elmfmt

A code formatter for the Elm programming language, built on Topiary.

Example

Following Elm code was formatted with elmfmt's default settings:

module Main exposing (main)

import Html exposing (Html, text, div, button)
import Html.Events exposing (onClick)


type Msg = Increment | Decrement


update msg model =
  case msg of
    Increment -> model + 1
    Decrement -> model - 1


view model =
  div
    [ style "font-size" "2rem" ]
    [ button [ onClick Decrement ] [ text "-" ]
    , div [] [ text (String.fromInt model) ]
    , button [ onClick Increment ] [ text "+" ]
    ]

Installation

From crates.io

cargo install elmfmt

From source

Requires Rust toolchain.

# Clone the repository
git clone https://github.com/your-username/elmfmt.git
cd elmfmt

# Install
cargo install --path .

Usage

# Format a file and print to stdout
elmfmt src/Main.elm

# Format a file in place
elmfmt -i src/Main.elm

# Write to a different file
elmfmt src/Main.elm -o formatted.elm

# Check if a file is formatted (exits with code 1 if not)
elmfmt -c src/Main.elm

# Format from stdin
cat src/Main.elm | elmfmt

Options

Option	Description
[FILE]	Input file (reads from stdin if not provided)
-o, --output <FILE>	Write output to a file
-i, --in-place	Modify the file in place
-c, --check	Check if file is formatted without modifying
--skip-idempotence	Skip idempotence check
-h, --help	Show help
-V, --version	Show version

Configuration

Create an elmfmt.yaml file in your project directory. The formatter searches for this file starting from the input file's directory and moving upward.

# Number of spaces for indentation (default: 2)
indentation: 2

# Style for if-then-else expressions: 'indented' or 'hanging' (default: indented)
if-style: indented

If-Style Options

Indented (default):

if condition then
    expr1

else
    expr2

Hanging:

if condition
    then expr1
    else expr2

Editor Integration

VSCode

To use elmfmt with the Elm Language Server extension, add the following to your VSCode settings:

{
  "elmLS.elmFormatPath": "/Users/your-username/.cargo/bin/elmfmt"
}

Replace /Users/your-username/.cargo/bin/elmfmt with the actual path to your elmfmt binary. You can find it by running which elmfmt after installation.