hx fmt

Format Haskell source code.

Synopsis

hx fmt [OPTIONS] [FILES]...

Description

The fmt command formats Haskell source files using a code formatter. By default, it uses Ormolu, but can be configured to use Fourmolu or other formatters.

Arguments

FILES...                Files to format (default: all .hs files)

Options

    --check             Check formatting without modifying files
    --diff              Show diff of changes
    --formatter <NAME>  Formatter to use [ormolu, fourmolu, stylish]
    --config <FILE>     Custom formatter config file
-v, --verbose           Show detailed output

Examples

Format All Files

hx fmt

Format Specific Files

hx fmt src/MyModule.hs app/Main.hs

Check Formatting (CI)

hx fmt --check

Exit code 0 if formatted, 1 if changes needed.

Show Diff

hx fmt --diff

Use Fourmolu

hx fmt --formatter fourmolu

Formatters

Ormolu (Default)

Opinionated, minimal configuration:

Consistent output
No configuration options
Widely adopted

Fourmolu

Ormolu fork with more options:

Configurable via fourmolu.yaml
More formatting options
Compatible with Ormolu style

Stylish Haskell

Traditional formatter:

Highly configurable
.stylish-haskell.yaml config
Modular formatting

Configuration

Configure in hx.toml:

[fmt]
# Formatter to use
formatter = "ormolu"

# Check only (don't modify)
check = false

# Exclude patterns
exclude = ["generated/**"]

Fourmolu Configuration

Create fourmolu.yaml:

indentation: 2
function-arrows: leading
comma-style: leading
import-export-style: leading
indent-wheres: true
record-brace-space: true
newlines-between-decls: 1

Stylish Haskell Configuration

Create .stylish-haskell.yaml:

steps:
  - imports:
      align: global
  - language_pragmas:
      style: vertical
  - trailing_whitespace: {}

Editor Integration

VS Code

Install “Haskell” extension, configure:

{
  "haskell.formattingProvider": "ormolu"
}

Vim/Neovim

With ALE:

let g:ale_fixers = {'haskell': ['ormolu']}
let g:ale_fix_on_save = 1

Emacs

With haskell-mode:

(setq haskell-mode-stylish-haskell-path "ormolu")

Pre-commit Hook

Format on commit:

# .git/hooks/pre-commit
#!/bin/sh
hx fmt --check || {
    echo "Run 'hx fmt' to format code"
    exit 1
}

Or use hx with git hooks:

# Create hook
echo '#!/bin/sh\nhx fmt --check' > .git/hooks/pre-commit
chmod +x .git/hooks/pre-commit

CI Integration

- name: Check formatting
  run: hx fmt --check