Generator (computer programming)

In computer science, a generator is a routine that can be used to control the iteration behaviour of a loop.

Generators can be implemented in terms of more expressive control flow constructs, such as coroutines or first-class continuations.

In more complicated situations, a generator may be used manually outside of a loop to create an iterator, which can then be used in various ways.

The final Common Lisp standard does not natively provide generators, yet various library implementations exist, such as SERIES documented in CLtL2 or pygen.

[10] On POSIX platforms, when the cost of context switching per iteration is not a concern, or full parallelism rather than merely concurrency is desired, a very simple generator function framework can be implemented using pthreads and pipes.

[11] The set of pre-processor macros defined in this source allow generators defined with the syntax as in the following example: This can then be iterated using: Moreover, C++11 allows foreach loops to be applied to any class that provides the begin and end functions.

Printing squares from 0 to 20 can be achieved by writing: However, most of the time custom generators are implemented with "gather" and "take" keywords in a lazy context.

In Haskell, with its lazy evaluation model, every datum created with a non-strict data constructor is generated on demand.

For example, where (:) is a non-strict list constructor, cons, and $ is just a "called-with" operator, used for parenthesization.

This uses the standard adaptor function, which walks down the list and stops on the first element that doesn't satisfy the predicate.

List comprehensions can be freely used: Racket provides several related facilities for generators.

Ruby supports generators (starting from version 1.9) in the form of the built-in Enumerator class.

Whenever next() is called on the iterator, Python resumes the frozen frame, which executes normally until the next yield statement is reached.

An infinite Fibonacci sequence can be written using a function generator: The iterators package can be used for this purpose.