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In other cases a future and a promise are created together and associated with each other: the future is the value, the promise is the function that sets the value – essentially the return value (future) of an asynchronous function (promise). Setting the value of a future is also called resolving, fulfilling, or binding it.
A function using async/await can use as many await expressions as it wants, and each will be handled in the same way (though a promise will only be returned to the caller for the first await, while every other await will utilize internal callbacks). A function can also hold a promise object directly and do other processing first (including ...
Often, consuming code registers a callback for a particular type of event. When that event occurs, the callback is called. Callbacks are often used to program the graphical user interface (GUI) of a program that runs in a windowing system. The application supplies a reference to a custom callback function for the windowing system to call.
Function pointers allow different code to be executed at runtime. They can also be passed to a function to enable callbacks. Function pointers are supported by third-generation programming languages (such as PL/I, COBOL, Fortran, [1] dBASE dBL [clarification needed], and C) and object-oriented programming languages (such as C++, C#, and D). [2]
In Python, functions are first-class objects, just like strings, numbers, lists etc. This feature eliminates the need to write a function object in many cases. Any object with a __call__() method can be called using function-call syntax. An example is this accumulator class (based on Paul Graham's study on programming language syntax and ...
This function has one argument of a function type; that function argument accepts the function too, which discards all computations going after its call. For example, let's break the execution of the pyth function if at least one of its arguments is negative returning zero:
The promise is the set of directed acyclic graphs. In this example, the promise is easy to check. In particular, it is very easy to check if a given graph is cyclic. However, the promised property could be difficult to evaluate. For instance, consider the problem "Given a Hamiltonian graph, determine if the graph has a cycle of size 4."
Example of a Structured Chart. [1] A structure chart (SC) in software engineering and organizational theory is a chart which shows the smallest of a system to its lowest manageable levels. [2] They are used in structured programming to arrange program modules into a tree. Each module is represented by a box, which contains the module's name.