Merge pull request #1056 from pre-commit/document_new_language

Document adding a supported language

CONTRIBUTING.md

@@ -2,8 +2,8 @@
 
 ## Local development
 
-- The complete test suite depends on having at least the following installed (possibly not
-  a complete list)
+- The complete test suite depends on having at least the following installed
+  (possibly not a complete list)
   - git (A sufficiently newer version is required to run pre-push tests)
   - python2 (Required by a test which checks different python versions)
   - python3 (Required by a test which checks different python versions)
@@ -30,7 +30,7 @@ Running a specific test with the environment activated is as easy as:
 
 ### Running all the tests
 
-Running all the tests can be done by running `tox -e py27` (or your
+Running all the tests can be done by running `tox -e py37` (or your
 interpreter version of choice).  These often take a long time and consume
 significant cpu while running the slower node / ruby integration tests.
 
@@ -49,5 +49,98 @@ Documentation is hosted at https://pre-commit.com
 This website is controlled through
 https://github.com/pre-commit/pre-commit.github.io
 
-When adding a feature, please make a pull request to add yourself to the
-contributors list and add documentation to the website if applicable.
+## Adding support for a new hook language
+
+pre-commit already supports many [programming languages](https://pre-commit.com/#supported-languages)
+to write hook executables with.
+
+When adding support for a language, you must first decide what level of support
+to implement.  The current implemented languages are at varying levels:
+
+- 0th class - pre-commit does not require any dependencies for these languages
+  as they're not actually languages (current examples: fail, pygrep)
+- 1st class - pre-commit will bootstrap a full interpreter requiring nothing to
+  be installed globally (current examples: node, ruby)
+- 2nd class - pre-commit requires the user to install the language globally but
+  will install tools in an isolated fashion (current examples: python, go, rust,
+  swift, docker).
+- 3rd class - pre-commit requires the user to install both the tool and the
+  language globally (current examples: script, system)
+
+"third class" is usually the easiest to implement first and is perfectly
+acceptable.
+
+Ideally the language works on the supported platforms for pre-commit (linux,
+windows, macos) but it's ok to skip one or more platforms (for example, swift
+doesn't run on windows).
+
+When writing your new language, it's often useful to look at other examples in
+the `pre_commit/languages` directory.
+
+It might also be useful to look at a recent pull request which added a
+language, for example:
+
+- [rust](https://github.com/pre-commit/pre-commit/pull/751)
+- [fail](https://github.com/pre-commit/pre-commit/pull/812)
+- [swift](https://github.com/pre-commit/pre-commit/pull/467)
+
+### `language` api
+
+here are the apis that should be implemented for a language
+
+Note that these are also documented in [`pre_commit/languages/all.py`](https://github.com/pre-commit/pre-commit/blob/master/pre_commit/languages/all.py)
+
+#### `ENVIRONMENT_DIR`
+
+a short string which will be used for the prefix of where packages will be
+installed.  For example, python uses `py_env` and installs a `virtualenv` at
+that location.
+
+this will be `None` for 0th / 3rd class languages as they don't have an install
+step.
+
+#### `get_default_version`
+
+This is used to retrieve the default `language_version` for a language.  If
+one cannot be determined, return `'default'`.
+
+You generally don't need to implement this on a first pass and can just use:
+
+```python
+get_default_version = helpers.basic_default_version
+```
+
+`python` is currently the only language which implements this api
+
+#### `healthy`
+
+This is used to check whether the installed environment is considered healthy.
+This function should return `True` or `False`.
+
+You generally don't need to implement this on a first pass and can just use:
+
+```python
+healthy = helpers.basic_healthy
+```
+
+`python` is currently the only language which implements this api, for python
+it is checking whether some common dlls are still available.
+
+#### `install_environment`
+
+this is the trickiest one to implement and where all the smart parts happen.
+
+this api should do the following things
+
+- (0th / 3rd class): `install_environment = helpers.no_install`
+- (1st class): install a language runtime into the hook's directory
+- (2nd class): install the package at `.` into the `ENVIRONMENT_DIR`
+- (2nd class, optional): install packages listed in `additional_dependencies`
+  into `ENVIRONMENT_DIR` (not a required feature for a first pass)
+
+#### `run_hook`
+
+This is usually the easiest to implement, most of them look the same as the
+`node` hook implementation:
+
+https://github.com/pre-commit/pre-commit/blob/160238220f022035c8ef869c9a8642f622c02118/pre_commit/languages/node.py#L72-L74