ffmt 0.1.0

An opinionated Fortran formatter with Fypp and OpenACC support

ffmt

An opinionated, idempotent Fortran formatter with first-class Fypp preprocessor and OpenACC/OpenMP directive support.

ffmt is a single-pass formatter written in Rust. It replaces fprettify and produces consistent output on the first run -- no multi-pass convergence needed.

Installation

From PyPI

pip install ffmt

From crates.io

cargo install ffmt

From source

git clone https://github.com/sbryngelson/ffmt
cd ffmt
cargo install --path .

Usage

# Format files in-place
ffmt file.fpp
ffmt src/

# Check mode (CI) -- exit 1 if any file would change
ffmt --check src/

# Show what would change (with color)
ffmt --diff src/

# Both
ffmt --check --diff src/

# Parallel formatting
ffmt -j 8 src/

# Read from stdin, write to stdout (for editor integration)
cat file.fpp | ffmt -
ffmt --stdin-filepath file.fpp - < file.fpp

# Exclude patterns
ffmt --exclude "autogen/**" src/

Stdin/stdout mode

Use - as the path to read from stdin and write formatted output to stdout. This enables editor integration (format-on-save) and piping:

# Format stdin to stdout
echo "INTEGER::x" | ffmt -
# Output: integer :: x

# Check stdin (exit 1 if it would change)
cat file.fpp | ffmt --check --stdin-filepath file.fpp -

# Diff stdin
cat file.fpp | ffmt --diff --stdin-filepath file.fpp -

The --stdin-filepath flag provides a filename for diagnostics.

.gitignore support

ffmt automatically respects .gitignore, .ignore, .fdignore, and .git/info/exclude when walking directories. Files ignored by git are skipped.

Editor integration

Vim/Neovim

" Format on save
autocmd BufWritePost *.fpp,*.f90 silent !ffmt %

" Or use as formatprg
set formatprg=ffmt\ -

VS Code

Add to settings.json:

{
    "[fortran]": {
        "editor.formatOnSave": true
    },
    "fortran.formatting.formatter": "ffmt",
    "fortran.formatting.path": "ffmt",
    "fortran.formatting.args": ["--stdin-filepath", "${file}", "-"]
}

CI integration

GitHub Actions

- uses: sbryngelson/ffmt@v1
  with:
    args: "--check src/"

Or install directly:

- run: pip install ffmt
- run: ffmt --check src/

pre-commit

Add to .pre-commit-config.yaml:

repos:
  - repo: https://github.com/sbryngelson/ffmt
    rev: v0.1.0
    hooks:
      - id: ffmt

Use ffmt-check instead of ffmt for check-only mode (no modifications).

What it does

ffmt formats Fortran free-form source files (.fpp, .f90, .f95, .f03) through a single-pass pipeline:

1. Indentation -- 4-space indentation based on scope tracking (if/do/subroutine/module/type/...)
2. Fypp block indentation -- #:if, #:for, #:call, #:def, etc. are indented as nested blocks
3. Directive indentation -- !$acc, !$omp, !DIR$ lines indented to match surrounding Fortran scope
4. Whitespace normalization -- consistent spacing around operators, commas, colons, parentheses
5. Case normalization -- Fortran keywords lowercased (IF -> if, END DO -> end do)

Formatting rules

Indentation:

• 4 spaces per scope level
• Fypp blocks (#:if/#:for/#:call/#:def/#:block/#:mute) create scope
• #ifdef/#endif preprocessor blocks do not change indentation
• Continuation lines (&) preserve their original spacing
• !! Doxygen continuation comments preserve their original alignment

Operators -- space each side: ==, /=, <=, >=, <, >, .and., .or., .not., = (assignment), =>, +, - (binary), // (concatenation), ::

Operators -- no spaces: *, /, **

Other:

• Commas: one space after, none before
• Array slices: no spaces around : -- a(1:n)
• Keyword arguments: no spaces around = -- call foo(bar=1)
• Parentheses: no internal padding -- f(x) not f( x )
• Trailing whitespace: always stripped
• Blank lines: 3+ consecutive collapsed to 2
• Blank lines after !$acc loop directives: removed

Preserved as-is:

• String literal contents ('...', "...")
• Inline Fypp expressions (${...}$, @{...}@)
• Comment contents
• Doxygen comment alignment (!<, !>, !!)
• Continuation line structure and alignment

Design

ffmt is opinionated like gofmt -- one style, no configuration file, minimal CLI flags.

Why not fprettify?

• Not idempotent -- fprettify requires 2-4 passes to converge
• No directive awareness -- requires a separate indenter script for !$acc/!$omp lines
• Fragile Fypp support -- treats Fypp lines as "not Fortran," causing inconsistent indentation
• Unmaintained -- last release circa 2020

Architecture

Source lines -> Join continuations -> Classify (LineKind enum) ->
Track scope depth -> Apply indentation -> Normalize whitespace ->
Normalize case -> Emit formatted lines

Module	Responsibility
reader.rs	Join & continuations, mark opaque regions (strings, ${...}$, comments)
classifier.rs	Classify lines into LineKind enum with disambiguation rules
scope.rs	Scope tracking state machine (push/pop on block open/close)
indent.rs	Apply 4 * depth leading spaces
whitespace.rs	Operator/comma/colon/paren spacing normalization
case_norm.rs	Lowercase Fortran keywords outside strings/Fypp/comments
formatter.rs	Pipeline orchestrator
cli.rs	CLI with stdin/stdout, .gitignore, color, exclude patterns

Exit codes

Code	Meaning
0	Success (all files formatted, or no changes needed in --check mode)
1	Files would change (--check mode only)
2	Usage error or I/O failure

License

MIT