c# - Why isn't a CancellationToken included in the Task<T> monad? -

task<t> neatly holds "has started, might finished" computation, can composed other tasks, mapped functions, etc. in contrast, f# async monad holds "could start later, might running now" computation, along with cancellationtoken. in c#, typically have thread cancellationtoken through every function works task. why did c# team elect wrap computation in task monad, not cancellationtoken?

more or less, encapsulated implicit use of cancellationtoken c# async methods. consider this:

var cts = new cancellationtokensource(); cts.cancel(); var token = cts.token;  var task1 = new task(() => token.throwifcancellationrequested()); task1.start(); task1.wait(); // task in faulted state  var task2 = new task(() => token.throwifcancellationrequested(), token); task2.start(); task2.wait(); // task in cancelled state  var task3 = (new func<task>(async() => token.throwifcancellationrequested()))(); task3.wait(); // task in cancelled state 

for non-async lambda, had explicitly associate token task2 cancellation propagate correctly, providing argument new task() (or task.run). async lambda used task3, happens automatically part of async/await infrastructure code.

moreover, any token propagate cancellation async method, while non-async computational new task()/task.run lambda has same token passed task constructor or task.run.

of course, still have call token.throwifcancellationrequested() manually implement cooperative cancellation pattern. can't answer why c# , tpl teams decided implement way, guess aimed not over-complicate syntax of async/await yet keep flexible enough.

as f#, haven't looked @ generated il code of asynchronous workflow, illustrated in tomas petricek's blog post linked. yet, far understand, token automatically tested @ locations of workflow, corresponding await in c# (by analogy, might calling token.throwifcancellationrequested() manually after every await in c#). means cpu-bound work still won't cancelled immediately. otherwise, f# have emit token.throwifcancellationrequested() after every il instruction, quite substantial overhead.