Cedar Schema Annotations

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+# Annotations for Cedar Schemas
+
+## Related issues and PRs
+
+- Reference Issues:
+- Implementation PR(s): https://github.com/cedar-policy/cedar/tree/feature/shaobo/rfc48
+
+## Timeline
+
+- Started: 2024-02-05
+
+## Summary
+
+Like Cedar policies, users may want to associate arbitrary, machine readable metadata with Schema objects.
+We solved this problem in Cedar policies by allowing for *annotations*: arbitrary key/value pairs that are attachable to policies.
+This could be extended to Cedar schemas, allowing users to attach attributes an entity type/common type/action declaration and attribute declaration.
+
+
+## Basic example
+
+Here is a basic example for doc comments.
+```
+@doc("this is the namespace")
+namespace TinyTodo {
+    @doc("a common type representing a task")
+    type Task = {
+        @doc("task id")
+        "id": Long,
+        "name": String,
+        "state": String,
+    };
+    @doc("a common type representing a set of tasks")
+    type Tasks = Set<Task>;
+
+    @doc("an entity type representing a list")
+    @docComment("any entity type is a child of type `Application`")
+    entity List in [Application] = {
+        @doc("editors of a list")
+        "editors": Team,
+        "name": String,
+        "owner": User,
+        @doc("readers of a list")
+        "readers": Team,
+        "tasks": Tasks,
+    };
+
+    @doc("actions that a user can operate on a list")
+    action DeleteList, GetList, UpdateList appliesTo {
+        principal: [User],
+        resource: [List]
+    };
+}
+```
+The `@id("...")` notation is similar to the notation used for policy annotations.
+
+## Motivation
+
+Users should be allowed to associate machine readable metadata with objects in a Schema.
+While we could create special syntax for associating particular kinds of metadata, we cannot
+predict all of the metadata uses that users will have.
+Thus providing a flexible system that users can adapt to their needs is preferable.
+This proposal re-uses the same syntax from Cedar Policies, creating a unified syntax.
+
+
+## Detailed design
+
+### Semantics
+Attributes have **no** impact on validation decisions.
+Attributes are arbitrary key/value pairs where:
+* 'key' is a valid Cedar identifier
+* 'value' is a Cedar string
+
+
+The Cedar spec takes no opinion or stance on the interpretation of annotations.
+The interpretation is entirely up to users of Cedar.
+
+### Cedar Schema Syntax
+Attributes in Cedar Schemas will mirror the syntax used for attributes in a policy: informally that's `@<key>("value")`.
+Formally the following rule is added to the Cedar grammar:
+```
+Annotation := '@' IDENT '(' STR ')'
+Annotations := {Annotations}
+```
+With an arbitrary number of them being able to prepend to a namespace declaration, entity type declaration, common type declaration, action declaration, and an attribute declaration.
+
+Thus the full schema syntax becomes:
+```
+Schema      := {Namespace}
+Namespace   := (Annotations 'namespace' Path '{' {Decl} '}') | {Decl}
+Decl        := Entity | Action | TypeDecl
+Entity      := Annotations 'entity' Idents ['in' EntOrTyps] [['='] RecType] ';'
+Action      := Annotations 'action' Names ['in' (Name | '[' [Names] ']')] [AppliesTo] [ActAttrs]';'
+TypeDecl    := Annotations 'type' IDENT '=' Type ';'
+Type        := PRIMTYPE | IDENT | SetType | RecType
+EntType     := Path
+SetType     := 'Set' '<' Type '>'
+RecType     := '{' [AttrDecls] '}'
+AttrDecls   := Annotations Name ['?'] ':' Type [',' | ',' AttrDecls]
+AppliesTo   := 'appliesTo' '{' AppDecls '}'
+ActAttrs    := 'attributes' '{' AttrDecls '}'
+AppDecls    := ('principal' | 'resource') ':' EntOrTyps [',' | ',' AppDecls]
+             | 'context' ':' RecType [',' | ',' AppDecls]
+Path        := IDENT {'::' IDENT}
+EntTypes    := Path {',' Path}
+EntOrTyps   := EntType | '[' [EntTypes] ']'
+Name        := IDENT | STR
+Names       := Name {',' Name}
+Idents      := IDENT {',' IDENT}
+Annotation  := '@' IDENT '(' STR ')
+Annotations := Annotation {Annotations}
+
+IDENT       := ['_''a'-'z''A'-'Z']['_''a'-'z''A'-'Z''0'-'9']* - PRIMTYPE
+STR         := Fully-escaped Unicode surrounded by '"'s
+PRIMTYPE    := 'Long' | 'String' | 'Bool'
+WHITESPC    := Unicode whitespace
+COMMENT     := '//' ~NEWLINE* NEWLINE
+```
+
+### JSON Syntax
+None of the three top-level constructs (EntityTypes, Actions, CommonTypes) in JSON schemas allow for arbitrary key/value pairs.
