RFC: Allow interfaces to implement other interfaces

Author: mike-marcacci

spec/Section 3 -- Type System.md

@@ -43,9 +43,10 @@ type in the system, allowing the definition of arbitrary type hierarchies.
 
 GraphQL supports two abstract types: interfaces and unions.
 
-An `Interface` defines a list of fields; `Object` types that implement that
-interface are guaranteed to implement those fields. Whenever the type system
-claims it will return an interface, it will return a valid implementing type.
+An `Interface` defines a list of fields; `Object` types and other `Interface`
+types that implement the interface are guaranteed to implement those fields.
+Whenever the type system claims it will return an interface, it will return a valid
+implementing `Object` type.
 
 A `Union` defines a list of possible types; similar to interfaces, whenever the
 type system claims a union will be returned, one of the possible types will be
@@ -553,11 +554,15 @@ of rules must be adhered to by every Object type in a GraphQL schema.
             the interface field type is either an Interface type or a Union type
             and the object field type is a possible type of the interface field
             type.
-         3. An object field type is a valid sub-type if it is a List type and
+         3. An object field type is a valid sub-type if it is an Interface type
+            and the interface field type is either an Interface type or a Union
+            type and the object field type is a possible type of the interface
+            field type.
+         4. An object field type is a valid sub-type if it is a List type and
             the interface field type is also a List type and the list-item type
             of the object field type is a valid sub-type of the list-item type
             of the interface field type.
-         4. An object field type is a valid sub-type if it is a Non-Null variant
+         5. An object field type is a valid sub-type if it is a Non-Null variant
             of a valid sub-type of the interface field type.
       2. The object field must include an argument of the same name for every
          argument defined in the interface field.
@@ -569,9 +574,9 @@ of rules must be adhered to by every Object type in a GraphQL schema.
 
 ### Interfaces
 
-GraphQL Interfaces represent a list of named fields and their arguments. GraphQL
-objects can then implement an interface, which guarantees that they will
-contain the specified fields.
+GraphQL interfaces represent a list of named fields and their arguments. GraphQL
+objects and interfaces can then implement an interface, which guarantees that
+they will contain the specified fields.
 
 Fields on a GraphQL interface have the same rules as fields on a GraphQL object;
 their type can be Scalar, Object, Enum, Interface, or Union, or any wrapping
@@ -652,6 +657,24 @@ interface. Querying for `age` is only valid when the result of `entity` is a
 }
 ```
 
+When defining an interface that implements another interface, the implementing
+interface must define each field that is specified by the implemented interface.
+For example, the interface `Resource` must define the field `id` to implement the
+`Node` interface:
+
+```graphql example
+interface Node {
+  id: ID!
+}
+
+interface Resource implements Node {
+  id: ID!
+  url: String
+}
+```
+
+
+
 **Result Coercion**
 
 The interface type should have some way of determining which object a given
@@ -672,6 +695,38 @@ Interface types have the potential to be invalid if incorrectly defined.
    type; no two fields may share the same name.
 3. Each field of an Interface type must not have a name which begins with the
    characters {"__"} (two underscores).
+4. An interface type may declare that it implements one or more unique
+   interfaces, but may not implement itself.
+5. An interface type must be a super-set of all interfaces it implements:
+   1. The implementing interface type must include a field of the same name for
+   every field defined in an implemented interface.
+      1. The implementing interface field must be of a type which is equal to or
+         a sub-type of the implemented interface field (covariant).
+         1. An implementing interface field type is a valid sub-type if it is
+            equal to (the same type as) the implemented interface field type.
+         2. An implementing interface field type is a valid sub-type if it is an
+            Object type and the implemented interface field type is either an
+            Interface type or a Union type and the implementing interface field
+            type is a possible type of the implemented interface field type.
+         3. An implementing interface field type is a valid sub-type if it is an
+            Interface type and the implemented interface field type is either an
+            Interface type or a Union type and the implementing interface field
+            type is a possible type of the implemented interface field type.
+         4. An implementing interface field type is a valid sub-type if it is a
+            List type and the implemented interface field type is also a List
+            type and the list-item type of the implementing interface field type
+            is a valid sub-type of the list-item type of the implemented
+            interface field type.
+         5. An implementing interface field type is a valid sub-type if it is a
+            Non-Null variant of a valid sub-type of the implemented interface
+            field type.
+      2. The implementing interface field must include an argument of the same
+         name for every argument defined in the implemented interface field.
+         1. The implementing interface field argument must accept the same type
+            (invariant) as the implemented interface field argument.
+      3. The implementing interface field may include additional arguments not
+         defined in the implemented interface field, but any additional argument
+         must not be required.
 
 
 ### Unions