Editing Generator (computer programming) (section)

==Uses==

Generators are usually [[execution (computers)|invoked]] inside loops.<ref name=icon>The [[Icon (programming language)|Icon Programming Language]] utilizes generators to implement its goal directed evaluation. In Icon, generators can be invoked in contexts outside of the normal looping control structures.</ref>  The first time that a generator invocation is reached in a loop, an iterator [[object (computer science)|object]] is created that encapsulates the state of the generator routine at its beginning, with arguments bound to the corresponding [[parameter (computer science)|parameter]]s.  The generator's body is then executed in the context of that iterator until a special ''yield'' action is encountered; at that time, the value provided with the ''yield'' action is used as the value of the invocation expression.  The next time the same generator invocation is reached in a subsequent iteration, the execution of the generator's body is resumed after the ''yield'' action, until yet another ''yield'' action is encountered.  In addition to the ''yield'' action, execution of the generator body can also be terminated by a ''finish'' action, at which time the innermost loop enclosing the generator invocation is terminated. 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.

Because generators compute their yielded values only on demand, they are useful for representing [[Stream (computing)|streams]], such as sequences that would be expensive or impossible to compute at once. These include e.g. infinite sequences and live data streams.

When eager evaluation is desirable (primarily when the sequence is finite, as otherwise evaluation will never terminate), one can either convert to a [[List (abstract data type)|list]], or use a parallel construction that creates a list instead of a generator. For example, in Python a generator <code>g</code> can be evaluated to a list <code>l</code> via <code>l = list(g)</code>, while in [[F Sharp programming language|F#]] the sequence expression <code>seq { ... }</code> evaluates lazily (a generator or sequence) but <code>[ ... ]</code> evaluates eagerly (a list).

In the presence of generators, loop constructs of a language – such as for and while – can be reduced into a single loop ... end loop construct; all the usual loop constructs can then be comfortably simulated by using suitable generators in the right way. For example, a ranged loop like <code>for x = 1 to 10</code> can be implemented as iteration through a generator, as in Python's <code>for x in range(1, 10)</code>. Further, <code>break</code> can be implemented as sending ''finish'' to the generator and then using <code>continue</code> in the loop.