Added enum type

graphql_schema/lib/src/enum.dart

@@ -0,0 +1,22 @@
+part of graphql_schema.src.schema;
+
+class GraphQLEnumType extends _GraphQLStringType {
+  final String name;
+  final List<String> values;
+  final String description;
+
+  GraphQLEnumType(this.name, this.values, {this.description}) : super._();
+
+  @override
+  ValidationResult<String> validate(String key, String input) {
+    var result = super.validate(key, input);
+
+    if (result.successful && !values.contains(result.value)) {
+      return result._asFailure()
+        ..errors.add(
+            '"${result.value}" is not a valid value for the enum "$name".');
+    }
+
+    return result;
+  }
+}

graphql_schema/lib/src/schema.dart

@@ -7,6 +7,8 @@ import 'package:source_span/source_span.dart';
 
 part 'argument.dart';
 
+part 'enum.dart';
+
 part 'field.dart';
 
 part 'gen.dart';

graphql_schema/lib/src/validation_result.dart

@@ -5,6 +5,8 @@ class ValidationResult<T> {
   final T value;
   final List<String> errors;
 
+  ValidationResult._(this.successful, this.value, this.errors);
+
   ValidationResult._ok(this.value)
       : errors = [],
         successful = true;
@@ -12,4 +14,8 @@ class ValidationResult<T> {
   ValidationResult._failure(this.errors)
       : value = null,
         successful = false;
+
+  ValidationResult<T> _asFailure() {
+    return new ValidationResult<T>._(false, value, errors);
+  }
 }