Recursive Descent Parser

A recursive descent parser is a top-down parser that processes input based on a set of recursive functions, where each function corresponds to a grammar rule. It parses the input from left to right, constructing a parse tree by matching the grammar's production rules. This parser is simple to implement and is suitable for LL(1) grammars, where decisions can be made based on a single lookahead token. While straightforward, recursive descent parsers struggle with left-recursive grammars and may require grammar transformations to handle such cases effectively.

A Predictive Parser is a special case of Recursive Descent Parser, where no Back Tracking is required. 
By carefully writing a grammar, means eliminating left recursion and left factoring from it, the resulting grammar will be a grammar that can be parsed by a recursive descent parser.

By carefully writing a grammar means eliminating left recursion and left factoring from it, the resulting grammar will be a grammar that can be parsed by a recursive descent parser.

Example:

Algorithm for Recursive Descent Parser

S()
{ Choose any S production, S ->X1X2…..Xk;
 for (i = 1 to k)
 {
 If ( Xi is a non-terminal)
 Call procedure Xi();
 else if ( Xi equals the current input, increment input)
 Else /* error has occurred, backtrack and try another possibility */
 }
}

Let's understand it better with an example:

The given grammar is:

E → i E'
E' → + i E' | ε

Function E()

E()
{ 
 if (input == 'i') { // If the input is 'i' (identifier) 
 input++; // Consume 'i' 
} 
 E'(); // Call E' to check for further expressions
}

• It checks for i (identifier).
• If found, it moves the input pointer ahead.
• Calls E'() to check if a + operation exists.

Function E'()

void E`() {
 if (input == '+') {
 input++; // Consume the '+'

 if (input == 'i') {
 input++; // Consume the 'i'
 }

 E`(); // Recursively process more additions
 } else {
 return; // If no '+', return (ε production)
 }
}

• It checks for + i.
• If found, it consumes them and calls E'() recursively.
• If no +, it returns (ε production).

Main Function

Main()
{ 
 E(); // Start parsing from E 
 if (input == '$') // If we reach end of input 
 Parsing Successful;
}

• Calls E() to start parsing.
• Checks if the input ends with $, which indicates a successful parse.

Example Input Parsing

Let’s consider the example input:

i + i $

Processing step by step:

1. E() starts → input == i, so consume i
2. Call E'() → input == +, so consume +
3. input == i, so consume i
4. Call E'() again → no +, so return.
5. Back to Main(), input == $ → Parsing Successful

Important points about recursive descent parsers

1. Top-Down Parsing: It starts from the start symbol and recursively applies grammar rules to break down the input.
2. One Function per Non-Terminal: Each grammar rule has a corresponding function in the parser, making the implementation straightforward.
3. Uses Recursion: The parser calls functions within themselves to process different parts of the input, matching the recursive nature of grammar rules.
4. Works Best with LL(1) Grammars: It’s most effective for grammars that can be parsed with a single token lookahead, typically simple, non-left-recursive grammars.
5. Easy to Implement: The structure is easy to follow and implement, making it a good choice for small compilers or interpreters.
6. Error Handling: It can detect syntax errors and report them, making it useful for debugging input strings.

Code Implementation of a Recursive Descent Parser