Readme.md

AsyncLazyPipeline Monad (C#)

I had the need to create an async pipeline that consisted of actions (methods) composing over each other. I also wanted the construction of this pipeline to not generate any side effects, including Exceptions, Object state mutations, IO operations, etc. I had the need for function composition and Monadic Composition (Kleisli Composition), and did not want to use Task.Run, I do not like doing that in ASP.NET applications.

Having worked in JavaScript with the IO Monad, I decided to do something similar in C# that would fulfill my needs. In here, I leave the implementation I've come up so far with, and below, a simple code example that shows how to use it, plus some other code for the sake of completeness.

AsyncLazyPipeline.cs

public class AsyncLazyPipeline<TSource>
{
    private Func<Task<TSource>> Expression { get; }

    public AsyncLazyPipeline(Func<Task<TSource>> expression)
    {
        Expression = expression;
    }

    public Task<TSource> Flatten() => Expression();

    public AsyncLazyPipeline<TDestination> Select<TDestination>(Func<TSource, Task<TDestination>> fn)
    {
        async Task<TDestination> CombineExpressions()
        {
            var result = await Expression();
            return await fn(result);
        }

        return CreatePipeLine.With(CombineExpressions);
    }

    public AsyncLazyPipeline<TDestination> Select<TDestination>(Func<TSource, TDestination> fn)
    {
        async Task<TDestination> CombineExpressions()
        {
            var result = await Expression();
            return fn(result);
        }

        return CreatePipeLine.With(CombineExpressions);
    }

    public AsyncLazyPipeline<TDestination> SelectMany<TDestination>(Func<TSource, AsyncLazyPipeline<TDestination>> fn)
    {
        async Task<TDestination> CombineExpressions()
        {
            var result = await Expression();
            return await fn(result).Flatten();
        }

        return CreatePipeLine.With(CombineExpressions);
    }

    public AsyncLazyPipeline<TDestination> SelectMany<TIntermediate, TDestination>(
            Func<TSource, AsyncLazyPipeline<TIntermediate>> fn, Func<TSource, TIntermediate, TDestination> select)
    => SelectMany(a => fn(a).Select(b => select(a, b)));
}

public static class CreatePipeLine
{
    public static AsyncLazyPipeline<TDestination> With<TDestination>(Func<Task<TDestination>> fn)
    => new AsyncLazyPipeline<TDestination>(fn);
    
    public static AsyncLazyPipeline<TDestination> With<TDestination>(Func<TDestination> fn)
    => new AsyncLazyPipeline<TDestination>(() => fn().AsTask());

    public static AsyncLazyPipeline<TDestination> Return<TDestination>(TDestination value)
    => new AsyncLazyPipeline<TDestination>(() => value.AsTask());
}

FinalNoteAppender.cs

public interface IFinalNoteAppender
{
    AsyncLazyPipeline<string> AppendFinalText(string value);
}

public class FinalNoteAppender : IFinalNoteAppender
{
    public AsyncLazyPipeline<string> AppendFinalText(string value) 
    => CreatePipeLine.With(() => $"{value} final text.".AsTask());
}

IOperations.cs

public interface IOperations
{
    Task<string> GetFirstFileNote();
    Task<string> CombineWithSecondFileNote(string firstNote);
}

Item.cs

public class Item
{
    public string Notes { get; private set; }

    public Item AddNotes(string notes)
    {
        Notes = notes;
        return this;
    }
}

ItemService.cs

public class ItemService
{
    private IFinalNoteAppender NoteAppender { get; }
    private IOperations Operations { get; }

    public ItemService(IOperations operations, IFinalNoteAppender noteAppender)
    {
        NoteAppender = noteAppender;
        Operations = operations;
    }

    public Task<Item> AddNotesToItem()
    {
        var combineNotes = CreatePipeLine
                            .With(Operations.GetFirstFileNote)
                            .Select(Operations.CombineWithSecondFileNote);

        var appendLastNote = from notes in combineNotes
                             from finalNote in NoteAppender.AppendFinalText(notes)
                             select finalNote;

        var combinedOperations = appendLastNote.Select(new Item().AddNotes);

        return combinedOperations.Flatten();
    }
}

ObjectExtensions.cs

public static class ObjectExtensions
{
    public static Task<T> AsTask<T>(this T that) => Task.FromResult(that);
}

Thank you @JesseXia, I am glad you like this approach.

Thanks for this. Very cool.

Thank you @damiensawyer. I am glad you find it cool.