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This project is a port of Kotlins Kotlin Standard library for Dart/Flutter projects. It's a useful addition to dart:core and includes collections (KtList, KtMap, KtSet) as well as and other packages which can improve every Dart/Flutter app.


Dart's dart:core package provides basic building blocks. But sometimes they are too low leven and not as straight-forward as Kotlins kotlin-stdlib.

Here are a few examples what this project offers: (click to expand)

Immutable collections by default

dart:core collections

Dart's List is mutable by default. The immutable List.unmodifiable is the same type but the mutation methods throw at runtime.

final dartList = [1, 2, 3];
dartList.add(4); // mutation is by default possible
assert(dartList.length == 4);

final immutableDartList = List.unmodifiable(dartList);
immutableDartList.add(5); // throws: Unsupported operation: Cannot add to an unmodifiable list

Dart's mutable List is undistinguishable from an immutable List which might cause errors.

void addDevice(List<Widget> widgets, Device device) {
  // no way to check whether widgets is mutable or not
  // add might or might now throw
  widgets.add(Divider(height: 1.0));

kt.dart collections

KtList and KtMutableList are two different Types. KtList is immutable by default and has no mutation methods (such as add). Methods like map((T)->R) or plusElement(T) return a new KtList leaving the old one unmodified.

final ktList = listOf(1, 2, 3);
// The method 'add' isn't defined for the class 'KtList<int>'.
ktList.add(4); // compilation error

// Adding an item returns a new KtList
final mutatedList = ktList.plusElement(4);
assert(ktList.size == 3);
assert(mutatedList.size == 4);

KtMutableList offers mutation methods where the content of that collection can be actually mutated. I.e. with remove(T) or add(T);

// KtMutableList allow mutation
final mutableKtList = mutableListOf(1, 2, 3);
mutableKtList.add(4); // works!
assert(mutableKtList.size == 4);

All collection types has mutable counterparts:

KtSetKtMutableSet, KtHashSet, KtLinkedSet
KtMapKtMutableMap, KtHashMap, KtLinkedMap
KtCollectionKtMutableCollection and all the above
KtIterableKtMutableIterable and all the above
Deep equals

dart:core collections

Dart's List works like a Array in Java. Equals doesn't compare the items, equals only checks the identity. To compare the contents you have to use helper methods methods from 'package:collection/collection.dart'.

// Comparing two Dart Lists works only by identity
final a = [1, 2, 3, 4];
final b = [1, 2, 3, 4];
print(a == b); // false, huh?

// content based comparisons require unnecessary glue code
Function listEq = const ListEquality().equals;
print(listEq(a, b)); // true

// MapEquality isn't deep by default
final x = {1: ["a", "b", "c"], 2: ["xx", "yy", "zz"]};
final y = {1: ["a", "b", "c"], 2: ["xx", "yy", "zz"]};
Function mapEq = const MapEquality().equals;
print(mapEq(x, y)); // false, wtf?!

Function deepEq = const DeepCollectionEquality().equals;
print(deepEq(x, y)); // true, finally

kt.dart collections

KtList and all other collection types implement equals by deeply comparing all items.

final a = listOf(1, 2, 3, 4);
final b = listOf(1, 2, 3, 4);
print(a == b); // true, as expected

final x = mapFrom({1: listOf("a", "b", "c"), 2: listOf("xx", "yy", "zz")});
final y = mapFrom({1: listOf("a", "b", "c"), 2: listOf("xx", "yy", "zz")});
print(x == y); // deep equals by default
Common methods

Some of Dart's method names feel unfamiliar. That's because modern languages and frameworks (Kotlin, Swift, TypeScript, ReactiveExtensions) kind of agreed on naming methods when it comes to collections. This makes it easy to switch platforms and discuss implementations with coworkers working with a different language.

expand -> flatMap

final dList = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];
final kList = listOf(listOf(1, 2, 3), listOf(4, 5, 6), listOf(7, 8, 9));

// dart:core
final dFlat = dList.expand((l) => l).toList();
print(dFlat); // [1, 2, 3, 4, 5, 6, 7, 8, 9]

// kt.dart
final kFlat = kList.flatMap((l) => l);
print(kFlat); // [1, 2, 3, 4, 5, 6, 7, 8, 9]

where -> filter

final dNames = ["Chet", "Tor", "Romain", "Jake", "Dianne"];
final kNames = listFrom(dNames);

// dart:core
final dShortNames = dNames.where((name) => name.length <= 4).toList();
print(dShortNames); // [Chet, Tor, Jake]

// kt.dart
final kShortNames = kNames.filter((name) => name.length <= 4);
print(kShortNames); // [Chet, Tor, Jake]

firstWhere -> first, firstOrNull

final dNames = ["Chet", "Tor", "Romain", "Jake", "Dianne"];
final kNames = listFrom(dNames);

// dart:core
dNames.firstWhere((name) => name.contains("k")); // Jake
dNames.firstWhere((name) => name.contains("x"), orElse: () => null); // null
dNames.firstWhere((name) => name.contains("x"), orElse: () => "Nobody"); // Nobody

// kt.dart
kNames.first((name) => name.contains("k")); // Jake
kNames.firstOrNull((name) => name.contains("x")); // null
kNames.firstOrNull((name) => name.contains("x")) ?? "Nobody"; // Nobody



import 'package:kt_dart/annotation.dart';

Annotations such as @nullable or @nonNull giving hints about method return and argument types


import 'package:kt_dart/collection.dart';

Collection types, such as KtIterable, KtCollection, KtList, KtSet, KtMap with over 150 methods as well as related top-level functions. The collections are immutable by default but offer a mutable counterpart i.e. KtMutableList.


Planned modules for the future are async, tuples, comparison, range, sequence, text


Copyright 2019 Pascal Welsch

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at


Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
limitations under the License.