resolve #82 -- add support for interface extensions

ast/ast.ast

@@ -20,6 +20,7 @@ O SchemaDefinition
 O ScalarTypeDefinition
 O ObjectTypeDefinition
 O InterfaceTypeDefinition
+O InterfaceExtensionDefinition
 O UnionTypeDefinition
 O EnumTypeDefinition
 O InputObjectTypeDefinition
@@ -175,6 +176,9 @@ S Name name
 P? Directive directives
 P FieldDefinition fields
 
+T InterfaceExtensionDefinition
+S InterfaceTypeDefinition definition
+
 T UnionTypeDefinition
 S Name name
 P? Directive directives

parser.ypp

@@ -61,6 +61,7 @@ using facebook::graphql::ast::SchemaDefinition;
 using facebook::graphql::ast::ScalarTypeDefinition;
 using facebook::graphql::ast::ObjectTypeDefinition;
 using facebook::graphql::ast::InterfaceTypeDefinition;
+using facebook::graphql::ast::InterfaceExtensionDefinition;
 using facebook::graphql::ast::UnionTypeDefinition;
 using facebook::graphql::ast::EnumTypeDefinition;
 using facebook::graphql::ast::InputObjectTypeDefinition;
@@ -125,6 +126,7 @@ union yystype {                                         \
     ScalarTypeDefinition *scalarTypeDefinition;                     \
     ObjectTypeDefinition *objectTypeDefinition;                     \
     InterfaceTypeDefinition *interfaceTypeDefinition;               \
+    InterfaceExtensionDefinition *interfaceExtensionDefinition;               \
     UnionTypeDefinition *unionTypeDefinition;                       \
     EnumTypeDefinition *enumTypeDefinition;                         \
     InputObjectTypeDefinition *inputObjectTypeDefinition;           \
@@ -266,6 +268,7 @@ union yystype {                                         \
 %type <scalarTypeDefinition> scalar_type_definition;
 %type <objectTypeDefinition> object_type_definition;
 %type <interfaceTypeDefinition> interface_type_definition;
+%type <interfaceExtensionDefinition> interface_extension_definition;
 %type <unionTypeDefinition> union_type_definition;
 %type <enumTypeDefinition> enum_type_definition;
 %type <inputObjectTypeDefinition> input_object_type_definition;
@@ -360,6 +363,7 @@ schema_gate:    schema_definition { $$ = static_cast<Definition *>($1); }
         |       enum_type_definition { $$ = static_cast<Definition *>($1); }
         |       input_object_type_definition { $$ = static_cast<Definition *>($1); }
         |       type_extension_definition { $$ = static_cast<Definition *>($1); }
+        |       interface_extension_definition { $$ = static_cast<Definition *>($1); }
         |       directive_definition { $$ = static_cast<Definition *>($1); }
         ;
 
@@ -644,6 +648,9 @@ input_value_definition: name ":" type default_value_opt directives_opt { $$ = ne
 interface_type_definition: INTERFACE name directives_opt "{" field_definition_list "}" { $$ = new InterfaceTypeDefinition(@$, $2, $3, $5); }
                 ;
 
+interface_extension_definition: EXTEND interface_type_definition { $$ = new InterfaceExtensionDefinition(@$, $2); }
+                ;
+
 union_type_definition: UNION name directives_opt "=" union_members { $$ = new UnionTypeDefinition(@$, $2, $3, $5); }
                 ;
 

test/ParserTests.cpp

@@ -347,6 +347,8 @@ TEST(SchemaParserTests, SimpleSchema) {
                       "otherField: otherType }");
   expectSchemaParsing("extend type SomeType " DIRECTIVES
                       "{ anotherField : AnotherType }");
+  expectSchemaParsing("extend interface SomeInterface " DIRECTIVES
+                      "{ anotherField : AnotherType }");
   expectSchemaParsing("directive @somedirective(a1 : t1 = 1 " DIRECTIVES
                       ", a2 : t2) on foo | bar");
 }

test/schema-kitchen-sink.graphql

@@ -64,6 +64,10 @@ extend type Foo {
   seven(argument: [String]): Type
 }
 
+extend interface Bar {
+  name: String
+}
+
 # NOTE: out-of-spec test cases commented out until the spec is clarified; see
 # https://github.com/graphql/graphql-js/issues/650 .
 # extend type Foo @onType {}