One of the basic combinators is map , which, given a future and a mapping function for the value of the future, produces a new future that is completed with the mapped value once the original future is successfully completed. You can reason about mapping futures in the same way you reason about mapping collections.
By using map on rateQuote we have eliminated one foreach callback and, more importantly, the nesting. If we now decide to sell some other currency, it suffices to use map on purchase again. But what happens if isProfitable returns false , hence causing an exception to be thrown? In that case purchase is failed with that exception. Furthermore, imagine that the connection was broken and that getCurrentValue threw an exception, failing rateQuote.
In conclusion, if the original future is completed successfully then the returned future is completed with a mapped value from the original future. If the mapping function throws an exception the future is completed with that exception. If the original future fails with an exception then the returned future also contains the same exception. This exception propagating semantics is present in the rest of the combinators, as well. One of the design goals for futures was to enable their use in for-comprehensions. For this reason, futures also have the flatMap and withFilter combinators. The flatMap method takes a function that maps the value to a new future g , and then returns a future which is completed once g is completed.
We have to fetch quotes for both currencies, and then decide on buying based on both quotes. Here is an example of flatMap and withFilter usage within for-comprehensions:. The purchase future is completed only once both usdQuote and chfQuote are completed— it depends on the values of both these futures so its own computation cannot begin earlier. The flatMap operation maps its own value into some other future. Once this different future is completed, the resulting future is completed with its value.
In our example, flatMap uses the value of the usdQuote future to map the value of the chfQuote into a third future which sends a request to buy a certain amount of Swiss francs. The resulting future purchase is completed only once this third future returned from map completes. This can be mind-boggling, but fortunately the flatMap operation is seldom used outside for-comprehensions, which are easier to use and understand. The filter combinator creates a new future which contains the value of the original future only if it satisfies some predicate.
Otherwise, the new future is failed with a NoSuchElementException. For futures calling filter has exactly the same effect as does calling withFilter. The relationship between the collect and filter combinator is similar to the relationship of these methods in the collections API. Since the Future trait can conceptually contain two types of values computation results and exceptions , there exists a need for combinators which handle exceptions.
The connection. It returns the amount bought. If the quote has changed in the meanwhile, it will throw a QuoteChangedException and it will not buy anything. If we want our future to contain 0 instead of the exception, we use the recover combinator:. The recover combinator creates a new future which holds the same result as the original future if it completed successfully.
If it did not then the partial function argument is applied to the Throwable which failed the original future.
- Michelangelo’s Creation of Adam.
- Fairy Dusted.
- The Last Judgment (Michelangelo) - Wikipedia.
- Working with Dolls by Berenguer Doll Kits (Excellence in Reborn Artistry).
- Destroyer Angel (Anna Pigeon Mysteries, Book 18): A suspenseful thriller of the American wilderness.
If it maps the Throwable to some value, then the new future is successfully completed with that value. If the partial function is not defined on that Throwable , then the resulting future is failed with the same Throwable. The recoverWith combinator creates a new future which holds the same result as the original future if it completed successfully. Otherwise, the partial function is applied to the Throwable which failed the original future.
If it maps the Throwable to some future, then this future is completed with the result of that future. Its relation to recover is similar to that of flatMap to map. Combinator fallbackTo creates a new future which holds the result of this future if it was completed successfully, or otherwise the successful result of the argument future. In the event that both this future and the argument future fail, the new future is completed with the exception from this future, as in the following example which tries to print US dollar value, but prints the Swiss franc value in the case it fails to obtain the dollar value:.
The andThen combinator is used purely for side-effecting purposes. It returns a new future with exactly the same result as the current future, regardless of whether the current future failed or not. Once the current future is completed with the result, the closure corresponding to the andThen is invoked and then the new future is completed with the same result as this future.
This ensures that multiple andThen calls are ordered, as in the following example which stores the recent posts from a social network to a mutable set and then renders all the posts to the screen:. In summary, the combinators on futures are purely functional. Every combinator returns a new future which is related to the future it was derived from.
To enable for-comprehensions on a result returned as an exception, futures also have projections. If the original future fails, the failed projection returns a future containing a value of type Throwable. If the original future succeeds, the failed projection fails with a NoSuchElementException. The following is an example which prints the exception to the screen:. Because f is unsuccessful here, the closure is registered to the foreach callback on a newly-successful Future[Throwable].
The following example does not print anything to the screen:. Support for extending the Futures API with additional utility methods is planned. This will allow external frameworks to provide more specialized utilities. Futures are generally asynchronous and do not block the underlying execution threads.
However, in certain cases, it is necessary to block. We distinguish two forms of blocking the execution thread: invoking arbitrary code that blocks the thread from within the future, and blocking from outside another future, waiting until that future gets completed. As seen with the global ExecutionContext , it is possible to notify an ExecutionContext of a blocking call with the blocking construct. The implementation is however at the complete discretion of the ExecutionContext. While some ExecutionContext such as ExecutionContext.
The blocking code may also throw an exception. In this case, the exception is forwarded to the caller.
Michelangelo’s Creation of Adam – exoxodiheqek.cf
As mentioned earlier, blocking on a future is strongly discouraged for the sake of performance and for the prevention of deadlocks. Callbacks and combinators on futures are a preferred way to use their results. However, blocking may be necessary in certain situations and is supported by the Futures and Promises API. In the currency trading example above, one place to block is at the end of the application to make sure that all of the futures have been completed.
Here is an example of how to block on the result of a future:. In the case that the future fails, the caller is forwarded the exception that the future is failed with.
