Functional Error Handling with Either and fpdart in Flutter: An Introduction

Nothing can be perfect, and coding is not an exception to it. Despite the best coding and technical fulfilment, errors may creep into the best codes. The ultimate solution to this is debugging and resolving the errors which running the program repeatedly follows.

Flutter, a web development application, is one of the most trending platforms. Organisations hire Flutter developers to take significant advantage. However, handling errors in coding on Flutter is extremely important. In this blog, we shall discuss the Error handling part with Either and fpdart while working with Flutter. 

Errors handling in Dart language 

Options such as try, throw, and catch are available in the dart language to handle the errors and exceptions. By utilizing a pre-existing exception system, writing codes that may become hazardous in the future becomes a simplified task. Nevertheless, bear in mind the following essential considerations when engaging in this task:

  • It is possible to find out the throwing possibility of a function only by reading its implementation and documentation.
  • It is tough to leverage static analysis and the type of system that handles the errors explicitly.

These are the key points to understand the whole journey of error handling. This calls for a different remedy which we shall discuss in the next section. Organizations should hire Flutter developers to learn and understand more about it.

Functional programming principles for error handling 

It is possible to manage and handle the issues mentioned in the above section using Functional Programming Language (FLP). Functional Programming Language is a set of codes that helps avoid errors by writing clean codes in a declarative programming style. 

It is opposite to that of the object-oriented programming style. Many modern languages provide support to object-oriented and functional paradigms. However, FLP finds wide usage in Dart and Flutter: 

  • Working with type and generics inference.
  • Using iterable operators such as reduce, map, and where, and iterable types such as steams and lists.
  • Passing callback functions as an argument to another function. 
  • It also helps in destructuring, higher types of orders, pattern matching, and multiple value returns.
  • Last but not least is error handling. For example, it can help in the execution of a program to parse a number effectively compared to its native code.

These are the reasons you may need to hire Flutter developers to learn about the FLP and its functions for your codes. 

The Either type and fdpart coding and errors 

The Either type lets one specify the failure and success of the signature function. Here is an example of coding in Either type with fdpart: 

import ‘package:fpdart/fpdart.dart’;

Either<FormatException, double> parseNumber(String value) {

  try {

    return Either.right(double.parse(value));

  } on FormatException catch (e) {

    return Either.left(e);




The code mentioned above may require more work to produce an output. Here is a solution to it, using the try-catch instructor of functional programming language: 

Either<FormatException, double> parseNumber(String value) {

  return Either.tryCatch(

    () => double.parse(value),

    (e, _) => e as FormatException,



The output that comes out is this: 

/// Try to execute `run`. No error, then return [Right].

/// Otherwise, return [Left] with the result of `onError`.

factory Either.tryCatch(

    R Function() run, 

L Function(Object ob, StackTrace st) onError) {

  try {

    return Either.of(run());

  } catch (e, s) {

    return Either.left(onError(e, s));



The functional style of this program is below. It is a code that describes this code in a fully functioning style: 

Iterable<Either<FormatException, String>>

    parseFizzBuzz(List<String> strings) =>

      (string) => parseNumber(string).map(

        (value) => fizzBuzz(value) // no tear-off



Using tear-off, there are chances for more simplicity and ease in this piece of coding:

Iterable<Either<FormatException, String>>

    parseFizzBuzz(List<String> strings) =>

      (string) => parseNumber(string).map(

        fizzBuzz // with tear-off



If it’s possible to do this by oneself, one should hire Flutter developers to achieve the maximum benefit. 

How else can Either can help? 

Here are some more functions of Either that can help manage and handle the Error in programs. The first code is for deciding the left and right direction of values: 

/// Create an instance of [Right]

final right = Either<String, int>.of(10);

/// Create an instance of [Left]

final left = Either<String, int>.left(‘none’);

This is how Either can help in mapping values: 

/// Mapping the right value to [String]

final mapRight = => ‘$a’);

/// Mapping the left value to an [int]

final mapLeft = right.mapLeft((a) => a.length);


Pattern matching programming with the help of Either: 

/// Pattern matching

final number = parseNumber(‘invalid’);

/// Unwrap error/success with the fold method

number.fold( // same as number.match()

  (exception) => print(exception),

  (value) => print(value),


Critical points while using Either and fdpart 

This coding is very versatile and works anywhere. You can try researching it, or the best way is to hire Flutter developers. However, here are some key points that one should remember about programming: 

  • In the self-documenting codes, it is possible to use Either + L or R. It is an alternative to throwing exceptions when the aim is to declare the signature explicitly of signature. 
  • The program would only be able to compile if errors remain. Therefore, either helps in this.
  • With the help of Either API, data manipulation is straightforward with the service of functional operators. For example, map, map left, fold, and others. 

It is easy to use this language package by knowing more about it. You can also check here how to handle error with Bloc pattern in Flutter?


The error handling stage is an integral part of the development cycle. It is essential to get a system that runs without any issues. It is possible only if one knows the art of scanning the errors and their resolutions. This factor would reduce errors at almost every stage.

Therefore, this blog has taken us on the journey of handling various fictional errors. It is necessary to save time, resources and get the codes to run effectively.
Image by pressfoto on Freepik


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