RLC.TaskChaining 0.1.0

RLC.TaskChaining

Monadic-style chaining for C# Tasks.

Rationale

Asynchronous code (particularly in C#) typically relies on using the async/await feature introduced in C# 5.0. This has a lot of benefits, but it unfortunately tends to push code into an imperative style. This library aims to make writing asychronous functional code easier, cleaner, and less error-prone using extensions to System.Threading.Tasks.

Installation

Install RLC.TaskChaining as a NuGet package via an IDE package manager or using the command-line instructions at nuget.org.

API

Chaining

Then

Once a Task<T> has been created, successive operations can be chained using the Then method.

HttpClient client;  // Assuming this is coming from an HttpClientFactory or injected or whatever

Task.FromResult("https://www.google.com")  // Task<string>
  .Then(client.GetAsync)                   // Task<HttpResponseMessage>
  .Then(response => response.StatusCode);  // Task<System.Net.HttpStatusCode>

Catch

When a Task<T> enters a faulted state, the Catch method can be used to return the Task<T> to a non-faulted state.

HttpClient client;  // Assuming this is coming from an HttpClientFactory or injected or whatever

Task.FromResult("not-a-url")              // Task<string>
  .Then(client.GetAsync)                  // Task<HttpResponseMessage> but FAULTED
  .Catch(exception => exception.Message)  // Task<string> and NOT FAULTED
  .Then(message => message.Length)        // Task<int>

Note that it's entirely possible for a Catch method to cause the Task<T> to remain in a faulted state, e.g. if you only wanted to recover into a non-faulted state if a particular exception type occurred.

HttpClient client;  // Assuming this is coming from an HttpClientFactory or injected or whatever

Task.FromResult("not-a-url")          // Task<string>
  .Then(client.GetAsync)              // Task<HttpResponseMessage> but FAULTED
  .Catch(exception => exception is NullReferenceException
    ? exception.Message
    : Task.FromException(exception))  // Task<string> but STILL FAULTED if anything other than NullReferenceException occurred
  .Then(message => message.Length)    // Task<int>

IfFulfilled/IfRejected/Tap

The IfFulfilled and IfRejected methods can be used to perform side effects such as logging when the Promise<T> is in the fulfilled or faulted state, respectively.

HttpClient client;  // Assuming this is coming from an HttpClientFactory or injected or whatever

Promise<string>.Resolve("https://www.google.com/")
  .Then(client.GetAsync)
  .IfFulfilled(response => _logger.LogDebug("Got response {Response}", response)
  .Then(response => response.StatusCode);

HttpClient client;  // Assuming this is coming from an HttpClientFactory or injected or whatever

Promise<string>.Resolve("not-a-url")
  .Then(client.GetAsync)
  .IfRejected(exception => _logger.LogException(exception, "Failed to get URL")
  .Catch(exception => exception.Message)
  .Then(message => message.Length);

The Tap method takes both an onFulfilled and onRejected Action in the event that you want to perform some side effect on both sides of the Promise at a single time.

HttpClient client;  // Assuming this is coming from an HttpClientFactory or injected or whatever

Promise<string>.Resolve(someExternalUrl)
  .Then(client.GetAsync)
  .Tap(
    response => _logger.LogDebug("Got response {Response}", response),
    exception => _logger.LogException(exception, "Failed to get URL")
  )

Static Methods

There are some convenience methods on TaskExtras that are useful when transitioning between the fulfilled and faulted states.

RejectIf

RejectIf can be used to transition into a faulted state based on some Predicate<T>.

Task.FromResult(1)
  .Then(TaskExtras.RejectIf(
    value => value % 2 == 0,
    value => new Exception($"{nameof(value)} was not even")
  ));

ResolveIf

ResolveIf can be used to transition from a faulted state to a fulfilled state based on some Predicate<Exception>.

Task.FromException<int>(new ArgumentException())
  .Catch(TaskExtras.ResolveIf(
    exception => exception is ArgumentException,
    exception => exception.Message.Length
  ))