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Re: [IndexedDB] Events and requests

From: Keean Schupke <keean@fry-it.com>
Date: Mon, 10 Jan 2011 22:26:28 +0000
Message-ID: <AANLkTimepQ_EnTDRRXsE+PWn3FCJNqvh_AyZH04x9cDR@mail.gmail.com>
To: Jonas Sicking <jonas@sicking.cc>
Cc: ben turner <bent.mozilla@gmail.com>, Jeremy Orlow <jorlow@chromium.org>, Webapps WG <public-webapps@w3.org>
Just to correct my cut and paste error, that was of course supposed to be:

var y = do {
    result1 <- db.transaction(["foo"]).objectStore("foo").getM(mykey1);
    result2 <- db.transaction(["foo"]).objectStore("foo").getM(mykey2);
    unit(result1 + result2);
}


Cheers,
Keean.

On 10 January 2011 22:24, Keean Schupke <keean@fry-it.com> wrote:

> Okay, sorry, the original change seemed sensible, I guess I didn't see how
> you got from there to promises.
>
>
> Here's some fun to think about as an alternative though:
>
>
> Interestingly the pattern of multiple callbacks, providing each callback is
> passed zero or one parameter forms a Monad.
>
> So for example if 'unit' is the constructor for the object returned from
> "get" then onsuccess it 'bind' and I can show that these obey the 3 monad
> laws. Allowing composability of callbacks. So you effectively have:
>
> var x = db.transaction(["foo"]).objectStore("foo").getM(mykey);
>
> var y =
> db.transaction(["foo"]).objectStore("foo").getM(mykey1).bind(function(result1)
> {
>
>  db.transaction(["foo"]).objectStore("foo").getM(mykey2).bind(function(result2)
> {
>         unit(result1 + result2);
>     });
> });
>
> The two objects returned "x" and "y" are both the same kind of object. y
> represents the sum or concatination of the results of the lookups "mykey1"
> and "mykey2". You would use it identically to using the result of a single
> lookup:
>
> x.bind(function(result) {... display the result of a single lookup ...});
>
> y.bind(function(result) {... display the result of both lookups ...});
>
>
> If we could then have some syntactic sugar for this like haskell's do
> notation we could write:
>
> var y = do {
>     db.transaction(["foo"]).objectStore("foo").getM(mykey1);
>     result1 <- db.transaction(["foo"]).objectStore("foo").getM(mykey2);
>     result2 <- db.transaction(["foo"]).objectStore("foo").getM(mykey2);
>     unit(result1 + result2);
> }
>
> Which would be a very neat way of chaining callbacks...
>
>
> Cheers,
> Keean.
>
>
> On 10 January 2011 22:00, Keean Schupke <keean@fry-it.com> wrote:
>
>> Whats wrong with callbacks? To me this seems an unnecessary complication.
>>
>> Presumably you would do:
>>
>> var promise = db.transaction(["foo"]).objectStore("foo").get(mykey);
>> var result = promise.get();
>> if (!result) {
>>     promise.onsuccess(function(res) {...X...});
>> } else {
>>     ...Y...
>> }
>>
>>
>> So you end up having to duplicate code at X and Y to do the same thing
>> directly or in the context of a callback. Or you define a function to
>> process the result:
>>
>> var f = function(res) {...X...};
>> var promise = db.transaction(["foo"]).objectStore("foo").get(mykey);
>> var result = promise.get();
>> if (!result) {
>>      promise.onsuccess(f);
>> } else {
>>     f(result)
>> };
>>
>> But in which case what advantage does all this extra clutter offer over:
>>
>> db.transaction(["foo"]).objectStore("foo").get(mykey).onsuccess(function(res)
>> {...X...});
>>
>>
>> I am just wondering whether the change is worth the added complexity?
>>
>>
>> Cheers,
>> Keean.
>>
>>
>> On 10 January 2011 21:31, Jonas Sicking <jonas@sicking.cc> wrote:
>>
>>> I did some outreach to developers and while I didn't get a lot of
>>> feedback, what I got was positive to this change.
>>>
>>> The basic use-case that was brought up was implementing a promises
>>> which, as I understand it, works similar to the request model I'm
>>> proposing. I.e. you build up these "promise" objects which represent a
>>> result which may or may not have arrived yet. At some point you can
>>> either read the value out, or if it hasn't arrived yet, register a
>>> callback for when the value arrives.
>>>
>>> It was pointed out that this is still possible with how the spec is
>>> now, but it will probably result in that developers will come up with
>>> conventions to set the result on the request themselves. This wouldn't
>>> be terribly bad, but also seems nice if we can help them.
>>>
>>> / Jonas
>>>
>>> On Mon, Jan 10, 2011 at 8:13 AM, ben turner <bent.mozilla@gmail.com>
>>> wrote:
>>> > FWIW Jonas' proposed changes have been implemented and will be
>>> > included in Firefox 4 Beta 9, due out in a few days.
>>> >
>>> > -Ben
>>> >
>>> > On Fri, Dec 10, 2010 at 12:47 PM, Jonas Sicking <jonas@sicking.cc>
>>> wrote:
>>> >> I've been reaching out to get feedback, but no success yet. Will
>>> re-poke.
>>> >>
>>> >> / Jonas
>>> >>
>>> >> On Fri, Dec 10, 2010 at 4:33 AM, Jeremy Orlow <jorlow@chromium.org>
>>> wrote:
