4 ms·

What is the role of the `async` keyword in Dart? I couldn't understand it from the article. > the outcome of the method is a Future Isn't the function signatu

What is the role of the `async` keyword in Dart? I couldn't understand it from the article. > the outcome of the method is a Future Isn't the function signature sufficient to express this? > it runs synchronously the code of that method up to the very first await keyword How is this different from non-async methods? > the next line of code will be run as soon as the Future, referenced by the await keyword, will have completed But the await keyword is in the caller which is not statically known. What is the difference between an async method and a method that returns a Future?

pxndx 8y ago
> What is the difference between an async method and a method that returns a Future? There's no difference for callers of the method, but you can only use await in methods that are async.
hbhakhra 8y ago
An async method can use await within it. A method can return a future but not be able to use the await keyword.
- ridiculous_fish 8y ago
  What is the point of this restriction? Why not just allow all Future-returning functions to use await?
  - qqqwww 8y ago
    Async methods also allow you to return values which will be returned as futures. For example: return 4; If the function is marked async, the return value should be Future<int>. If you allow a non-async method use await, you get into this situation: Future<int> foo(){ int result = await something (); // How to return "result" and still make compiler happy? }
    - ridiculous_fish 8y ago
      Why not just some generic function T -> Future<T>? It sounds like the async keyword allows for a particular implicit type conversion within the function body, but on return statements only. That's a pretty thin justification for a keyword.
krzat 8y ago
It works like in every other language that supports async/await. Compiler will convert code inside async method into a state machine to prevent it from locking thread during await calls. If you don't uses async you have to do compiler's job yourself.
- ridiculous_fish 8y ago
  But why do we need a new keyword to express this? Isn't the return type sufficient?
  - krzat 8y ago
    No, because you can write async functions without using async/await. Using callback for example: Future<int> foo() { return Future.delayed(Duration(seconds: 1)).then((x) => 1); } In this case there is no compiler magic needed. If you ask why async keyword is needed if we can just infer it from await inside the method body - probably only for readability.
    - ridiculous_fish 8y ago
      async is part of the method signature, the public facing part of the method. But it controls an implementation detail, namely whether the function body may use await (as you say asynchronous functions are not required to use await). I found a link [1] that described async as a compatibility affordance in C# 5: legacy code may use await as a variable name, so async opts into the contextual await keyword. But Dart doesn't have such legacy, so the keyword doesn't seem as well motivated. https://blogs.msdn.microsoft.com/ericlippert/2010/11/11/asynchrony-in-c-5-part-six-whither-async/ https://blogs.msdn.microsoft.com/ericlippert/2010/11/11/asyn...

pxndx 8y ago

> What is the difference between an async method and a method that returns a Future? There's no difference for callers of the method, but you can only use await in methods that are async.

hbhakhra 8y ago

An async method can use await within it. A method can return a future but not be able to use the await keyword.

ridiculous_fish 8y ago
What is the point of this restriction? Why not just allow all Future-returning functions to use await?
- qqqwww 8y ago
  Async methods also allow you to return values which will be returned as futures. For example: return 4; If the function is marked async, the return value should be Future<int>. If you allow a non-async method use await, you get into this situation: Future<int> foo(){ int result = await something (); // How to return "result" and still make compiler happy? }
  - ridiculous_fish 8y ago
    Why not just some generic function T -> Future<T>? It sounds like the async keyword allows for a particular implicit type conversion within the function body, but on return statements only. That's a pretty thin justification for a keyword.

krzat 8y ago

It works like in every other language that supports async/await. Compiler will convert code inside async method into a state machine to prevent it from locking thread during await calls. If you don't uses async you have to do compiler's job yourself.

ridiculous_fish 8y ago
But why do we need a new keyword to express this? Isn't the return type sufficient?
- krzat 8y ago
  No, because you can write async functions without using async/await. Using callback for example: Future<int> foo() { return Future.delayed(Duration(seconds: 1)).then((x) => 1); } In this case there is no compiler magic needed. If you ask why async keyword is needed if we can just infer it from await inside the method body - probably only for readability.
  - ridiculous_fish 8y ago
    async is part of the method signature, the public facing part of the method. But it controls an implementation detail, namely whether the function body may use await (as you say asynchronous functions are not required to use await). I found a link [1] that described async as a compatibility affordance in C# 5: legacy code may use await as a variable name, so async opts into the contextual await keyword. But Dart doesn't have such legacy, so the keyword doesn't seem as well motivated. https://blogs.msdn.microsoft.com/ericlippert/2010/11/11/asynchrony-in-c-5-part-six-whither-async/ https://blogs.msdn.microsoft.com/ericlippert/2010/11/11/asyn...