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Then that's a breaking change of the someBigFunction()s interface and it should be treated with care, not just quickly patching on a promise and hope for the best. Javascript is single threaded so when you wrote someBigFunction() you didn't have to think of parallel side effects. If you suddenly pause the execution of someBigFunc() and allow the user to mess around with the interface, the global state that someBigFunc() is working on might become changed.

Simply slabbing on a promise inside such a function only works if the function is pure to start with. Usually that's not the case with slow functions in javascript since the slowest thing in javascript is messing with the DOM, i.e global state.

The fact that I changed a calculation from a constant to reading its value from indexeddb shouldnt need to be an intrusive change. It is makes things more likely to break, there isnt some 'oh you decided to do io so you deserve to do a big refactor' point

And this is why handling state in imperative programs is such a mess. You are fundamentally changing the way state is treated in that application - why shouldn't it require a massive refactor?

Functional programs don't make this easier.

And don't say "monad transformer stacks" somehow solve this problem in an easier way.

Not necessarily because it is functional, but Erlang makes this vastly easier.

I find the "oh you changed tiny piece of logic, you should totally have to refactor every line of code that follows it" very strange. It obviously sucks, there are far better ways to handle it and they are slowly making it into the language.

Erlang makes it easier because it enforces no shared state between actors, and each actor has it's own independent thread of control.

Haskell definitely makes it easier. You can use preemptive green threads with almost all the ease of cooperatively multitasked code because of the prevalence of immutability.

You get the performance benefits of non-blocking code. The simplicity benefits of blocking code. And (almost) none of the threading hell you get in imperative languages.

Are they really preemptive? How does the scheduler decide when to switch?

Yeah, they are preemptive (though there had been a long-standing bug where threads that have no allocations don't get preempted, I believe it is fixed now).

This is some documentation of the scheduler: http://blog.ezyang.com/2013/01/the-ghc-scheduler/

Fair enough. It might be a breaking change but wouldn't it be nice if instead of having to rewrite everything into continuation passing style (a boring and error prone process!) you could just do something simple like

    var a = await someBigFunc()

      a <- someBigFunc()
Javascript in its current state is not built for supporting CPS programming very well. You often end up needing to reimplement control flow primitives like while loops, break statements and exception handling since those don't play nice with async stuff.

This syntax already exists in LiveScript (http://livescript.net/#backcalls) which compiles to straightforward JS.

    a <- doFirstThing()
    b, c <- doSecondThing(a)
    doFinalThing(b + c)

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