>>> >>> Any additional thoughts on this?  If no one else cares, then we can
>>> go with
>>> >>> Jonas' proposal (and we should file a bug).
>>> >>> J
>>> >>>
>>> >>> On Thu, Nov 11, 2010 at 12:06 PM, Jeremy Orlow <jorlow@chromium.org>
>>> wrote:
>>> >>>>
>>> >>>> On Tue, Nov 9, 2010 at 11:35 AM, Jonas Sicking <jonas@sicking.cc>
>>> wrote:
>>> >>>>>
>>> >>>>> Hi All,
>>> >>>>>
>>> >>>>> One of the things we briefly discussed at the summit was that we
>>> >>>>> should make IDBErrorEvents have a .transaction. This since we are
>>> >>>>> allowing you to place new requests from within error handlers, but
>>> we
>>> >>>>> currently provide no way to get from an error handler to any useful
>>> >>>>> objects. Instead developers will have to use closures to get to the
>>> >>>>> transaction or other object stores.
>>> >>>>>
>>> >>>>> Another thing that is somewhat strange is that we only make the
>>> result
>>> >>>>> available through the success event. There is no way after that to
>>> get
>>> >>>>> it from the request. So instead we use special event interfaces
>>> with
>>> >>>>> supply access to source, transaction and result.
>>> >>>>>
>>> >>>>> Compare this to how XMLHttpRequests work. Here the result and error
>>> >>>>> code is available on the request object itself. The 'load' event,
>>> >>>>> which is equivalent to our 'success' event didn't supply any
>>> >>>>> information until we recently added progress event support. But
>>> still
>>> >>>>> it only supplies information about the progress, not the actual
>>> value
>>> >>>>> itself.
>>> >>>>>
>>> >>>>> One thing we could do is to move
>>> >>>>>
>>> >>>>> .source
>>> >>>>> .transaction
>>> >>>>> .result
>>> >>>>> .error
>>> >>>>>
>>> >>>>> to IDBRequest. Then make "success" and "error" events be simple
>>> events
>>> >>>>> which only implement the Event interface. I.e. we could get rid of
>>> the
>>> >>>>> IDBEvent, IDBSuccessEvent, IDBTransactionEvent and IDBErrorEvent
>>> >>>>> interfaces.
>>> >>>>>
>>> >>>>> We'd still have to keep IDBVersionChangeEvent, but it can inherit
>>> >>>>> Event directly.
>>> >>>>>
>>> >>>>> The request created from IDBFactory.open would return a IDBRequest
>>> >>>>> where .transaction and .source is null. We already fire a IDBEvent
>>> >>>>> where .source is null (actually, the spec currently doesn't define
>>> >>>>> what the source should be I see now).
>>> >>>>>
>>> >>>>>
>>> >>>>> The only major downside with this setup that I can see is that the
>>> >>>>> current syntax:
>>> >>>>>
>>> >>>>> db.transaction(["foo"]).objectStore("foo").get(mykey).onsuccess =
>>> >>>>> function(e) {
>>> >>>>>  alert(e.result);
>>> >>>>> }
>>> >>>>>
>>> >>>>> would turn into the slightly more verbose
>>> >>>>>
>>> >>>>> db.transaction(["foo"]).objectStore("foo").get(mykey).onsuccess =
>>> >>>>> function(e) {
>>> >>>>>  alert(e.target.result);
>>> >>>>> }
>>> >>>>>
>>> >>>>> (And note that with the error handling that we have discussed, the
>>> >>>>> above code snippets are actually plausible (apart from the alert()
>>> of
>>> >>>>> course)).
>>> >>>>>
>>> >>>>> The upside that I can see is that we behave more like
>>> XMLHttpRequest.
>>> >>>>> It seems that people currently follow a coding pattern where they
>>> >>>>> place a request and at some later point hand the request to another
>>> >>>>> piece of code. At that point the code can either get the result
>>> from
>>> >>>>> the .result property, or install a onload handler and wait for the
>>> >>>>> result if it isn't yet available.
>>> >>>>>
>>> >>>>> However I only have anecdotal evidence that this is a common coding
>>> >>>>> pattern, so not much to go on.
>>> >>>>
>>> >>>> Here's a counter proposal:  Let's add .transaction, .source, and
>>> .result
>>> >>>> to IDBEvent and just specify them to be null when there is no
>>> transaction,
>>> >>>> source, and/or result.  We then remove readyState from IDBResult as
>>> it
>>> >>>> serves no purpose.
>>> >>>> What I'm proposing would result in an API that's much more similar
>>> to what
>>> >>>> we have at the moment, but would be a bit different than XHR.  It is
>>> >>>> definitely good to have similar patterns for developers to follow,
>>> but I
>>> >>>> feel as thought the model of IndexedDB is already pretty different
>>> from XHR.
>>> >>>>  For example, method calls are supplied parameters and return an
>>> IDBRequest
>>> >>>> object vs you using new to create the XHR object and then making
>>> method
>>> >>>> calls to set it up and then making a method call to start it.  In
>>> fact, if
>>> >>>> you think about it, there's really not that much XHR and IndexedDB
>>> have in
>>> >>>> common except that they use event handlers.
>>> >>>> As for your proposal, let me think about it for a bit and forward it
>>> on to
>>> >>>> some people I know who are playing with IndexedDB already.
>>> >>>> J
>>> >>>
>>> >>
>>> >>
>>> >
>>>
>>>
>>
>
Received on Monday, 10 January 2011 22:27:02 GMT

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