Productions use two kinds of symbols, terminals and nonterminals. Terminals are also called tokens:
Given a properly defined grammar, you write a class for each production in the grammar, and you write one fairly simple method in each class. In those methods, you can identify errors and throw exceptions as necessary.
After parsing, you have a syntax tree sometimes called a parse tree that you can examine or modify. This blog is inactive. You can find the complete list of posts here.
For a typical professional developer, there are lots of benefits to understanding grammars, recursive descent parsers, and syntax trees. At one point in my process of learning CI read the specification of the C language, which includes the grammar for the language. Understanding the grammar, and correlating the grammar to the text verified that I understood each construct, and reduced the chance that I had any misconceptions about the exact semantics of the language.
Going through the process of writing a small recursive descent parser is very helpful too. This makes sure that you fully understand how grammars are put together, and what they mean.
Finally, as a professional developer, you may come across situations where this information is useful.
Perhaps you need to develop a small domain-specific language DSL. There are a number of approaches to developing DSLs, each appropriate in their own situation, and there are a few situations where it is most appropriate to define a grammar and write a parser.
As an Open XML developer, understanding recursive descent parsers is useful. Formulas in a spreadsheet are stored as strings. Without parsing those strings, those formulas are opaque. Fortunately, with Open XML, we have a grammar for those formulas. J As I was contemplating implementing some functionality searching for all formulas across a range of workbooks and worksheets that reference a specific cellI started contemplating writing a recursive descent parser in C and LINQ.
It promised to be a super-fun programming project. There are some huge benefits that we gain by writing the parser using LINQ. Many of those methods that we need to write in the production classes are significantly simplified by the use of LINQ.
Just to make it clear why we need this parser: In order to examine references to cells in formulas, we must parse those formulas according to the grammar. For instance, if we want to search for all references to cell A3, we may see the following formula in cell A1: There are practical reasons for writing this code.
I want to be able to look inside them and process Open XML spreadsheets in new and interesting ways. However, the most important reason to write these posts is for the sheer, absolute fun of it.
This whole project will be fun. It will be fun explaining how recursive descent parsers work. It will be fun to explore how LINQ makes it easy to write recursive descent parsers. And it will be fun to be able to query Open XML spreadsheets in cool ways.Since that will clearly work without backtracking, it can serve as the basis for how to write an LL(1) grammar.
Is it possible to simplify this grammar to ease the recursive-descent approach? I'm not sure if the following grammar is any simpler, but it does correspond to .
Parsing expressions by recursive descent poses two classic problems how to get the abstract syntax tree (or other output) to follow the precedence and associativity of operators and We can transform this grammar to a parser written in pseudo code.
Eparser is var t: Tree t:= E expect We use the following grammar G3. Jul 13, · To learn how recursive descent parsers work, it is helpful to implement a very simple grammar, so for pedagogical purposes, I’ve defined a grammar for simple arithmetic expressions.
The parser will construct a syntax tree from expressions that we can then examine as necessary.
Just for fun, after implementing the parser, we will write . This concludes the coding of a recursive descent parser.
If you haven’t figured it out by now, the name “recursive descent” comes from the fact that the parser performs a depth first search by recursively calling the same methods. Feb 22, · Use any programming language you prefer to write a recursive-descent parser that parses the language generated by the following EBNF descriptions.
Your parser should detect whether or not the input program has any syntax errors. Recursive Descent Parsing Parsing, you will recall, is the process of turning a stream of tokens into an abstract syntax tree. Any parsing technique requires a grammar --a formal, detailed definition of what sequence of symbols constitutes a syntactically correct program.