Module maps 

Map

Maps are key-value pairs with two extra properties:

1. The None/null key is written outside the map in the "if_none" field. This is because JSON doesn’t let you use null as a key in a map.

2. Maps have a fallback value, called else, that is returned when using a key not otherwise in the map.

Components that use maps “flatten” them, so while Action::PartMap contains an Action::PartMap::map field of type Map, you would use it as follows:

{"PartMap": {
  "part": "NormalizedHost",
  "map": {
    "example.com": "The Action to take for example.com URLs",
    "example2.com": "The Action to take for example2.com URLs"
  },
  "if_none": "Optional Action to take if the NormalizedHost is somehow None. This is usually useful for parts like {\"QueryParam\": \"abc\"} that can be expected to be None",
  "else": "Optional Action to take if the Normalized Host is not in the map or if the NormalizedHost is None and the if_none field is absent"
}}

Maps excel at flattening certain long if-then-else chains. For example, take this Action:

{"If": {
  "if": {"PathIs": "/search"},
  "then": "Do a thing",
  "else": {"If": {
    "if": {"PathStartsWith": "/user/"},
    "then": "Do a different thing",
    "else": {"If": {
      "if": {"PathStartsWith": "/post/"},
      "then": "Do yet another thing"
    }}
  }}
}}

While not every situation allows for this, the above example can be simplified dramatically by using maps and, in this case specifically, Action::PartMap.

{"PartMap": {
  "part": {"PathSegment": 0},
  "map": {
    "search": "Do a thing",
    "user": "Do a different thing",
    "post": "Do yet another thing"
  }
}}

While this is simpler and usually faster (It’s a HashMap), the behavior has been changed slightly.

1. For the first branch, the scope of matching URLs was expanded from only URLs with a path of /search to also including URLs with paths like /search/, /search/abc, and so on.

2. For both the user and post branches, the change results in accepting not just paths of /user/ and /post/, but paths of /user and /post.

While most websites are unlikely to care about these changes (what website has both /user/user1234 and /user as valid paths?), care should be taken to ensure these sorts of changes are relegated only to “invalid” URLs.