Implementation Notes

Status and known limitations for the Fortran grammar collection. This is the canonical status reference for parser/grammar coverage in this repository.

Grammar Status

All grammars are complete and tested. Each implements the full syntax of its standard.

Standard	Status	Tests	Notes
FORTRAN 1957	Complete	Yes	Strict fixed-form validator in tools/strict_fixed_form.py
FORTRAN II	Complete	Yes	SUBROUTINE, FUNCTION, COMMON, Hollerith
FORTRAN 66	Complete	Yes	First ANSI standard
FORTRAN 77	Complete	Yes	CHARACTER, IF-THEN-ELSE, last fixed-form-only
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, global scope, auto realloc

Status boundaries:

• "Complete" means grammar-level parsing support in this repository.
• Semantic checks (trait conformance, inheritance restrictions, signature matching) are intentionally deferred to semantic analysis.
• See LFortran Design: Deferred Trait Items (Semantic Phase) for the deferred trait list.

Fortran 2028 Delta Audit (J3/26-007)

In-scope syntax delta implemented on top of Fortran 2023:

• Tokens: TEMPLATE, REQUIREMENT, REQUIRE, REQUIRES, INSTANTIATE, DEFERRED, CONSTANT, {, }
• Constructs:
  • template / end template
  • requirement / end requirement
  • require and requires statements
  • instantiate statements (including ONLY rename and procedure alias form)
  • templated function and templated subroutine program units
  • deferred type/constant/procedure declarations

Missing productions/tokens in this in-scope F2028 template family: none.

Semantic Validation

Grammar-level parsing is complete. Semantic validators exist in tools/ for:

• Fixed-form column layout (strict_fixed_form.py) - FORTRAN 1957/II/90
• Hollerith length validation - 1957/II
• Numeric constant ranges - 1957
• FORMAT descriptor validation - 1957
• Statement ordering - FORTRAN 66/77, Fortran 90
• Expression precedence - Fortran 90+
• CYCLE/EXIT context - Fortran 90+
• ALLOCATE constraints - Fortran 2003/2008
• BLOCK construct semantics - Fortran 2008

Design Decisions

Fixed-Form Model

Grammars use layout-lenient parsing:

• Tokens parsed by order, not column position
• Strict validation available via tools/strict_fixed_form.py
• Supports both legacy .f and modern .f90 layouts

Expression Grammar

Operator precedence fully encoded for Fortran 90+ (issue #678 resolved).

Scope Boundary

Grammars are syntactic. Type checking and interface matching are out of scope.

References

Standard	Document
FORTRAN 1957	IBM C28-6003
FORTRAN II	IBM C28-6000-2
FORTRAN 66	ANSI X3.9-1966
FORTRAN 77	ANSI X3.9-1978
Fortran 90	ISO/IEC 1539:1991
Fortran 95	ISO/IEC 1539-1:1997
Fortran 2003	ISO/IEC 1539-1:2004
Fortran 2008	ISO/IEC 1539-1:2010
Fortran 2018	ISO/IEC 1539-1:2018
Fortran 2023	ISO/IEC 1539-1:2023
Fortran 2028 (WD)	J3/26-007
LFortran	J3/24-107r1 + traits proposal

PDFs in validation/pdfs/.