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High performance System.Threading.(Value)Task computation expressions for F#
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Ply.fsproj Update Apr 14, 2019


Ply is a high performance TPL library for F#.

The goal of Ply is to be a very low overhead Task abstraction like it is in C#.


see benchmark code

Method Mean Error StdDev Ratio RatioSD Gen 0/1k Op Gen 1/1k Op Gen 2/1k Op
C# Async Await 24.59 us 0.8028 us 0.8923 us 1.00 0.00 0.2136 - -
Ply 24.60 us 1.1610 us 1.3371 us 1.00 0.07 0.3052 - -
TaskBuilder.fs v2.1.0 26.86 us 0.6751 us 0.6932 us 1.09 0.03 0.5798 - -

Allocated Memory/Op is removed as it isn't correct on .NET Core, Gen 0/1k Op is the relevant metric.


Ply comes bundled with these builders:

builder return type tfm namespace
task Task<'T> netstandard2.0, netcoreapp2.1 FSharp.Control.Tasks.Builders
vtask ValueTask<'T> netcoreapp2.1 FSharp.Control.Tasks.Builders
unitTask Task netstandard2.0, netcoreapp2.1 FSharp.Control.Tasks.Builders
unitVtask ValueTask netcoreapp2.1 FSharp.Control.Tasks.Builders
uvtask ValueTask<'T> netcoreapp2.1 FSharp.Control.Tasks.Builders.Unsafe
uunitTask Task netcoreapp2.1 FSharp.Control.Tasks.Builders.Unsafe
uunintVtask ValueTask netcoreapp2.1 FSharp.Control.Tasks.Builders.Unsafe
uply Ply<'T> netstandard2.0,netcoreapp2.1 FSharp.Control.Tasks.Builders.Unsafe

More information on when to use which builder:

builder description
vtask Near zero allocation CE, allocates one object for the execution bubble at the start, and two objects per bind if the Task we bind to isn't completed yet.
unitTask As the TPL doesn't know about F#'s unit type Task.FromResult(()) won't ever return a cached task. On netcoreapp we can check for a succesful completion instead to directly return a CompletedTask removing the task allocation.
unitVtask CE shorthand for doing if vtask.IsCompletedSuccessfully then ValueTask() else ValueTask(vtask.AsTask() :> Task)
uvtask An unsafe version of vtask and one of the few zero allocation* CEs Ply comes with. Read about the trade-off under execution bubble
uunitTask An unsafe version of unitTask and one of the few zero allocation* CEs Ply comes with. Read about the trade-off under execution bubble
uunitVtask An unsafe version of unitVtask and one of the few zero allocation* CEs Ply comes with. Read about the trade-off under execution bubble
uply Can be enqueued directly onto the caller's state machine, skips Task and execution bubble.

*zero allocation only when any Task (or Task-like) you bind against is already completed.

Execution bubble

An execution bubble is made by any C# async-await method for capturing and restoring async local and synchronization context changes. Any changes would otherwise escape onto the caller context.

It's rare that methods do anything with async locals or synchronization contexts, even in C#. So if you know anything you use doesn't do that either then there's nothing inherently unsafe about using the Unsafe CEs as you don't need any execution bubble for correctness.

Special Thanks

Thanks to @gusty for very valuable SRTP advice, it helped me tremendously to narrow down what specifically was wrong about an earlier approach I took.

Thanks to @rspeele TaskBuilder.fs was a great inspiration in developing Ply.

Next Steps and Improvements

  • Finish up the experimental branch.
  • On master we are at 2 allocations per bind, which are the focus of upcoming work. Then there are the few constant factor allocations which are inevitable and equivalent to C# semantics for async methods.
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