Decaf Compiler — Python-Based Educational Compiler

This project implements a fully functional compiler for the Decaf programming language — a simplified, Java-like language designed for teaching compiler construction. The compiler is structured into four main phases: Lexical Analysis, Parsing, Semantic Analysis, and groundwork for Code Generation.

It demonstrates fundamental compiler design concepts through modular, readable, and testable Python code.

🗓️ Created: December 2024 | 🎓 5th Semester, Compiler Design Project: A Full Pipeline for a Simplified Java-like Language

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Project Structure

File	Description
phase0_lexer_manual.py	Manual lexer built using regular expressions
phase1_lexer_plyLib.py	Lexer using the PLY (Python Lex-Yacc) library
phase2_parser_yacc.py	Parser built using CFG and PLY's yacc module
phase3_semantic_analysis.py	Performs semantic checks, builds symbol tables
decaf.txt, input.txt	Sample Decaf programs for input testing
grammar.txt, updated_grammar.txt	Define Decaf grammar used by parser
parsetab.py	Auto-generated parse table (PLY-generated)
TABLE1.csv	token metadata for manual lexer

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Compiler Phases Overview

🔹 Phase 0: Manual Lexical Analyzer (phase0_lexer_manual.py)

• Implements tokenization logic manually using regular expressions.
• Identifies Decaf keywords, operators, identifiers, literals, and delimiters.
• Ideal for educational demonstration of how a lexer operates under the hood.

🔹 Phase 1: Lexer with PLY (phase1_lexer_plyLib.py)

• Uses the PLY library to define token rules.
• Simplifies tokenization via Python classes and pattern-matching.
• Outputs structured tokens including type and line number metadata.

🔹 Phase 2: Syntax Parser (phase2_parser_yacc.py)

• Parses tokens using a Context-Free Grammar defined in Python.
• Constructs a parse tree with full structure of the input program.
• Handles constructs like classes, methods, conditionals, loops, and expressions.

🔹 Phase 3: Semantic Analyzer (phase3_semantic_analysis.py)

• Walks the parse tree to validate type rules, function calls, variable scopes.
• Tracks declared identifiers and their scopes via symbol tables.
• Reports semantic errors with descriptive messages and location info.

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Technologies Used

Area	Tools
Programming Language	Python 3
Lexer	Regular Expressions, PLY (Python Lex-Yacc)
Parser	PLY Yacc
Semantic Analysis	Custom logic in Python
Input/Output	Text-based input files, console output

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Sample Decaf Input

Here's a sample Decaf program (from decaf.txt):

class program {
  int x, y;
  
  void foo(int a, int b) {
    if (x == 10) {
      return;
    }
    
    while (y >= 5) {
      // do something
    }

    return;
  }
}