The 'Vsync' (AKA, 'Variable Synchronization') computation expression that coheres into the Sync comp. expr. when SYNC is #defined, and into the Async comp. expr. otherwise.

Vsync.fs

namespace YourNamespaceHere
open System
open System.Diagnostics
open System.Threading
open System.Threading.Tasks

/// The 'Sync' builder for evaluating expressions in a synchronous style to aid debugging.
type [<Sealed>] Sync () =
    member inline this.Bind (x, f) = f x
    member inline this.Return x = x
    member inline this.ReturnFrom x = x
    member inline this.Zero () = ()
    member inline this.Combine ((), _) = ()
    member inline this.Yield x = x
    member inline this.YieldFrom x = x
    member inline this.Run f = f ()
    member inline this.Delay f = fun () -> f ()
    member inline this.For (s, f) = for x in s do f x
    member inline this.While (g, b) = while g () do this.Run b
    member inline this.TryWith (b, h) = try this.Run b with exn -> h exn
    member inline this.TryFinally (b, c) = try this.Run b finally c ()
    member inline this.Using (d, b) = use u = d in b u
    static member inline Ignore _ = ()
    static member inline Sleep (t : int) = Thread.Sleep t
    static member inline RunSynchronously x = x
    static member inline Start x = x
    static member inline StartAsTask x : _ Task = Task.Factory.StartNew (fun () -> x)
    static member inline AwaitTask (t : _ Task) = t.Result
    static member inline Catch x = Choice1Of2 x
    static member inline Parallel (s : _ seq) = s |> Seq.map (fun x -> Thread.Sleep 10; x) |> List.ofSeq |> Seq.ofList // Thread.Sleep keeps from soaking CPU

[<AutoOpen; CompilationRepresentation (CompilationRepresentationFlags.ModuleSuffix)>]
module Sync =

    /// The Sync builder instance.
    /// Used like: sync { return 0 }
    let sync = Sync ()

/// The 'Vsync' (AKA, 'Variable Synchronization') cexpr that coheres into the Sync cexpr when SYNC is #defined, and
/// into the Async cexpr otherwise.
/// TODO: forward documentation from FSharp.Core.
type [<Sealed>] Vsync () =

    member inline this.Builder =
#if SYNC
        sync
#else
        async
#endif

    member inline this.Bind (m, f) = this.Builder.Bind (m, f)
    member inline this.Return x = this.Builder.Return x
    member inline this.ReturnFrom m = this.Builder.ReturnFrom m
    member inline this.Zero () = this.Builder.Zero ()
    member inline this.Combine (a, b) = this.Builder.Combine (a, b)
#if SYNC
    member inline this.Yield x = this.Builder.Yield x
    member inline this.YieldFrom x = this.Builder.YieldFrom x
    member inline this.Run f = this.Builder.Run f
#endif
    member inline this.Delay f = this.Builder.Delay f
    member inline this.For (m, f) = this.Builder.For (m, f)
    member inline this.While (g, b) = this.Builder.While (g, b)
    member inline this.TryWith (b, h) = this.Builder.TryWith (b, h)
    member inline this.TryFinally (b, c) = this.Builder.TryFinally (b, c)
    member inline this.Using (d, b) = this.Builder.Using (d, b)

    static member inline Ignore m =
#if SYNC
        Sync.Ignore m
#else
        Async.Ignore m
#endif

    static member inline Sleep t =
#if SYNC
        Sync.Sleep t
#else
        Async.Sleep t
#endif

    static member inline RunSynchronously m =
#if SYNC
        Sync.RunSynchronously m
#else
        Async.RunSynchronously m
#endif

    static member inline Start m =
#if SYNC
        Sync.Start m
#else
        Async.Start m
#endif

    static member inline StartAsTask m : _ Task =
#if SYNC
        Sync.StartAsTask m
#else
        Async.StartAsTask m
#endif

    static member inline AwaitTask (t : _ Task) =
#if SYNC
        Sync.AwaitTask t
#else
        Async.AwaitTask t
#endif

    /// Catch an exception while binding m.
    /// NOTE: the semantics between a Sync.Catch and an Async.Catch are remarkably different. Most notably, Sync.Catch
    /// is just the application of Choice1Of2 to m since there is no evaluation taking place inside of it.
    static member inline Catch m =
#if SYNC
        Sync.Catch m
#else
        Async.Catch m
#endif

    static member inline Parallel s =
#if SYNC
        Sync.Parallel s
#else
        Async.Parallel s
#endif

[<AutoOpen; CompilationRepresentation (CompilationRepresentationFlags.ModuleSuffix)>]
module Vsync =

    /// The Vsync builder instance.
    /// Used like: vsync { return 0 }
    let vsync = Vsync ()

/// The Vsync alias.
type Vsync<'a> =
#if SYNC
    'a
#else
    Async<'a>
#endif