Purpose
Nested functions are a form of information hiding and are useful for dividing procedural tasks into sub tasks which are only meaningful locally; it avoids cluttering other parts of the program with functions, variables, etc. unrelated to those parts. Nested functions therefore complement other structuring possibilities such as records and objects.
In languages with nested functions, functions may normally also contain local constants, and types (in addition to local variables, parameters, and functions), encapsulated and hidden in the same nested manner. This may further enhance the code structuring possibilities.
Nested functions can also be used to terminate a nested for loop early.
Read more about this topic: Nested Function
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