interfaces themselves are actually trivial!

generate/testdata/InterfaceNoFragments.graphql

@@ -0,0 +1,10 @@
+{
+  root {
+    id
+    name
+    children {
+      id
+      name
+    }
+  }
+}

generate/testdata/InterfaceNoFragments.graphql.go

@@ -0,0 +1,12 @@
+type Response struct {
+	Root struct {
+		Id       string    `json:"id"`
+		Name     string    `json:"name"`
+		Children []content `json:"children"`
+	} `json:"root"`
+}
+
+type content struct {
+	Id   string `json:"id"`
+	Name string `json:"name"`
+}

generate/testdata/schema.graphql

@@ -26,6 +26,34 @@ type User {
   authMethods: [AuthMethod!]!
 }
 
+interface Content {
+  id: ID!
+  name: String!
+  parent: Topic
+}
+
+type Article implements Content {
+  id: ID!
+  name: String!
+  parent: Topic!
+  text: String!
+}
+
+type Video implements Content {
+  id: ID!
+  name: String!
+  parent: Topic!
+  duration: Int!
+}
+
+type Topic implements Content {
+  id: ID!
+  name: String!
+  parent: Topic
+  children: [Content!]!
+}
+
 type Query {
   user(query: UserQueryInput): User
+  root: Topic!
 }

generate/types.go

@@ -222,7 +222,7 @@ func (builder *typeBuilder) writeType(typ *ast.Type, selectionSet []selection, i
 
 func (builder *typeBuilder) writeTypedef(typedef *ast.Definition, selectionSet []selection) error {
 	switch typedef.Kind {
-	case ast.Object, ast.InputObject:
+	case ast.Object, ast.InputObject, ast.Interface:
 		builder.WriteString("struct {\n")
 		for _, field := range selectionSet {
 			err := builder.writeField(field)
@@ -244,7 +244,7 @@ func (builder *typeBuilder) writeTypedef(typedef *ast.Definition, selectionSet [
 		}
 		builder.WriteString(")\n")
 		return nil
-	case ast.Scalar, ast.Union, ast.Interface:
+	case ast.Scalar, ast.Union:
 		// TODO(benkraft): Handle custom scalars, unions, and interfaces.
 		return fmt.Errorf("not implemented: %v", typedef.Kind)
 	default: