name: "using-fpdart" description: "Functional error handling in Dart using the fpdart package with Either, Option, TaskEither, and Do notation. Use when replacing try-catch with type-safe error handling or composing async operations functionally." metadata: last_modified: "2026-04-01 14:35:00 (GMT+8)"

Functional Programming with fpdart

Goal

Implement robust, type-safe, and predictable business logic using functional programming principles. Replace side-effect-prone imperative code with pure functions and composable monads (Option, Either, Task), ensuring that error handling and null-safety are enforced at the type level rather than through runtime exceptions.

Process

Phase 1: Basic Data Containment

Use primitives to handle common edge cases without branching logic:

• Option<T>: Replaces T?. Use Option.of(val) or Option.none(). Access via match or getOrElse.
• Either<L, R>: Replaces try-catch. L (Left) is for errors, R (Right) is for success.
• Unit: Replaces void to allow functional composition.

Phase 2: Execution & Async Flow

Manage side effects and asynchronous operations through wrappers that defer execution:

• IO<T>: Synchronous wrapper for pure/safe side effects.
• Task<T>: Asynchronous wrapper for Future that never fails.
• TaskEither<L, R>: The standard for API requests. Composes Future and Either into a single monadic flow.

Phase 3: Advanced Linear Chaining

Utilize the Do Notation to flatten nested flatMap calls into readable, imperative-style blocks while preserving monadic safety:

return TaskEither.Do(($) async {
  final user = await $(repository.getUser(id));
  final profile = await $(api.getProfile(user.code));
  return profile.toDomain();
});

⚡ Quick Reference

Error Handling (Either)

Either<String, int> checkValue(int v) => 
  v > 10 ? Either.of(v) : Either.left('Too small');

final result = checkValue(5).match(
  (error) => 'Failure: $error',
  (value) => 'Success: $value',
);

Async Operations (TaskEither)

TaskEither<Exception, User> fetchUser(String id) =>
  TaskEither.tryCatch(
    () => api.getUser(id),
    (error, stack) => Exception('Fetch failed: $error'),
  );

Collections Extension

fpdart extends Iterable with safer methods:

• list.head -> Option<T> (Safe first)
• list.all((e) => ...) -> bool

Constraints

• Zero Exceptions: Prohibit the use of throw for expected business errors. Force use of Either or TaskEither.
• Monadic Integrity: Never use ! (bang operator) or force-casts on functional types. Always use safe extraction methods (match, getOrElse).
• Linear Logic: Mandatory use of Do notation when chaining more than two monadic operations to prevent "callback hell."
• Immutability First: Combine with fast_immutable_collections for all state definitions.