elmjson

Elm json, commonly written as elm.json, is the project manifest used by the Elm tooling to describe an Elm project’s metadata, dependencies, and build settings. Introduced with Elm 0.19, it replaced the older elm-package.json approach and is typically located at the root of the project. The file can be created with the elm init command and later edited or updated as needed.

The elm.json file is a JSON object that can contain several fields. Core fields indicate the project type (such as application or package), the Elm version to target (elm-version), and source directories (source-directories). In addition, it may include metadata fields like name, version, summary, license, and repository. These metadata fields help identify the project when published as a package or shared with others.

Dependencies are a central part of elm.json. They are organized under dependencies, which itself has direct and indirect sections. Each dependency is paired with a version constraint, using syntax such as "1.0.0 <= v < 2.0.0" to express compatible ranges. Direct dependencies are the packages that the project directly uses, while indirect dependencies are those brought in by the direct ones. There is also a test-dependencies section for dependencies needed to run tests.

Elm tooling uses elm.json to resolve and install dependencies, to build code with elm make, and to ensure reproducible builds. While it can be edited manually, changes are typically made via elm init and elm install to maintain valid constraints and structure.