Editing Eiffel (programming language) (section)

===Once routines===
A routine's result can be cached using the <code>once</code> keyword in place of <code>do</code>. Non-first calls to a routine require no additional computation or resource allocation, but simply return a previously computed result. A common pattern for "once functions" is to provide shared objects; the first call will create the object, subsequent ones will return the reference to that object. The typical scheme is:

<syntaxhighlight lang="eiffel">
shared_object: SOME_TYPE
    once
        create Result.make (args)
             -- This creates the object and returns a reference to it through `Result'.
    end
</syntaxhighlight>

The returned object—<code>Result</code> in the example—can itself be mutable, but its reference remains the same.

Often "once routines" perform a required initialization: multiple calls to a library can include a call to the initialization procedure, but only the first such call will perform the required actions. Using this pattern initialization can be decentralized, avoiding the need for a special initialization module. "Once routines" are similar in purpose and effect to the [[singleton pattern]] in many programming languages, and to the [[b:Computer Science Design Patterns/Singleton#Python|Borg pattern]] used in Python.

By default, a "once routine" is called ''once per thread''. The semantics can be adjusted to ''once per process'' or ''once per object'' by qualifying it with a "once key", e.g. <code>once ("PROCESS")</code>.