MSC3887: List matching push rules

proposals/3887-push-rule-list-matching.md

@@ -0,0 +1,50 @@
+# MSC3887: List matching push rules
+
+This MSC proposes a way to match against a list within event content.
+[MSC1796](https://github.com/matrix-org/matrix-spec-proposals/pull/1796) is an example change that
+would require this list matching.
+
+
+## Proposals
+
+### Handling of lists within event data
+
+We propose that conditons simply match any value within a list. Conditions should be evaluated against
+every item in the list and return a match if any of the items meet the condition. This is compatible
+with [all condition types](https://spec.matrix.org/v1.3/client-server-api/#conditions-1). For example an event:
+
+```json
+{
+  "content": {
+    "mentions": ["@user-a:beeper.com", "@user-b:beeper.com"]
+  }
+}
+```
+
+The following condition would match:
+
+```json
+{
+  "kind": "event_match",
+  "key": "event.content.mentions",
+  "value": "@user-a:beeper.com"
+}
+```
+
+
+## Alternatives
+
+A new condition kind designed specifically for matching against lists could provide more options, such
+as specifying if one, all or some items must match.
+
+TODO: consider use-cases different to the example that would require more in-depth forms of matching.
+
+
+## Security considerations
+
+None.
+
+
+## Unstable prefix
+
+Not required.