Finished coerceArgumentValues

graphql_schema/lib/src/argument.dart

@@ -4,5 +4,10 @@ class GraphQLFieldArgument<Value, Serialized> {
   final String name;
   final GraphQLType<Value, Serialized> type;
   final Value defaultValue;
-  GraphQLFieldArgument(this.name, this.type, {this.defaultValue});
+
+  /// If [defaultValue] is `null`, and `null` is a valid value for this argument, set this to `true`.
+  final bool defaultsToNull;
+
+  GraphQLFieldArgument(this.name, this.type,
+      {this.defaultValue, this.defaultsToNull: false});
 }

graphql_schema/lib/src/field.dart

@@ -4,12 +4,17 @@ typedef FutureOr<Value> GraphQLFieldResolver<Value, Serialized>(
     Serialized serialized);
 
 class GraphQLField<Value, Serialized> {
+  final List<GraphQLFieldArgument> arguments = <GraphQLFieldArgument>[];
   final String name;
-  final GraphQLFieldArgument argument;
   final GraphQLFieldResolver<Value, Serialized> resolve;
   final GraphQLType<Value, Serialized> type;
 
-  GraphQLField(this.name, {this.argument, this.resolve, this.type});
+  GraphQLField(this.name,
+      {Iterable<GraphQLFieldArgument> arguments: const <GraphQLFieldArgument>[],
+      this.resolve,
+      this.type}) {
+    this.arguments.addAll(arguments ?? <GraphQLFieldArgument>[]);
+  }
 
   FutureOr<Serialized> serialize(Value value) {
     return type.serialize(value);

graphql_schema/lib/src/type.dart

@@ -5,6 +5,8 @@ abstract class GraphQLType<Value, Serialized> {
   Value deserialize(Serialized serialized);
   ValidationResult<Serialized> validate(String key, Serialized input);
   GraphQLType<Value, Serialized> nonNullable();
+
+  bool get isNullable => false;
 }
 
 /// Shorthand to create a [GraphQLListType].
@@ -64,6 +66,9 @@ class _GraphQLNonNullableType<Value, Serialized>
   final GraphQLType<Value, Serialized> type;
   _GraphQLNonNullableType._(this.type);
 
+  @override
+  bool get isNullable => true;
+
   @override
   GraphQLType<Value, Serialized> nonNullable() {
     throw new UnsupportedError(

graphql_server/lib/graphql.dart

@@ -33,8 +33,7 @@ class GraphQL {
           if (customTypes.containsKey(ctx.typeName.name))
             return customTypes[ctx.typeName.name];
           throw new ArgumentError(
-              'Unknown GraphQL type: "${ctx.typeName.name}"\n${ctx.span
-                  .highlight()}');
+              'Unknown GraphQL type: "${ctx.typeName.name}"\n${ctx.span.highlight()}');
           break;
       }
     } else {
@@ -168,9 +167,36 @@ class GraphQL {
     var argumentValues = field.field.arguments;
     var fieldName = field.field.fieldName.name;
     var desiredField = objectType.fields.firstWhere((f) => f.name == fieldName);
+    var argumentDefinitions = desiredField.arguments;
 
-    // TODO: Multiple arguments?
-    var argumentDefinitions = desiredField.argument;
+    for (var argumentDefinition in argumentDefinitions) {
+      var argumentName = argumentDefinition.name;
+      var argumentType = argumentDefinition.type;
+      var defaultValue = argumentDefinition.defaultValue;
+      var value = argumentValues.firstWhere((a) => a.name == argumentName,
+          orElse: () => null);
+
+      if (value != null) {
+        var variableName = value.name;
+        var variableValue = variableValues[variableName];
+
+        if (variableValues.containsKey(variableName)) {
+          coercedValues[argumentName] = variableValue;
+        } else if (defaultValue != null || argumentDefinition.defaultsToNull) {
+          coercedValues[argumentName] = defaultValue;
+        } else if (!argumentType.isNullable) {
+          throw new GraphQLException(
+              'Missing value for argument "$argumentName".');
+        }
+      } else {
+        if (defaultValue != null || argumentDefinition.defaultsToNull) {
+          coercedValues[argumentName] = defaultValue;
+        } else if (!argumentType.isNullable) {
+          throw new GraphQLException(
+              'Missing value for argument "$argumentName".');
+        }
+      }
+    }
 
     return coercedValues;
   }