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

  kt_dart: ^1.1.0
import 'package:kt_dart/kt.dart';


Dart's dart:core package provides basic building blocks. But sometimes they are too low level and not as straightforward as Kotlin's kotlin-stdlib.

Here are a few examples of 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 indistinguishable 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 have mutable counterparts:

Immutable Mutable
KtList KtMutableList
KtSet KtMutableSet, KtHashSet, KtLinkedSet
KtMap KtMutableMap, KtHashMap, KtLinkedMap
KtCollection KtMutableCollection and all the above
KtIterable KtMutableIterable and all the above
Deep equals

dart:core collections

Dart's List works like a Array in Java. Equals doesn't compare the items; it 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


KtList is a read-only list of elements. It is immutable because it doesn't offer mutation methods such as remove or add. Use KtMutableMap if you want to use a mutable list.

To create a KtList/KtMutableList use the KtList.of constructor or convert an existing Dart List to a KtList with the list.toImmutableList() extension.

Create a KtList

// Create a KtList from scratch
final beatles = KtList.of("John", "Paul", "George", "Ringo");

// Convert a existing List to KtList
final abba = ["Agnetha", "Björn", "Benny", "Anni-Frid"];
final immutableAbba = abba.toImmutableList();

Create a KtMutableList

KtList is immutable by default, which means it doesn't offer methods like add or remove. To create mutable list with kt_dart use the KtMutableList constructor.

// Create a KtMutableList from scratch
final beatles = KtMutableList.of("John", "Paul", "George", "Ringo");
print(beatles); // [Paul, George, Ringo]

Mutable/Immutable conversion

Conversions between KtList and KtMutableList can be done with KtList.toMutableList() and KtMutableList.toList();

final beatles = KtList.of("John", "Paul", "George", "Ringo");
final mutable = beatles.toMutableList();
print(mutable); // [Paul, George, Ringo]
print(beatles); // [John, Paul, George, Ringo]

for loop

kt_dart collections do not implement Iterable. It is therefore not possible to directly iterate over the entries of a KtList.

All kt_dart collections offer a .iter property which exposes a Dart Iterable. For-loops therefore don't look much different.

final beatles = KtList.of("John", "Paul", "George", "Ringo");
for (final member in beatles.iter) {

Yes, alternatively you could use .asList() instead which returns a Dart List.

Kotlin syntax

Kotlin users might be more familiar with the listOf() and mutableListOf() functions. Use them if you like but keep in mind that the dart community is much more used to use constructors instead of top-level functions.

final beatles = listOf("John", "Paul", "George", "Ringo");
final abba = mutableListOf("Agnetha", "Björn", "Benny", "Anni-Frid");


A KtSet is a unordered collection of elements without duplicates.

Creating a KtSet/KtMutableSet is very similar to the KtList API.

// Create a KtSet from scratch
final beatles = KtSet.of("John", "Paul", "George", "Ringo");

// Convert a existing Set to KtSet
final abba = {"Agnetha", "Björn", "Benny", "Anni-Frid"};
final immutableAbba = abba.toImmutableSet();


To create a KtMap/KtMutableMap start with Dart Map and then convert it to a KtMap with either:

  • pokemon.toImmutableMap(): KtMap (since Dart 2.7)
  • KtMap.from(pokemon): KtMap
  • pokemon.kt: KtMutableMap (since Dart 2.7)
  • KtMutableMap.from(pokemon): KtMutableMap
// immutable
final pokemon = {
  1: "Bulbasaur",
  2: "Ivysaur",
  3: "Stegosaur",

final newPokemon = KtMap.from({
  152: "Chikorita",
  153: "Bayleef",
  154: "Meganium",

// mutable
final mutablePokemon = {
  1: "Bulbasaur",
  2: "Ivysaur",
  3: "Stegosaur",

final newMutablePokemon = KtMutableMap.from({
  152: "Chikorita",
  153: "Bayleef",
  154: "Meganium",

KtHashMap and KtLinkedMap

You may want to use a specific Map implementation. kt_dart offers:

  • KtLinkedMap - based on Darts LinkedHashMap where the insertion order of keys is remembered and keys are iterated in the order they were inserted into the map
  • KtHashMap - based on Darts HashMap where keys of a HashMap must have consistent [Object.==] and [Object.hashCode] implementations. Iterating the map's keys, values or entries (through [forEach]) may happen in any order.

KtPair, KtTriple

kt_dart offer two types of tuples, KtPair with two elements and KtTriple with three elements. They are used by some collection APIs and prevent a 3rd party dependency.

final beatles = KtList.of("John", "Paul", "George", "Ringo");
final partitions = beatles.partition((it) => it.contains("n"));
print(partitions.first); // [John, Ringo]
print(partitions.second); // [Paul, George]

There won't be a KtQuadruple or TupleN in this library. If you want to use tuples heavily in you application consider using the tuple package. Better, use freezed to generated data classes which makes for a much better API.



Kotlin already has Non-Nullable types, something which is coming to Dart soon™. kt_dart already makes use of Non-Nullable Types and never returns null unless a method is annotated with @nullable.


There isn't any tooling which will warn you about the wrong usage but at least it's documented. And once nnbd lands in Dart it will be fairly easy to convert.


This annotation annotates methods which never return null. Although this is the default in kt_dart, is makes it very obvious for methods which sometimes return null in other languages.


A method/class annotated with @experimental marks the method/class as experimental feature. Experimental APIs can be changed or removed at any time.


Copyright 2019 Pascal Welsch

Licensed under the Apache License, Version 2.0 (the "License");
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