-terminal (if any) using C++ "bitwise or" operator:

		rExpression = rNumber | '(' >> rExpression >> ')' |
			      rExpression >> '+' >> rExpression;
		rNumber = digit_r >> *digit_r;

You are done converting your BNF to Hapy rules. Note how C++ "right shift" operator is used to concatenate rule components. Also note that each Hapy Rule object got exactly one assignment operator, defining the corresponding parsing rule. The BNF-to-Hapy conversion merged multiple "expression" definitions in BNF into one C++ assignment. The order of assignments is not important.

Let's add a tiny bit of realism to our example. The rExpression grammar specifies a single valid expression. In reality, a valid input probably can have one or more expressions:

		Rule rGrammar;
		rGrammar = +(rExpression >> ';');

The above addition specifies that a valid input must contain at least one expression and expressions are delimited using semicolons.

Whitespace handling rules

Some grammars have explicit built-in rules to handle whitespace, comments, and such. If your grammar is like that, you can skip this section.

Most grammars, however, have a prose statement saying that every grammar token or terminal can be surrounded by any amount of whitespace. While you can always express such a statement using explicit grammar rules, the resulting noise makes the grammar unreadable. A better approach is to tell Hapy to trim whitespace automagically:


The R.trim() method specifies the default "whitespace trimming" rule for rule R and all rules R uses. By default, trimming rule propagates all the way down to grammar terminals (e.g., string constants or character class rules like digit_r), surrounding each terminal by a copy of the specified trimming rule. However, rules marked as verbatim do not propagate trimming to their subrules. We marked rNumber as a verbatim rule because numbers should not have spaces inside them (e.g., "1 2 + 3 4" is not a valid expression in our grammar).

The "whitespace" rule can be anything, of course. We used *space_r as a typical example. Note that specifying just space_r would require a single whitespace character as a terminal separator, making expressions like "(3)" or "1 + 2" invalid. Sometimes, that is exactly what you want!

Rules like rNumber are really terminals from high-level grammar and interpretation points of view: they do not have internal whitespaces, and you are unlikely to be interested in their character-level decomposition. It is a good idea to mark such nodes as "leaf" nodes:


Optimizing parsing speed

The last, and often optional, step is to optimize your grammar for parsing speed and memory usage by inserting "commit points". By default, a Hapy parser always tries all alternatives specified by the grammar. In most cases, that is exactly what you want from the correctness point of view. However, it may take a long time for the parser to try all alternatives in complicated rules, especially when dealing with invalid input.

If you mark a rule as a commit point, the rule will never backtrack. That is, once the committed rule finds the first match, the rule will optimize the corresponding part of the parsing tree and will not try to find other matches, even if all other rules fail because of that.


Committing the top rule (rGrammar) gives Hapy parser a chance to optimize the entire parsing tree once parsing is completed. However, it prevents you from finding other alternatives with Parser::nextMatch() if your grammar is ambiguous.

Use this optimization with care! If you add too many commit points, you may disable backtracking where it is essential. As a rule of thumb, it is usually safe to commit a rule without subrule repetition or or-ed subrules. It is often safe to commit a rule terminated by a relatively unique expression (once that expression is parsed, it is unlikely that a different alternative would match). For example, if we had isolated the (rExpression >> ";") rule into an explicit Rule object, we could commit that rule.

N.B. It is also possible to use commit points to remove grammar ambiguities.


With the previous step behind us, we are ready to generate a parser and parse input using our grammar:

		Parser parser;
		if (!parser.parse(content)) {
			cerr << parser.result().location() << ": syntax error" << endl;
		//parser.result().pree.print(cout << "success; parsing tree:" << endl);


The above code fragment assumes that the input has been assigned to a string called "content". The Parser::parse() method returns false if there is not match or if there were errors interpreting the grammar itself. The current error reporting code is ugly and related interface will be improved in future Hapy releases. The existing approach is OK to start with.

The commented out C++ line at the end of the code fragment prints the parsing tree. It's handy for debugging. The last line calls the interpreter code described below.

At this point, we have a working parser and can detect valid input. If your job is to build a document validator, you are done. Otherwise, the next section illustrates parsing tree interpretation techniques.

Results interpretation

A Hapy parser produces a parsing tree (called "pree" in Hapy code). Interpretation involves traversing that tree and/or searching for particular nodes on that tree. Tree node interface is documented elsewhere.

Let's see how we can compute and print the value of every input expression using the parsing tree. Let's start with traversing the tree to find and print top-level expressions (those delimited by semicolons). We will implement expression value computation later.

		// extract top-level expressions (magical version)
		void interpret(const Pree &result) {
			for_some(result, rExpression.id(), ptr_fun(&intrpTop));

		// interpret and report a single top-level expression
		void intrpTop(const Pree &top) {
			const int value = intrpExpr(expr);
			cout << expr.image() << " = " << value << endl;

If the interpret() code looks to cryptic, read on: The for_some() function does the left-to-right depth-first traversal of the parsing tree and calls the specified function for each node that was produced by a rule with the specified ID. Note that for_some() does not go deeper if a match is found. Thus, only top-level expressions will match.

The intrpTop() function extracts the expression, evaluates it using yet unspecified intrpExpr() function, and prints expression image and interpreted value.

We could have avoided the for_some() magic in interpret() by manually locating and iterating top-level expressions: We know that a plus rule in rGrammar, +(rExpression >> ";"), yields a single parsing tree node, with one child for every subrule match. Children are accumulated in an STL container, with the usual begin() and end() iterators. The interpretToo() function below iterates through those children using STL's standard for_each() call. It calls the same intrpTop() for every child.

		// extract top-level expressions (less magical version)
		void interpretToo(const Pree &result) {
			// rGrammar = +(rExpression >> ";");
			const Pree &topExs = result[0];
			for_each(topExs.begin(), topExs.end(), ptr_fun(&intrpTop));

Interpreting an expression is relatively straight forward. The recursive intrpExpr() function does the job by distinguishing three possible cases:

		// calculate expression value
		int intrpExpr(const Pree &expr) {
			// rExpression = rNumber | parens | sum
			const Pree &alt = expr[0];
			if (alt.rid() == rNumber.id()) // rNumber
				return intrpNumber(alt);
			if (alt[0].image() == "(") // parens
				return intrpExpr(alt[1]);
			else // sum
				return intrpExpr(alt[0]) + intrpExpr(alt[2]);

Interpreting the number node is trivial provided you do not need to check for overflows and such:

		// extract top-level expressions (less magical version)
		int intrpNumber(const Pree &num) {
			return atoi(num.image().c_str());

This is it. We are now ready to calculate a few expressions. The source code of the example can be found in the doc/calc.cc file in the Hapy distribution. It is built automatically (but not installed) when you build Hapy.

		% echo '1;' | doc/calc
		  1 = 1
		% echo '1+1;' | doc/calc
		  1+1 = 2
		% echo '1+(1);' | doc/calc
		  1+(1) = 2
		% echo '1+1;2+3;' | doc/calc
		  1+1 = 2
		  2+3 = 5
		% echo '1+;' | doc/calc
		  parsing failure after 2 bytes, near ;

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