Ambient-Oriented Programming

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at:tutorial:actors [2007/07/18 10:09] – elisagat:tutorial:actors [2007/07/18 10:51] – elisag
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 ===== Futures ===== ===== Futures =====
  
-As you may have noticed previously, asynchronous message sends do not return any value (that is, they return ''nil''). Quite often, the developer is required to work around this lack of return values by means of e.g. explicit customer objects, as shown previously. This, however, leads to less expressive, more difficult to understand code, where the control flow quickly becomes implicit.+As you may have noticed previously, asynchronous message sends do not return any value (that is, they return ''nil''). Quite often, the developer is required to work around this lack of return values by means of e.g. explicit customer objects, as shown previously in the calculator example. This, however, leads to less expressive, more difficult to understand code, where the control flow quickly becomes implicit.
  
 ==== The Concept ==== ==== The Concept ====
  
-The most well-known language feature to reconcile return values with asynchronous message sends is the notion of a [[Wp>Future_(programming)|future]]. Futures are objects that represent return values that may not yet have been computed. Once the asynchronously invoked method has completed, the future is replaced with the actual return value, and objects that referred to the future transparently refer to the return value.+The most well-known language feature in concurrent and distributed languages (for example, in ABCL, the actor-based concurrent language) to reconcile return values with asynchronous message sends is the notion of a  [[Wp>Future_(programming)|future]]. Futures are also commonly known by the name of //promises// (this is how they are called in the [[http://www.erights.org|E language]] and in Argus). Futures are objects that represent return values that may not yet have been computed. Once the asynchronously invoked method has completed, the future is replaced with the actual return value, and objects that referred to the future transparently refer to the return value.
  
 Using futures, it is possible to re-implement the previous example of requesting our calculator actor to add two numbers as follows: Using futures, it is possible to re-implement the previous example of requesting our calculator actor to add two numbers as follows:
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 ==== Enabling futures ==== ==== Enabling futures ====
  
-Futures are a frequently recurring language feature in concurrent and distributed languages (for example, in ABCL, the actor-based concurrent language). They are also commonly known by the name of //promises// (this is how they are called in the [[http://www.erights.org|E language]] and in Argus). In AmbientTalk, futures are not native to the language. However, because of AmbientTalk's reflective infrastructure, it is possible to build futures on top of the language. The system library shipped with AmbientTalk contains exactly this: a reflective implementation that adds futures to the language kernel. This implementation can be found in the file ''at/lang/futures.at''.+In AmbientTalk, futures are not native to the language. However, because of AmbientTalk's reflective infrastructure, it is possible to build futures on top of the language. The system library shipped with AmbientTalk contains exactly this: a reflective implementation that adds futures to the language kernel. This implementation can be found in the file ''at/lang/futures.at''.
  
 To enable futures, it suffices to import the futures module and to enable it, as follows: To enable futures, it suffices to import the futures module and to enable it, as follows:
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 When a future eventually becomes resolved with a value, any messages that were accumulated by the future are forwarded asynchronously to the actual return value, such that it appears as if the original object had sent the messages to the actual return value in the first place. When a future eventually becomes resolved with a value, any messages that were accumulated by the future are forwarded asynchronously to the actual return value, such that it appears as if the original object had sent the messages to the actual return value in the first place.
  
 +<note important>
 AmbientTalk only allows one method to be synchronously invoked on a future, the ''=='' method. A word of warning though: equality on futures is defined as pointer equality, so a future will only be equal to itself. It does not compare the parameter object with its actual value, if it would be resolved. AmbientTalk only allows one method to be synchronously invoked on a future, the ''=='' method. A word of warning though: equality on futures is defined as pointer equality, so a future will only be equal to itself. It does not compare the parameter object with its actual value, if it would be resolved.
 +</note>
  
 ==== Working with Resolved Futures ==== ==== Working with Resolved Futures ====
at/tutorial/actors.txt · Last modified: 2020/02/05 21:26 by elisag