Promise

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A [[promise]], in [[E]], is a kind of [[unresolved ref|unresolved]] [[eventual ref]]. It is a [[ref]] which will [[resolve]] to some other ref later; therefore all [[send]]s it receives are stored, then forwarded at the time of the resolution. Every promise has a corresponding [[resolver]] (though it might be implicit), which constitutes the [[permission]] to cause it to resolve to a given other reference (which may itself be a promise).
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A [[promise]], in [[E]], is a [[ref]] whose referent has not yet been determined -- it is an [[ref states|unresolved reference]].
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A '''local promise''' is a promise whose resolver is in this vat; local promises are reliably implemented by the local [[ELib]]. A [[remote promise]] is a promise whose resolver is in a different [[vat]]; remote promises are implemented by [[Proxy|proxies]].
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Every promise has a corresponding [[resolver]] (though it may not be explicitly used), which constitutes the [[permission]] to cause it to resolve to a given other reference (which may itself be a promise).
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==Local promises==
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Promises are often created by performing [[eventual send]]s or by explicit [[Ref (object)#promise/0|promise pair creation]].
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The general-purpose operation to create a local promise is [[Object Ref#promise/0|Ref#promise/0]].
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A [[remote promise]] is a promise whose resolver is in a different [[vat]].
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? def [promise, resolver] := Ref.promise()
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{{stub}}
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# value: [<Promise>, <Resolver>]
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? Ref.state(promise)
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# value: "EVENTUAL"
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? Ref.isResolved(promise)
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# value: false
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Once a promise (local or remote) is resolved, it is indistinguishable from the reference which it is resolved to, whether that reference is [[Selfless]],
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? resolver.resolve(99)
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? promise
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# value: 99
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? promise + 1
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# value: 100
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? promise == 99
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# value: true
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or [[Selfish]].
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? def [promise, resolver] := Ref.promise()
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> resolver.resolve(def x {})
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> promise == x
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# value: true
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{{XXX}} write tests and doc from the LocalResolver's perspective (isDone, resolveRace/smash, behavior after it is resolved) -- EoCL has some in ref.updoc
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==Calls and sends==
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? def [promise, resolver] := Ref.promise(); null
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As with any [[unresolved ref]], a promise cannot be [[call]]ed.
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? promise(1)
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# problem: not synchronously callable: <Promise>.run(1)
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However, any [[send]]s to the promise are accepted and buffered in the resolver, in time order.
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? promise <- (2)
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# value: <Promise>
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? E.sendOnly(promise, "run", [3])
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If the resolver becomes garbage, then so does the buffer; future messages to the promise are silently discarded and do not accumulate. Note that this implies that the promise does not [[strongly refer to]] the resolver. ({{XXX}} write tests for this using finalizers.)
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At the instant a local promise is resolved to another ref (which is known as its [[resolution]]), the buffered messages are sent, in the order they arrived, to the resolution.
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? var items := []
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> def resolution(x) { items with= x }
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> resolution <- (4)            # demonstrating exact ordering
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> def resolveReturn := resolver.resolve(resolution)
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> resolution <- (5)
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> resolveReturn
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? items
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# value: [4, 2, 3, 5]
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[[Category:ELib_specification]]
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Revision as of 19:20, 14 October 2008

A promise, in E, is a ref whose referent has not yet been determined -- it is an unresolved reference.

Every promise has a corresponding resolver (though it may not be explicitly used), which constitutes the permission to cause it to resolve to a given other reference (which may itself be a promise).

Promises are often created by performing eventual sends or by explicit promise pair creation.

A remote promise is a promise whose resolver is in a different vat.

This page is a stub; it should be expanded with more information. If doing so, check the original E web site and the mailing list archives for content which could be moved into this page.
Retrieved from "http://wiki.erights.org/wiki/Promise"
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