Fortran Programming Guidelines

Basic Principles

• Use modern Fortran standards such as Fortran 2003, 2008, or newer.
• Use implicit none in every program unit.
• Put procedures in modules to provide explicit interfaces.
• Keep modules focused and place each major module in its own file.
• Prefer clear, structured code over clever language tricks.
• Avoid obsolete features such as COMMON blocks, GOTO-heavy control flow, and numeric labels.

Kinds and Types

• Define numeric kind parameters in one shared module, such as kind_mod.
• Use real(kind=dp) or the project-approved real kind for floating point values.
• Use integer(kind=i4) or the project-approved integer kind for integer values.
• Define constants such as pi explicitly.
• Include units in comments for physical quantities.
• Use derived types to group related data instead of passing many primitive arguments.

Naming and Style

• Use lowercase for language keywords and most identifiers.
• Use underscores for multi-word names.
• Avoid names that differ only by case.
• Use descriptive names for procedures and state.
• Repeat the procedure or module name after end statements.
• Keep indentation consistent in do, if, select case, and module blocks.

Procedures

• Keep subroutines and functions short and single-purpose.
• Use intent(in), intent(out), or intent(inout) for every dummy argument.
• Keep functions free of side effects whenever possible.
• Prefer early validation and clear returns over deep nesting.
• Use use, only: when importing from modules.

Memory and Arrays

• Prefer allocatable arrays over pointers unless pointer semantics are required.
• Check allocation state and array sizes before use.
• Deallocate allocatable arrays when their lifetime is not naturally scoped.
• Specify array bounds clearly when they matter.
• Avoid unnecessary dynamic allocation in hot loops.

Testing and Build

• Use CMake, fpm, Make, or the project-standard build system consistently.
• Compile with warnings enabled and treat important warnings as failures in CI.
• Add unit tests for public procedures and integration tests for numerical workflows.
• Test boundary conditions, invalid inputs, and representative scientific cases.
• Verify numerical tolerances explicitly rather than relying on exact floating point equality.

Common Mistakes

• Do not declare variables after executable code unless using a block construct.
• Do not assume random_number is a function; it is a subroutine.
• Do not write to stdout from pure procedures.
• Do not declare the same variable twice in the same scope.
• Do not assume pi, dp, or project kinds already exist without importing or defining them.