Recursive Descent Parser - Explained in Points

1. Definition

A Recursive Descent Parser is a top-down parser built from a set of mutually recursive procedures, where each procedure implements one non-terminal of the grammar.

2. Parsing Strategy

• Top-down approach: Starts from the start symbol and attempts to derive the input string by recursively expanding non-terminals.
• Each production rule is translated into a function.
• Uses lookahead (typically one token) to decide which rule to apply.

3. Suitable Grammar

• Best suited for LL(1) grammars (Left-to-right scan, Leftmost derivation, 1-token lookahead).
• Left recursion must be eliminated before using recursive descent.

4. Basic Grammar Example

E  → T E'
E' → + T E' | ε
T  → F T'
T' → * F T' | ε
F  → ( E ) | id

5. Corresponding Pseudocode Functions

def E():
    T()
    E_prime()

def E_prime():
    if lookahead == '+':
        match('+')
        T()
        E_prime()

def T():
    F()
    T_prime()

def T_prime():
    if lookahead == '*':
        match('*')
        F()
        T_prime()

def F():
    if lookahead == '(':
        match('(')
        E()
        match(')')
    elif lookahead == 'id':
        match('id')
    else:
        error()

6. Key Components

• lookahead: Stores the current token.
• match(token): Verifies and consumes expected tokens.
• ε-production: Represented by a function that does nothing.

7. Advantages

• Simple to implement manually.
• Modular structure – each grammar rule corresponds to one function.
• Ideal for small language interpreters and educational use.

8. Disadvantages

• Cannot handle left-recursive grammars directly.
• May need backtracking if multiple alternatives exist without distinct lookaheads.
• Not scalable for complex or ambiguous grammars.

9. Applications

• Hand-written parsers for compilers, interpreters, DSLs.
• Used in early stages of language processing or in teaching compiler design.

10. Summary

A Recursive Descent Parser provides a clear, structured method for parsing source code when the grammar is well-behaved (LL(1)). Its recursive nature aligns closely with grammar rules, making it a fundamental technique in parsing theory.

Example 2 of Recursive Descent Parser

Grammar

We want to parse simple expressions like:

a + b * c

So we define a grammar:

E  → T E'
E' → + T E' | ε
T  → F T'
T' → * F T' | ε
F  → a | b | c

Sample Input

a + b * c

Parse Steps

1. Start with E
2. E → T E'
3. T → F T' → a T' (matched a)
4. T' → ε (no * after a)
5. E' → + T E' (matched +)
6. T → F T' → b T' (matched b)
7. T' → * F T' → * c T' (matched * and c)
8. T' → ε
9. E' → ε

Final Result

Input a + b * c is successfully parsed using recursive calls matching grammar rules.

Summary

This is another example of how a recursive descent parser handles expressions using function-like rule expansion.