fpkit #

Ultimate functional programming toolkit for Dart 3+ & Flutter. A modern successor to dartz / fpdart, built on sealed classes, pattern matching and records.

Types #

Plus: function.dart (pipe, compose/then, curry2, memoize, tap, also, let), predicate.dart (and/or/neg, contramap, allPass/anyPass), extensions.dart (nullable toOption/toEither, Iterable.traverseEither, Future.toTask, string parsers, ...).

Quick start #

import 'package:fpkit/fpkit.dart';

// Option
final name = Option.fromNullable(maybeName)
  .map((s) => s.toUpperCase())
  .getOrElse(() => 'GUEST');

// Either
Either<String, int> parseAge(String s) {
  final v = int.tryParse(s);
  return v == null ? left('bad age: $s') : right(v);
}
final age = parseAge('42').map((n) => n + 1).getOrElse((_) => 0);

// Validation accumulates errors (Either short-circuits)
final v = Validation.valid<String, int>(1)
  .zipWith(Validation.invalid<String, int>('boom'), (a, b) => a + b);

// Async fallible
TaskEither<String, int> fetchAge() =>
  TaskEither.tryCatch(() async => 42, (e, _) => e.toString());

// Reader for DI
final greeting = Reader<Map<String, String>, String>(
  (env) => 'Hello ${env['name']}',
);

// State
final counter = State<int, int>((s) => (s + 1, s));

// Pipe
final r = 1.pipe((x) => x + 1).pipe((x) => x * 10); // 20

Why fpkit over dartz/fpdart? #

• Dart 3 native: sealed, exhaustive switch, records (S, A).
• One import, consistent naming (map/flatMap/andThen/zipWith/sequence/traverse).
• Both short-circuit (Either) and accumulating (Validation) error models.
• Effects covered: IO / Task / TaskEither / TaskOption / Reader / Writer / State.
• Flutter-ready: pure Dart, no Flutter SDK dependency, works on all platforms.

Running tests #

dart test
# or
flutter test