F# Async Cache

Async.Cache.fs

let cache (a:Async<'a>) : Async<'a> =
  let tcs = TaskCompletionSource<'a>()
  let state = ref 0
  async {
   if (Interlocked.CompareExchange(state, 1, 0) = 0) then
     Async.StartWithContinuations(
       a, 
       tcs.SetResult, 
       tcs.SetException, 
       (fun _ -> tcs.SetCanceled()))
   return! tcs.Task |> Async.AwaitTask }

Passing in the cancellation token:

open System.Threading
open System.Threading.Tasks

let cache (workflow : Async<'a>) : Async<'a> =
  let tcs = TaskCompletionSource<'a>()
  let mutable state = 0

  async {
    if state = 0 && Interlocked.CompareExchange(&state, 1, 0) = 0 then
      let! ct = Async.CancellationToken

      Async.StartWithContinuations(
        workflow,
        tcs.SetResult,
        tcs.SetException,
        (fun _ -> tcs.SetCanceled()),
        ct
      )

    return! Async.AwaitTask tcs.Task
  }