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.
Productions use two kinds of symbols, terminals and nonterminals. Terminals are also called tokens: A nonterminal represents some sequence of tokens in the string that is being parsed. A production has a left-hand-side, which is always a nonterminal. We will use upper-case letters to represent nonterminals and other characters to represent terminals.
For example, the following are productions: Since n actually stands for a number, real examples of inputs would have numbers in them, e.
A grammar has a start symbol, usually a nonterminal. A legal string as defined by the grammar is anything that can be generated from the start symbol using productions. One way to think about this is that we start with a string containing just the start symbol, and use productions to change nonterminals into whatever appears on the right-hand side of the production, until there are no nonterminals left.
Recursive descent parsing and exceptions Many grammars can be parsed using recursive descent parsing, as we saw in class. Recursive descent parsers have one function method per nonterminal, and the job of that method is to read in all the tokens for that nononterminal. In order for this to work, the method must be able to decide what production to use based on the input it sees.
For example, with the grammar above, the code to parse an E looks very roughly like: If we have a method nextToken that reads consumes the next token from the scanner, we can implement checkToken as follows: If some other token was seen, there must have been a syntax error in the input, and the code throws an exception, which will propagate up the stack of method calls until it hits an exception handler.
If there is no exception, the program will stop and print an error message. Notice that we had to add the declaration throws Exception to the top of the method to indicate that it could throw an exception.
If we take this approach to handling errors, we'll have to add similar declarations to all the parsing methods. We can handle exceptions using a try For example, we could parse an expression as follows: Notice that the exception handler declares a variable exc of type Exception.
This variable refers to the new exception object that was created in the throw statement, and exc.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. The dynamic call graph of a recursive descent parser corresponds exactly to the parse tree of input Call graph of input string 1+2*3 Exercise 1: Write a regular expression to capture the format of floating point constants in C/C++.
Writing a Simple Recursive Descent Parser 30 July — A simple implementation of a field-based query string, with binary operations, using a recursive descent parser — 5-minute read Someone asked a question recently on the local ruby list.
I'm trying to write a (scannerless) recursive descent parser with a "catch all" rule for the following "Mustache template" grammar (simplified here).
Use proper grammar (like capital letter after punctuation). This makes automatically generated documentation look great, and further normalizes how Go code looks. TokenStack represents the input stack. is a good first sentance to start with.
is and in to a was not you i of it the be he his but for are this that by on at they with which she or from had we will have an what been one if would who has her.