Parser

Overview

The Parser is the second stage of the CodeSage pipeline.
It takes the list of tokens produced by the Scanner and organizes them into a structured representation known as the Abstract Syntax Tree (AST).

The Parser in CodeSage uses a Recursive Descent Parsing approach — a top-down method where each grammar rule corresponds to a Python method.
This makes it intuitive, modular, and easy to debug.

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Responsibilities

• Convert the flat list of tokens into a hierarchical tree structure (AST).
• Detect and report syntax errors.
• Implement error recovery using panic mode to continue parsing after an error.
• Pass the generated AST to other components (Resolver, Interpreter, and Summarizer).

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Example

Input Code:

a=25
if a >=18:
    print("Adult")
else:
    print("Child")

Parser Output

[Parser Output]:
ExpressionStmt(
  Assign(a)
    Literal(25.0)
)

IfStmt(
  Condition:
    Binary(>=)
      Variable(a)
      Literal(18.0)
  Then:
    BlockStmt(
      PrintStmt(
        Literal(Adult)
      )
    )
  Else:
    BlockStmt(
      PrintStmt(
        Literal(Child)
      )
    )
)

Error Handling in Parser

The Parser detects syntax errors, i.e., problems with the order or structure of tokens.

Common Parser Errors

Unexpected Token Unmatched Parentheses Invalid Expression
Invalid Statement

How Parser handles them

• Raise Exception: When a rule fails, the parser raises a syntax exception.
• Panic Mode: The synchronize() method skips tokens until a safe point (e.g., semicolon or start of a new statement) to continue parsing.
• Multiple Error Reporting: This ensures the parser reports all errors in a single run rather than stopping at the first one.

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Example

Input code

d=5
print(a)

x=105
print(x

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The parser continues parsing after errors, allowing multiple mistakes to be detected.