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A comprehensive, modular ANTLR4-based implementation of FORTRAN/Fortran language standards from 1957 to LazyFortran2025.

This repository implements an ANTLR4 grammar hierarchy for many FORTRAN/Fortran standards, designed with:

• Modular inheritance - Each standard builds upon its predecessor
• Reuse - Later grammars import and extend earlier ones
• Historical orientation - Grammars are organized by language standard
• Clean architecture - Source/build separation, proper dependencies

FORTRAN (1957) → FORTRAN II (1958) → FORTRAN 66 (1966) → FORTRAN 77 (1977)                                                                      ↓                                                                 Fortran 90 (1990)                                                                      ↓                                          Fortran 95 → 2003 → 2008 → 2018 → 2023

This repository currently implements grammars for:

• FORTRAN (1957, IBM 704)
• FORTRAN II (1958)
• FORTRAN 66 (ANSI X3.9‑1966)
• FORTRAN 77 (1977)
• Fortran 90, 95, 2003, 2008, 2018 and 2023

FORTRAN IV (1962) is not implemented as a separate grammar; its functionality is represented in the FORTRAN 66 grammar, reflecting the fact that FORTRAN 66 largely standardized the then‑current FORTRAN IV practice.

standard/├── grammars/           # Grammar source files (.g4)├── tests/              # Test suites grouped by standard├── scripts/            # Helper / validation scripts├── tools/              # Extra utilities└── validation/         # External grammar validation tools

• Python 3.8+
• ANTLR4 (Java version)
• Git

# Clone the repositorygit clone https://github.com/lazy-fortran/standard.gitcd standard# Install ANTLR4 (Arch Linux)sudo pacman -S antlr4# Install Python dependenciespip install antlr4-python3-runtime pytest

The project includes a comprehensive Makefile for building all grammars:

# Build all grammars in dependency ordermake all# Build specific standardmake Fortran2003# Clean generated filesmake clean# Show available targets and helpmakehelp# Run tests after buildingmaketest

# Run all testspython -m pytest tests/ -v

This project provides grammars and tests for many Fortran standards, but it doesnot implement every feature from each ISO standard and doesnot contain a separate FORTRAN IV grammar. The table below describes the state of the implementation in this repository (not the full language specifications):

At the moment the test suite consists of roughly 270 tests across these standards, and all of them pass after generating the grammars with ANTLR.

Historically, the original IBM 704 FORTRAN provided fixed‑form source code, arithmetic expressions, DO loops, conditional branching with arithmetic IF, computed GOTOs, FORMAT‑driven I/O and related features. In this repository the FORTRAN grammar focuses on that core statement set and serves as the base of the inheritance chain.

FORTRAN II added user‑written subprograms and shared storage:

• SUBROUTINE /FUNCTION /CALL /RETURN statements for separately compiled procedures
• COMMON blocks for sharing storage between program units
• Hollerith constants (nHtext) for representing character data

All features implemented using TDD:

1. RED - Write failing test
2. GREEN - Implement to pass
3. REFACTOR - Clean up

SeeCONTRIBUTING.md for guidelines.

1. Each grammar only defines NEW features
2. Import from predecessor in chain
3. No duplication of rules or tokens
4. Historical accuracy maintained
5. Clean separation of concerns

The project includes a growing test suite:

• On the order of 270 unit tests across the implemented standards (all currently passing)
• Historical code examples from different eras
• Operator‑precedence validation in the older dialects
• Parse‑tree checks for selected modern constructs
• Cross‑standard compatibility testing where inheritance is involved

MIT License

• Original FORTRAN team at IBM (John Backus et al.)
• ANTLR4 project for grammar tooling
• Historical FORTRAN documentation from IBM archives

• LazyFortran Compiler
• LazyFortran Type Inference

• GitHub Issues:Report bugs or request features
• Project Lead: @krystophny

Building the future of Fortran through understanding its past.