DoWhatNow.Dart

A Result package for Dart (and Flutter of course) designed around fluent APIs and a functional approach to error handling, because things go wrong! This package will help you process the DO without verbosity while forgetting all about the WHAT.
The DO being the happy path an application may take, and the WHAT being short for WTF. Complete your process OR find out what went wrong.

  final doWhat = combine(
    getUserAccount('apple'), 
    getUserAccount('bannana'))
    .ensure(
      (users) => users.$1.name == 'apple' && users.$2.name == 'bannana', 
      (users) => message('One or More Users are Not Value'))
    .map<Villian>((users) => Villian.eatUsers(users.$1, users.$2))
    .ensure(
      (villian) => villian.user1.eaten && villian.user2.eaten, 
      (villian) => message('The villian didn\'t eat the users'));

Table of Contents

1. Files
2. DoWhat
3. What
4. Extension Methods
5. Usage

The Files

located in the lib/src directory you will find a few files exported by this library.

/lib/src/do_what.dart contains the main type defined by this library as well as various factory methods. /lib/src/do_state.dart contains an enum used internally by the DoWhat class. /lib/src/what.dart contains error definitions, that WTF /lib/src/why.dart contains an enum used internally by the What error types. /lib/src/extensions.dart contains all DoWhat extension methods required to create a fluent API.

The DoWhat Object

The DoWhat type is an immutable class that stores a collection of Whats or a valid <TValue> object. The DoWhat class can only ever be a success or failure. Each DoWhat instance holds a propery value of <T?>, List<What> errors storing any errors, isSuccess boolean, and isFalure boolean. The value property will never be null if the isSuccessfull property is true, whilst the errors property will be populated with at least one What what the isSuccessfull property is false. The isFailure property is only every be true when the isSuccess property is false. And guess what, the collection of Whats errors property will have one or more errors. Bottom line, A DoWhat can only be 'Doing' or 'Whating'.

To begin using the DoWhat class create an instance with one of the factories provided.

Factories

create<TValue> can be used to evaluate an instance of the defined type. If the TValue instance is not not the create method will return a successfull result.

Create a valid DoWhat, note the String 'valid' is NOT NULL

// Don't forget to define the generic Type to avoid usage of dynamic
final doWhat = create<String>('value');

if(doWhat.isSuccess){
// this is now true as the 'value' string was a valid String
}

Create an invalid DoWhat with a null value.

// Don't forget to define the generic Type to avoid usage of dynamic
final doWhat = create<String>(_oppsReturnedNull());

if(doWhat.isFailure){
// this is now true as the bogus method returned a null string
}

...
String? _oppsReturnedNull(){
    //... some error returned null;
    return null;
}

from<TValue> can be used to evaluate a predicate to return a DoWhat based on the result.

final result = from<int>(2, (value) => value == 2);
result.isSuccess // this is true as the value passed into the from method is '2' and predicate evaluates to true.

In contrast the below example produces a failed DoWhat as the predicate returns false.

final result = from<int>(22, (value) => value == 2);
result.isFailure // this is true as the value passed into the from method is '12' and predicate evaluates to false.

Should you need to create a successfull DoWhat you can use the success<TValue> factory. Pass any instance of <TValue> into the method and begin chaining you validations.

final Thing thing = Thing();
final doWhat = success(thing); //this is now a successfull dowhat and can be processed.

Should you need to create a failed DoWhat you can use the failure<TValue> or failures factory. Pass any instance of What or List<What> into the method.

final doWhat = failure(message('This is a Failed thing')); 

The What Object

Stores the failures. Failures can be as complex as required. A What is an abstract class, several concrete Whats have been provided for you that store either messages, exceptions, and objects. Factory methods have been provided to streamline the API.

Create (or use a provided implmentation) a class the implements or extends the What class.

class WhatMessage extends What<String> {
  WhatMessage(String message) : super(Why.message, message);
  @override
  String _getMessage() => error!;
}

WhatMessage message(String message) => WhatMessage(message);

Provide this new What to the results of an error lambda.

final doWhat = create<String>('http://getusers')                              
    .ensure(
      (url) => url.startsWith('http://'),                   
      (url) => message('$url does not start with http://'))        

Extension Methods

DoWhat Usage

So whats the point. Well now you have a DoWhat and you can use this as a return type on potentially destructive methods. The DoWhat will allow you to gracfully handle these errors, process the results, return messages to UI, while manintaining readability. Below are some complex use cases.

Sanitizing Inputs

The following code snip demonstrates how we could handle an HTTP request (not this is not async delibratly to provide a simply example). We will start by sanitizing the input to our method. Right away the result will be a failure if the URL is null, we then validate the string starts with a proper http:// prefix. We then map it to a HTTP Reponse. At ths point the DoWhat has mutated to a DoWhat<HttpResponse>. Imediatly validate a returned 200 and ensure the body response is not empty. After completly sanitixing things we can map it to a User.

  final doWhat = create<String>('http://getusers')                              
    .ensure(
      (url) => url.startsWith('http://'),                     // Validate
      (url) => message('$url does not start with http://'))   // Provide Error  
    .tryMap<HttpResponse>((url) => getRequest(url))           // Convert                 
    .ensure(
      (httpResponse) => httpResponse.statusCode == 200,       // Validate
      (httpResponse) => message('Status Code ${httpResponse.statusCode}'))
    .ensure(
      (httpResponse) => httpResponse.body != null,                      // Validate
      (httpResponse) => message('Http response contains no data'))
    .tryMap<User>((httpResponse) => User.fromMap(httpResponse.body!));  //When KNOW the body is NOT Null!
  

Returning From a Function

Of course like all things dart, we can return a DoWhat from an function. This provides a fluent API to chain functions together. The combine example below calls two methods that return a DoWhat. The results are combined as a Tuple and evaluated. If the returned results are valid, both users are Eaten and a SuperVillian is created.

void combineExample(){

  final doWhat = combine(
    getUserAccount('apple'), 
    getUserAccount('bannana'))
    .ensure(
      (users) => users.$1.name == 'apple' && users.$2.name == 'bannana', 
      (users) => message('One or More Users are Not Value'))
    .map<Villian>((users) => Villian.eatUsers(users.$1, users.$2))
    .ensure(
      (villian) => villian.user1.eaten && villian.user2.eaten, 
      (villian) => message('The villian didn\'t eat the users'));

    print(doWhat);
}

DoWhat<User> getUserAccount(String name) =>
  create(name)
    .map<String>((name) => 'http://getusers/$name')                              
    .ensure(
      (url) => url.startsWith('http://'),                     
      (url) => message('$url does not start with http://'))    
    .tryMap<HttpResponse>((url) => getRequest(url, name: name))                           
    .ensure(
      (httpResponse) => httpResponse.statusCode == 200,       
      (httpResponse) => message('Status Code ${httpResponse.statusCode}'))
    .ensure(
      (httpResponse) => httpResponse.body != null,                      
      (httpResponse) => message('Http response contains no data'))
    .tryMap<User>((httpResponse) => User.fromMap(httpResponse.body!));