Fortran Standard Specifications

Note: This project is experimental and subject to major changes.

This repository contains:

1. ANTLR4 grammars for all Fortran standards from 1957 to 2028 (working draft)
2. LFortran extensions: F2028 generics base, inline instantiation, and infer mode
3. Specifications for LFortran Standard and LFortran Infer mode

It is the Lazy Fortran ecosystem source of truth for intended language behavior. Implementation work in fortfront, ffc, fortrun, and fortnb should align with these specifications instead of inventing separate dialect rules.

Standards Hierarchy

ISO Fortran 2023 (ISO/IEC 1539-1:2023)
    |
    v
Fortran 2028 Working Draft (J3/26-007)
    |
    v
LFortran Standard (--std=lf)
    |   - Fortran 2028 base + LFortran extensions
    |   - Inline instantiation: `name{T}(...)` and `name^(T)(...)`
    |   - Stricter defaults (bounds checking ON, implicit none)
    |   - 8-byte reals, 4-byte integers
    |
    v
LFortran Infer (--infer)
    - Type inference (`:=` and first assignment with `--infer`)
    - Automatic array reallocation
    - Interactive REPL mode

Ecosystem Alignment

• fortfront should treat this repo as the target for Lazy Fortran/Inferred source semantics and document any temporary divergence.
• ffc should compile the subset defined here, starting with a small executable scalar subset.
• fortrun and fortnb should not define separate language behavior; they should delegate parsing/compilation to FortFront/ffc.

Implementation Status

The grammar documents are intended to be complete for the tracked standards and extensions. This table tracks repository parser/grammar coverage; semantic/runtime behavior is tracked separately and is out of scope for this parser repository.

Status source of truth:

• Implementation Notes: Grammar Status
• Implementation Notes: Fortran 2028 Delta Audit
• LFortran Design: Deferred Trait Items (Semantic Phase)

Standard	Grammar	Tests	Key Features
FORTRAN 1957	Complete	Yes	Arithmetic IF, DO loops, FORMAT I/O
FORTRAN II	Complete	Yes	SUBROUTINE, FUNCTION, COMMON
FORTRAN 66	Complete	Yes	First ANSI standard, LOGICAL/COMPLEX
FORTRAN 77	Complete	Yes	CHARACTER, IF-THEN-ELSE, PARAMETER
Fortran 90	Complete	Extensive	Free-form, modules, derived types, arrays
Fortran 95	Complete	Yes	FORALL, PURE/ELEMENTAL
Fortran 2003	Complete	Extensive	OOP, C interop, PDTs, IEEE
Fortran 2008	Complete	Yes	Coarrays, submodules, DO CONCURRENT
Fortran 2018	Complete	Yes	Teams, events, atomics
Fortran 2023	Complete	Yes	Conditional expressions, TYPEOF/CLASSOF
Fortran 2028 (WD)	Complete	Yes	TEMPLATE/REQUIREMENT/REQUIRE(S)/INSTANTIATE facility
LFortran	Complete	Yes	F2028 base + inline instantiation ({} and ^()); traits syntax (implements, sealed, initial)
LFortran Infer	Complete	Yes	Type inference (:=, --infer), global scope

Quick Start

# Clone
git clone git@github.com:lazy-fortran/standard.git
cd standard

# Build grammars
make all

# Run tests
make test

Prerequisites

• Python 3.8+
• ANTLR4
• Git

Documentation

Document	Description
LFortran Standard	Stricter Fortran dialect specification (F2028 base)
LFortran Infer	Type inference and infer mode specification
Traits Proposal	Traits, nominal conformance, and {T} procedure generics
External Sources	Sync policy for Traits repo + Fortran 2028 draft
Design Rationale	Explains key design decisions
Implementation Notes	Status and known limitations

Directory Structure

standard/
├── docs/               # Specifications and documentation
├── grammars/src/       # ANTLR4 grammar files (.g4)
├── tests/              # Test suites by standard
├── tools/              # Semantic validators
└── validation/         # Reference corpora, standards PDFs, and validation tooling

Grammar Inheritance

Each grammar extends its predecessor, defining only NEW features:

FORTRAN 1957 -> FORTRAN II -> FORTRAN 66 -> FORTRAN 77
                                                 |
                                            Fortran 90
                                                 |
                              Fortran 95 -> 2003 -> 2008 -> 2018 -> 2023 -> 2028
                                                                              |
                                                                      LFortran -> Infer

License

MIT License

Related Projects

• LFortran - Modern Fortran compiler
• J3 Generics - Fortran 202Y generics proposal
• Traits for Types (J3/20-109) - Trait proposal track reflected in the LFortran grammar
• Traits-for-Fortran - Community traits/generics design document