explore options for value expansion

[mattsb42-aws]

There are many cases where what you want to define is "everything the same for each of these [X] actions, but change this one thing".

Ideally, this should not require someone to re-write the entire action definition, but instead simply provide some syntax to expand.

Some prior art for this type of behavior includes Tox's factor expansion[1] and Travis CI's build matrix[2].

There are a few different options that I am considering for how to support this.

We need to determine what route we want to take here and how to approach that.

List Expansion

List expansion works similarly to Tox's factor expansion.

If a string that is a member of a list contains values enclosed by braces, that string is removed from the list and replaced with a member for each possible combination of values defined in the original string.

# Source
value:
  - 3.5.{4,5,6}
  - 3.{6,7}.{0,1}
 
# Result
value:
  - 3.5.4
  - 3.5.5
  - 3.5.6
  - 3.6.0
  - 3.6.1
  - 3.7.0
  - 3.7.1

List Expansion

List expansion is more like an in-line version of Travis CI's build matrix.

Any values that expect strings and are provided lists of strings are used to all possible combinations.

# Source
example:
  name: MyName
  value:
    - value 1
    - value 2
  another:
    - value 3
    - value 4
 
# Result
example:
  -
    name: MyName
    value: value 1
    another: value 3
  -
    name: MyName
    value: value 1
    another: value 4
  -
    name: MyName
    value: value 2
    another: value 3
  -
    name: MyName
    value: value 2
    another: value 3

Template Definitions

Provide a way of defining an action template, then provide an action plugin (dependent on #27) that takes a template name an a list of values.

templates:
    run-python:
        parameters:
            - toxenv
            - image
        template:
            provider: CodeBuild
            image: {PARAMETER:image}
            buildspec: buildspec.yaml
            env:
                TOXENV: {PARAMETER:toxenv}
            inputs:
                - SourceOutput

pipeline:
    source:
        ...
    build:
        - provider: TemplateLoader
          template: run-python
          parameters:
              - toxenv: py36-local
                image: aws/codebuild/python:3.6.5
              - toxenv: py36-integ
                image: aws/codebuild/python:3.6.5

[1] https://tox.readthedocs.io/en/latest/config.html#complex-factor-conditions
[2] https://docs.travis-ci.com/user/build-matrix/

[mattsb42-aws]

Closing down project: #52