Add Elm documentation

_data/languages.yml

@@ -6,8 +6,9 @@ Clojure: "/user/languages/clojure/"
 Crystal: "/user/languages/crystal/"
 D: "/user/languages/d/"
 Dart: "/user/languages/dart/"
-Erlang: "/user/languages/erlang/"
 Elixir: "/user/languages/elixir/"
+Elm: "/user/languages/elm/"
+Erlang: "/user/languages/erlang/"
 F#: "/user/languages/csharp/"
 Go: "/user/languages/go/"
 Groovy: "/user/languages/groovy/"

user/languages/elm.md

@@ -0,0 +1,134 @@
+---
+title: Building an Elm Project
+layout: en
+
+---
+
+### What This Guide Covers
+
+This guide covers build environment and configuration topics specific to
+[Elm](https://elm-lang.org/) projects. Please make sure to read our
+[Tutorial](/user/tutorial/) and
+[general build configuration](/user/customizing-the-build/) guides first.
+
+<aside markdown="block" class="ataglance">
+
+| Elm                                         | Default                                   |
+|:--------------------------------------------|:------------------------------------------|
+| [Default `install`](#Dependency-Management) | `npm install`                             |
+| [Default `script`](#Default-Build-Script)   | `elm-format --validate . && elm-test`     |
+| [Matrix keys](#Build-Matrix)                | `elm`, `node_js`                          |
+| Support                                     | [Community Support](https://travis-ci.community/c/languages/elm) |
+
+Minimal example:
+
+```yaml
+  language: elm
+  elm:
+    - "0.19.0"
+```
+{: data-file=".travis.yml"}
+
+</aside>
+
+### Community-Supported Warning
+
+Travis CI support for Elm is contributed by the community and may be removed
+or altered at any time. If you run into any problems, please report them in the
+[Travis CI Community Forum](https://travis-ci.community/c/languages/elm).
+
+## Choosing Elm versions to test against
+
+You can specify the Elm language versions to test your project against with the
+`elm` key.
+This may be a single string value (e.g., `0.19.0`), or an array of string, in
+which case a build matrix consisting of jobs running each of the versions
+specified in the array.
+
+```yaml
+language: elm
+elm:
+  - elm0.19.0 # default, or equivalently, '0.19.0'
+  - elm0.18.0
+```
+{: data-file=".travis.yml"}
+
+### Support tools `elm-test` and `elm-format`
+
+Elm jobs will also install `elm-test` and `elm-format`.
+By default, these tools correspond to the `elm` version specified, with the
+prefix `elm`.
+For example, with `elm: 0.19.0`, the version of `elm-test` and `elm-format`
+with the tag `elm0.19.0` will be installed.
+You may also override them with `elm-test` and `elm-format` keys independently:
+
+```yaml
+language: elm
+elm-test: 0.19.0-rev3
+elm-format: 0.8.0
+```
+{: data-file=".travis.yml"}
+
+Notice that these values are passed to `npm`.
+This means that they can be specified as a version string (e.g., `0.19.0`)
+or a tag (e.g., `elm0.19.0`).
+`elm-test` and `elm-format` default to tags with `elm` prefix, derived from the
+`elm` value.
+
+### Choosing Node.js version to test against
+
+Elm projects may also specify Node.js version, as done for
+[Node.js](/user/languages/javascript-with-nodejs/) projects.
+
+```yaml
+language: elm
+node_js: '10' # latest 10.x
+```
+{: data-file=".travis.yml"}
+
+The default Node.js version is `10.13.0`.
+
+## Dependency Management
+
+For dependency management, Elm projects use the same logic as Node.js projects.
+See [Node.js documentation](/user/languages/javascript-with-nodejs/#dependency-management)
+for details.
+
+## Default Build Script
+
+The default command for testing Elm projects is:
+
+```bash
+elm-format --validate . && elm-test
+```
+
+You may override this with `script`:
+
+```yaml
+language: elm
+script: elm-test
+```
+{: data-file=".travis.yml"}
+
+## Build Matrix
+
+For Elm projects, `env`, `elm`, and `node_js` can be used as arrays
+to construct a build matrix.
+
+## Environment Variables
+
+* The version of Elm a job is using is available as `TRAVIS_ELM_VERSION`.
+  This defualts to `elm0.19.0`.
+* `TRAVIS_ELM_TEST_VERSION` points to the `elm-test` version in use.
+  If you override `elm-test`, that value is used.
+  If `elm-test` is not overridden but `elm` is, that value with the `elm`
+  prefix is used. (For example, `elm: 0.18.0` chooses `[email protected]`.)
+  If neither `elm-test` nor `elm` is overridden, the default value, `elm0.19.0`
+  is used.
+* `TRAVIS_ELM_FORMAT_VERSION` points to the `elm-format` version in use.
+  If you override `elm-format`, that value is used.
+  If `elm-format` is not overridden but `elm` is, that value with the `elm`
+  prefix is used. (For example, `elm: 0.18.0` chooses `[email protected]`.)
+  If neither `elm-format` nor `elm` is overridden, the default value, `elm0.19.0`
+  is used.
+* `TRAVIS_NODE_VERSION` points to the `node_js` version in use.