- From: Mark S. Miller <erights@google.com>
- Date: Mon, 6 May 2013 11:19:45 -0700
- To: "Tab Atkins Jr." <jackalmage@gmail.com>
- Cc: Jonas Sicking <jonas@sicking.cc>, Sam Tobin-Hochstadt <samth@ccs.neu.edu>, Domenic Denicola <domenic@domenicdenicola.com>, "public-script-coord@w3.org" <public-script-coord@w3.org>
- Message-ID: <CABHxS9g5EAMfiG+fTvQjRyz06jsNYaM=bUSRnJwE79BBHpSeqQ@mail.gmail.com>
On Mon, May 6, 2013 at 10:57 AM, Tab Atkins Jr. <jackalmage@gmail.com>wrote: > On Mon, May 6, 2013 at 10:39 AM, Mark S. Miller <erights@google.com> > wrote: > > I'm surprised that no one has come up with *any* use cases. As I > mentioned > > before, I have encountered some use cases that individually seem to argue > > for promises-for-promises. But I don't find them compelling. Given the > > absence of promises-for-promises, these can easily work around their > > absence. Here are two use cases. One of them even cuts both ways. > > > > * The generic contract host defined in Figure 3 of > > <http://research.google.com/pubs/pub40673.html> requires its contract > > function to be a function of resolved arguments. It cannot handle a > contract > > function that could validly take a pending promise as argument because it > > uses a Q.all to determine when all the arguments have arrived. > > In this case, you mean that the contract function could take a > fulfilled promise for a pending promise, but it couldn't do so it > promises were auto-flattened (so that it was just a pending promise)? > No. The problem is created merely by defining and trying to use a contract function that takes a pending promise as argument. Promise parameters aside, if a contract function takes a parameter x, the Q.all within makeContractHost will be .then-ing a promise-for-x. If promises-for-promises were possible, then x could be a pending promise and the Q.all would be .then-ing a promise-for-a-pending promise. When this outer promise becomes a fulfilled-promise-for-a-pending-promise, the Q.all would fire providing the contract function the outer promise's fulfillment value, which is the pending promise. Given that promises-for-promises are not possible, the only working contract functions that can be provided to makeContractHost are those whose arguments are anything other than pending promises. > > > * The Infinite Queue at > > < > http://wiki.ecmascript.org/doku.php?id=strawman:concurrency#infinite_queue > > > > cuts both ways. If the producer enqueues a pending promise, the consumer > > cannot detect this as different from the producer not yet having enqueued > > anything. For most purposes, this is the more convenient behavior. > However, > > for flow control purposes, the consumer might want to limit how far > ahead of > > the producer it dequeues, and this ambiguity would cause the consumer to > > stall inappropriately. > > I believe this relies on auto-flattening, no? "front ! head" returns > a promise for front.head, which is assumed to be a plain value. If > you enqueue a promise, assuming monadic flattening, I think you should > just get the promise back out. (However, I'm not entirely clear on > the semantics of !.) > "p ! foo" is just syntactic sugar for "Q(p).get('foo')". In the absence of extended or remote promises, this is essentially equivalent to Q(p).then(v => v.foo) > > ~TJ > -- Cheers, --MarkM
Received on Monday, 6 May 2013 18:20:12 UTC