This includes the failed projection— blocking on it results in a NoSuchElementException being thrown if the original future is completed successfully. Alternatively, calling Await. In the same way, calling that method will not throw an exception if the future is failed.
The Future trait implements the Awaitable trait with methods ready and result. These methods cannot be called directly by the clients— they can only be called by the execution context. When asynchronous computations throw unhandled exceptions, futures associated with those computations fail. Failed futures store an instance of Throwable instead of the result value.
Future s provide the failed projection method, which allows this Throwable to be treated as the success value of another Future. The following special exceptions are treated differently:. Typically, return constructs in method bodies are translated to throw s with this exception. Instead of keeping this exception, the associated value is stored into the future or a promise. ExecutionException - stored when the computation fails due to an unhandled InterruptedException , Error or a scala. In this case the ExecutionException has the unhandled exception as its cause.
The rationale behind this is to prevent propagation of critical and control-flow related exceptions normally not handled by the client code and at the same time inform the client in which future the computation failed.
Art as expression
Fatal exceptions as determined by NonFatal are rethrown in the thread executing the failed asynchronous computation. This informs the code managing the executing threads of the problem and allows it to fail fast, if necessary. See NonFatal for a more precise description of the semantics. So far we have only considered Future objects created by asynchronous computations started using the Future method.
However, futures can also be created using promises. Conversely, a promise can also be used to complete a future with an exception, by failing the promise, using the failure method. A promise p completes the future returned by p. This future is specific to the promise p. Depending on the implementation, it may be the case that p. Consider the following producer-consumer example, in which one computation produces a value and hands it off to another computation which consumes that value.
This passing of the value is done using a promise. Here, we create a promise and use its future method to obtain the Future that it completes. Then, we begin two asynchronous computations. The first does some computation, resulting in a value r , which is then used to complete the future f , by fulfilling the promise p.
The second does some computation, and then reads the result r of the completed future f. Note that the consumer can obtain the result before the producer task is finished executing the continueDoingSomethingUnrelated method. As mentioned before, promises have single-assignment semantics. As such, they can be completed only once. Calling success on a promise that has already been completed or failed will throw an IllegalStateException.
The Birth of Mount Rushmore
In this case, the associated future f is failed. Otherwise, the producer continues its computation, and finally completes the future f with a valid result, by completing promise p. Promises can also be completed with a complete method which takes a potential value Try[T] — either a failed result of type Failure[Throwable] or a successful result of type Success[T]. Analogous to success , calling failure and complete on a promise that has already been completed will throw an IllegalStateException. One nice property of programs written using promises with operations described so far and futures which are composed through monadic operations without side-effects is that these programs are deterministic.
Deterministic here means that, given that no exception is thrown in the program, the result of the program values observed in the futures will always be the same, regardless of the execution schedule of the parallel program. In some cases the client may want to complete the promise only if it has not been completed yet e. For these reasons methods tryComplete , trySuccess and tryFailure exist on promise.
The client should be aware that using these methods results in programs which are not deterministic, but depend on the execution schedule. The method completeWith completes the promise with another future. After the future is completed, the promise gets completed with the result of that future as well.
The following program prints 1 :. When failing a promise with an exception, three subtypes of Throwable s are handled specially. If the Throwable used to break the promise is a scala. NonLocalReturnControl , then the promise is completed with the corresponding value. If the Throwable used to break the promise is an instance of Error , InterruptedException , or scala. ControlThrowable , the Throwable is wrapped as the cause of a new ExecutionException which, in turn, is failing the promise. Using promises, the onComplete method of the futures and the future construct you can implement any of the functional composition combinators described earlier.
Note that in this implementation, if neither f nor g succeeds, then first f, g never completes either with a value or with an exception. To simplify handling of time in concurrent applications scala. Like all idioms, its meaning is nonliteral Ambiguous reference to time is found throughout the Bible. For example, kings' empires were often referred to in this way, "In the days of King Josiah And, it is stated in the New Testament, "But do not overlook this one fact, beloved, that with the Lord one day is as a thousand years, and a thousand years as one day.
The point is, God's sense of time is quite different from ours. Our definition of day is a full rotation of the earth in hours, but during creation, who's to say? If the Hebrew were specific, then certainly we would need to hold to whatever it says. However, the end of Genesis chapter one could just as accurately be tranlated, "And there was evening and there was morning, the sixth time period. Thus the heavens and the earth were finished The text does not obligate us to conclude that God completed his creation in six, hour time slots, nor does the book of Genesis necessarily conflict with scientific conclusions that formation of the earth and universe lasted billions of years.
It makes little sense fixating on the length of time when the Hebrew is going to remain ambiguous anyway , instead of focusing on the far more important message of what was actually happening?! Again, the important point is that Scripture is very clear that in real history, God chose to create the earth, the sun and stars, all aspects of the universe, the oceans, the vegetation, the sea life, the animals, and humanity. God, unequivocally, is the creator and originator of it all. Footnotes: 1 Genesis 2 Genesis 3 Genesis 4 Dr. Otto J. Helweg, quoted from the site, Reason.
Spiritual Adventure Pack. Ask a Question! Get the Spiritual Adventure Pack. See God in the Gospel of John. Get our free Every Student app.
- International Religious Freedom Report Country of Eritrea;
- How To Manage Money; Take Control Of Your Money By Setting Financial Goals, Repairing Credit, Creating Savings When You’re Broke and More!
- Forcing Gravity;
- What is realization? definition and meaning - exoxodiheqek.cf!
- Michelangelo’s Creation of Adam – exoxodiheqek.cf!
- Lesson Plans Times Arrow;
- What is meant by for ever and ever in the Bible?!