FSharp.Control.TaskSeq 0.2.2

TaskSeq

An implementation of IAsyncEnumerable<'T> as a computation expression: taskSeq { ... } with an accompanying TaskSeq module.

Table of contents

• Overview
  • taskSeq computation expressions
  • Examples
  • TaskSeq module functions
• More information
  • Futher reading IAsyncEnumerable
  • Futher reading on resumable state machines
  • Further reading on computation expressions

Overview

The IAsyncEnumerable interface was added to .NET in .NET Core 3.0 and is part of .NET Standard 2.1. The main use-case was for iterative asynchronous enumeration over some resource. For instance, an event stream or a REST API interface with pagination, asynchronous reading over a list of files and accumulating the results, where each action can be modeled as a MoveNextAsync call on the IAsyncEnumerator<'T> given by a call to GetAsyncEnumerator().

Since the introduction of task in F# the call for a native implementation of task sequences has grown, in particular because proper iterating over an IAsyncEnumerable has proven challenging, especially if one wants to avoid mutable variables. This library is an answer to that call and implements the same resumable state machine approach with taskSeq.

As with seq and Seq, this library comes with a bunch of well-known collection functions, like TaskSeq.empty/isEmpty or TaskSeq.map/iter/collect and TaskSeq.find/pick/choose/filter. Where applicable, these come with async variants, like TaskSeq.mapAsync/iterAsync/collectAsync and TaskSeq.findAsync/pickAsync/chooseAsync/filterAsync, which allows the apply function to be asynchronous.

See below for a full list of currently implemented functions.

taskSeq computation expressions

The taskSeq computation expression can be used just like using seq. On top of that, it adds support for working with tasks through let! and looping over a normal or asynchronous sequence (one that implements IAsyncEnumerable<'T>'). You can use yield! and yield and there's support for use and use!, try-with and try-finally and while loops within the task sequence expression:

Examples

open System.IO

open FSharp.Control

// singleton is fine
let hello = taskSeq { yield "Hello, World!"" }

// can be mixed with normal sequences
let oneToTen = taskSeq { yield! [1..10] }

// returns a delayed sequence of IAsyncEnumerable<string>
let allFilesAsLines() = taskSeq {
    let files = Directory.EnumerateFiles(@"c:\temp")
    for file in files do
        // await
        let! contents = File.ReadAllLinesAsync file
        // return all lines
        yield! contents
}

let write file = 
    allFilesAsLines()
    
    // synchronous map function on asynchronous task sequence
    |> TaskSeq.map (fun x -> x.Replace("a", "b"))

    // asynchronous map
    |> TaskSeq.mapAsync (fun x -> task { return "hello: " + x })
    
    // asynchronous iter
    |> TaskSeq.iterAsync (fun data -> File.WriteAllTextAsync(fileName, data))


// infinite sequence
let feedFromTwitter user pwd = taskSeq {
    do! loginToTwitterAsync(user, pwd)
    while true do
       let! message = getNextNextTwitterMessageAsync()
       yield message
}

TaskSeq module functions

We are working hard on getting a full set of module functions on TaskSeq that can be used with IAsyncEnumerable sequences. Our guide is the set of F# Seq functions in F# Core and, where applicable, the functions provided from AsyncSeq. Each implemented function is documented through XML doc comments to provide the necessary context-sensitive help.

The following is the progress report:

Note 1

These functions require a form of pre-materializing through TaskSeq.cache, similar to the approach taken in the corresponding Seq functions. It doesn't make much sense to have a cached async sequence. However, AsyncSeq does implement these, so we'll probably do so eventually as well.

Note 2

Because of the async nature of TaskSeq sequences, iterating from the back would be bad practice. Instead, materialize the sequence to a list or array and then apply the xxxBack iterators.

Note 3

The motivation for readOnly in Seq is that a cast from a mutable array or list to a seq<_> is valid and can be cast back, leading to a mutable sequence. Since TaskSeq doesn't implement IEnumerable<_>, such casts are not possible.

More information

Futher reading IAsyncEnumerable

• A good C#-based introduction can be found in this blog.
• An MSDN article written shortly after it was introduced.
• Converting a seq to an IAsyncEnumerable demo gist as an example, though TaskSeq contains many more utility functions and uses a slightly different approach.
• If you're looking for using IAsyncEnumerable with async and not task, the excellent AsyncSeq library should be used. While TaskSeq is intended to consume async just like task does, it won't create an AsyncSeq type (at least not yet). If you want classic Async and parallelism, you should get this library instead.

Futher reading on resumable state machines

• A state machine from a monadic perspective in F# can be found here, which works with the pre-F# 6.0 non-resumable internals.
• The original RFC for F# 6.0 on resumable state machines
• The original RFC for introducing task to F# 6.0.
• A pre F# 6.0 TaskBuilder that motivated the task CE later added to F# Core.
• MSDN Documentation on task and async.

Further reading on computation expressions

• Docs on MSDN form a good summary and starting point.
• Arguably the best step-by-step tutorial to using and building computation expressions by Scott Wlaschin.