+This means a new key can be safely added while preserving backwards compatibility.
+The same fact also applies to entity attribute declarations.
+This proposal reserves the `annotations` key at the top level of each of those constructs, which contains an Object, containing each annotation key as an Object key, associated with the annotation string.
+The only oddness here is Common Types, whose toplevel is a regular type. While this should still be backwards compatible, it will look a little odd to have annotations in some types and not in others.
+
+A corresponding JSON schema for the above example is as follows.
+```JSON
+{
+    "": {
+    "annotations": {
+        "doc": "this is the namespace"
+    },
+    "commonTypes": {
+        "Task": {
+        "annotations": {
+            "doc": "a common type representing a task"
+        },
+        "type": "Record",
+        "attributes": {
+            "id": {
+            "type": "Long",
+            "annotations": {
+                "doc": "task id"
+            },
+            },
+            "name": {
+            "type": "String"
+            },
+            "state": {
+            "type": "String"
+            }
+        }
+        },
+        "Tasks": {
+        "type": "Set",
+        "element": {
+            "type": "Task"
+        }
+        }
+    },
+    "entityTypes": {
+        "Application": {},
+        "List": {
+        "annotations": {
+            "doc": "an entity type representing a list",
+            "docComment": "any entity type is a child of type `Application`"
+        },
+        "memberOfTypes": [
+            "Application"
+        ],
+        "shape": {
+            "type": "Record",
+            "attributes": {
+            "editors": {
+                "type": "Team"
+            },
+            "name": {
+                "type": "String"
+            },
+            "owner": {
+                "type": "User"
+            },
+            "readers": {
+                "type": "Team"
+            },
+            "tasks": {
+                "type": "Tasks"
+            }
+            }
+        }
+        }
+    },
+    "actions": {
+        "CreateList": {
+         "annotations": {
+            "doc": "actions that a user can operate on a list"
+        },
+        "appliesTo": {
+            "resourceTypes": [
+            "Application"
+            ],
+            "principalTypes": [
+            "User"
+            ]
+        }
+       }
+    }
+  }
+}
+```
+
+## Drawbacks
+
+1. Complexity: adds more complexity to schema
+2. Oddness around syntax for Common Types in JSON form
+3. By not taking a stance on annotation meanings, it makes it harder for a standard to form around them (ex: for doc strings)
+4. Multi-line docstrings are technically valid but awkward.
+
+## Alternatives
+
+### Take a stance
+Reverse our decision around annotations and start taking stances on what annotations mean.
+This lets us standardize certain annotations, like `doc`.
+This probably can't happen unless we also do this for policies, which we've said we don't want to do.
+### Doc Strings as comments
+Instead of annotations, we could add "doc-strings" as a first class feature.
+Could look like this:
+```
+/# Stop users from accessing a high security document unless:
+/#  A) The principal and user are at the same location
+/#  B) The principal has a job level greater than  4
+forbid(principal, action, resource) when {
+    resource.security_level == "HIGH"
+unless {
+    (resource.location == principal.location) || (principal.job_level > 4 )
+};
+```
+This has nice and easy multi-line syntax, but is special cased and not as general.
+
+## Unresolved questions
+
+* If this RFC is accepted, do we want to patch [RFC24 (Schema Syntax](https://github.com/cedar-policy/rfcs/issues/24") to contain the full new grammar so it's in one place?