Update README with improved API and action constraints.

README.md

@@ -64,30 +64,44 @@ should not require general ternary matches on the destination address; the
 constraint documents this intend and let's us catch accidental ternary matches
 installed by the control plane at runtime.
 
+## Example - Action Restrictions
+
+An *action restriction* is similar to an entry restriction, but is placed on a
+P4 action:
+```p4
+// Disallow mulitcast group ID 0, since it indicates "no mulitcast" in 
+// `v1model.p4`.
+@action_restriction("multicast_group_id != 0")
+action multicast(bit<16> multicast_group_id) {
+  standard_metadata.mcast_grp = multicast_group_id;
+}
+```
+
 ## API
 
 At a high level, p4-constraint's API consists of only two functions:
 one function for parsing constraints and one function for checking them.
 ```C++
 /* p4_constraints/backend/constraint_info.h */
 
-// Translates P4Info to ConstraintInfo.
+// Translates `P4Info` to `ConstraintInfo`.
 //
-// Parses all tables and their constraint annotations into an in-memory
-// representation suitable for constraint checking. Returns parsed
-// representation, or a nonempty list of error statuses if parsing fails.
-absl::variant<ConstraintInfo, std::vector<absl::Status>> P4ToConstraintInfo(
-    const p4::config::v1::P4Info& p4info);
+// Parses all tables and actions and their p4-constraints annotations into an
+// in-memory representation suitable for constraint checking. Returns parsed
+// representation, or an error status if parsing fails.
+absl::StatusOr<ConstraintInfo> P4ToConstraintInfo(
+    const p4::config::v1::P4Info &p4info);
 ```
 ```C++
 /* p4_constraints/backend/interpreter.h */
 
-// Checks if a given table entry satisfies the entry/action(s) constraint(s)
-// attached to its associated table/action(s). Returns the empty string if this
-// is the case or a human-readable nonempty string explaining why it is not the
-// case otherwise. Returns an InvalidArgument Status if the entry/action(s)
-// don't belong in ConstraintInfo or is inconsistent with the table/action(s)
-// definition in ConstraintInfo.
+// Checks if a given table entry satisfies the constraints attached to its
+// associated table/action.
+//
+// Returns the empty string if this is the case, or a human-readable nonempty
+// string explaining why it is not the case otherwise. Returns an
+// `InvalidArgument` if the entry's table or action is not defined in
+// `ConstraintInfo`, or if `entry` is inconsistent with these definitions.
 absl::StatusOr<std::string> ReasonEntryViolatesConstraint(
     const p4::v1::TableEntry& entry, const ConstraintInfo& constraint_info);
 ```

p4_constraints/backend/constraint_info.h

@@ -104,11 +104,11 @@ struct ConstraintInfo {
   absl::flat_hash_map<uint32_t, TableInfo> table_info_by_id;
 };
 
-// Translates P4Info to ConstraintInfo.
+// Translates `P4Info` to `ConstraintInfo`.
 //
-// Parses all tables and their constraint annotations into an in-memory
-// representation suitable for constraint checking. Returns parsed
-// representation, or an error statuses if parsing fails.
+// Parses all tables and actions and their p4-constraints annotations into an
+// in-memory representation suitable for constraint checking. Returns parsed
+// representation, or an error status if parsing fails.
 absl::StatusOr<ConstraintInfo> P4ToConstraintInfo(
     const p4::config::v1::P4Info& p4info);
 

p4_constraints/backend/interpreter.h

@@ -36,12 +36,13 @@
 
 namespace p4_constraints {
 
-// Checks if a given table entry satisfies the entry/action(s) constraint(s)
-// attached to its associated table/action(s). Returns the empty string if this
-// is the case or a human-readable nonempty string explaining why it is not the
-// case otherwise. Returns an InvalidArgument Status if the entry/action(s)
-// don't belong in ConstraintInfo or is inconsistent with the table/action(s)
-// definition in ConstraintInfo.
+// Checks if a given table entry satisfies the constraints attached to its
+// associated table/action.
+//
+// Returns the empty string if this is the case, or a human-readable nonempty
+// string explaining why it is not the case otherwise. Returns an
+// `InvalidArgument` if the entry's table or action is not defined in
+// `ConstraintInfo`, or if `entry` is inconsistent with these definitions.
 absl::StatusOr<std::string> ReasonEntryViolatesConstraint(
     const p4::v1::TableEntry& entry, const ConstraintInfo& constraint_info);