And draft ES6 tried for monadic, but compatibility with Promises libraries
(more than "convenience") prevailed.
/be
On Sun, Feb 7, 2016 at 11:35 AM Raphael Mu <encryptedre...@gmail.com> wrote:
> In theory it's possible, but Promise.resolve automatically joins Promises
> for the sake of ergonomics.
>
> On Sun, Feb 7, 2016 at 1:15 PM Jordan Harband <ljh...@gmail.com> wrote:
>
>> How is Promise an instance of Monad, if you can't ever have a Promise of
>> a Promise?
>>
>> On Sun, Feb 7, 2016 at 9:59 AM, Raphael Mu <encryptedre...@gmail.com>
>> wrote:
>>
>>> The `a < -b` issue could be solved by using a different operator, like
>>> `</` or `<#`.
>>>
>>> On Sun, Feb 7, 2016 at 12:35 PM Kevin Smith <zenpars...@gmail.com>
>>> wrote:
>>>
>>>> Why not just use await within `async do`?
>>>>
>>>> On 12:19PM, Sun, Feb 7, 2016 Rick Waldron <waldron.r...@gmail.com>
>>>> wrote:
>>>>
>>>>> What does this do?
>>>>>
>>>>>
>>>>> let finalPromise = do {
>>>>> let a;
>>>>> a <- b;
>>>>> }
>>>>>
>>>>>
>>>>> Currently, that's an expression that means "a less than negated b"
>>>>>
>>>>> Rick
>>>>>
>>>>> On Sun, Feb 7, 2016 at 12:07 PM Raphael Mu <encryptedre...@gmail.com>
>>>>> wrote:
>>>>>
>>>>>> The ES Promise is an instance of Monad, a property that implies a
>>>>>> much more concise and expressive syntax for using Promise, by exploiting
>>>>>> its monadic properties. I've seen a lot of people complain about Promises
>>>>>> having too clumsy a syntax, and likewise for async/await.
>>>>>>
>>>>>> We now have the do-notation proposal (
>>>>>> http://wiki.ecmascript.org/doku.php?id=strawman:do_expressions), and
>>>>>> monadic assignment would fit well into the syntax.
>>>>>>
>>>>>> The extension would allow use of `<-` within a do-expression for
>>>>>> binding Promise values to variables, and the computation would behave
>>>>>> most
>>>>>> similarly to the Either monad in Haskell (in the following code, if
>>>>>> promiseA or promiseB reject, the result of the entire expression would
>>>>>> be a
>>>>>> rejected Promise).
>>>>>>
>>>>>> (monadic extension)
>>>>>> ```javascript
>>>>>> let finalPromise = do {
>>>>>> let a <- promiseA;
>>>>>> let b <- promiseB;
>>>>>> let c = f(a, b);
>>>>>> g(a, b, c)
>>>>>> }
>>>>>> ```
>>>>>>
>>>>>> (desugared to async/await)
>>>>>> ```javascript
>>>>>> let finalPromise = (async () => {
>>>>>> let a = await promiseA;
>>>>>> let b = await promiseB;
>>>>>> let c = f(a, b);
>>>>>> return g(a, b, c);
>>>>>> })();
>>>>>> ```
>>>>>>
>>>>>> (desugared to ES6)
>>>>>> ```javascript
>>>>>> let finalPromise = promiseA.then(a =>
>>>>>> promiseB.then(b => {
>>>>>> let c = f(a, b);
>>>>>> return g(a, b, c);
>>>>>> })
>>>>>> );
>>>>>> ```
>>>>>>
>>>>>> The do-notation would apply Promise.resolve to the last expression
>>>>>> (like async and Promise#then), effectively returning another Promise. The
>>>>>> current proposal doesn't specify this behavior, but there are two ways
>>>>>> about this collision:
>>>>>>
>>>>>> 1. use an explicit version of do-notation, e.g. `async do { ... }` or
>>>>>> `do* { ... }`
>>>>>> 2. revise the do-notation proposal to always apply Promise.resolve to
>>>>>> the last expression
>>>>>>
>>>>>> Both choices give us a new Promise literal for free: `do* { x }` (1)
>>>>>> or `do { x }` (2) would be functionally equivalent to
>>>>>> `Promise.resolve(x)`.
>>>>>>
>>>>>> The idea was briefly mentioned several years ago, but didn't attract
>>>>>> much attention (
>>>>>> https://mail.mozilla.org/pipermail/es-discuss/2012-March/021624.html
>>>>>> ).
>>>>>>
>>>>>> This is an earlier draft of this proposal:
>>>>>> https://github.com/edge/es-monadic
>>>>>> _______________________________________________
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>>>>>> es-discuss@mozilla.org
>>>>>> https://mail.mozilla.org/listinfo/es-discuss
>>>>>>
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