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The [core](https://developers.urbit.org/reference/glossary/core) is one of the key elements of Urbit's subject-oriented programming paradigm. As a way of organizing code and data in a homoiconic language, the core pattern allows Hoon developers to encapsulate processes and lexically scope code while relying on a well-structured fundament. Simple core patterns like gates and traps have only one arm in their `battery` and possibly no `sample` at all (for a trap). More complex core patterns (such as the `|^` barket core and doors) can consist of inner cores and outer cores, as well as stacking `sample`s (such as for a gate in a door). This pattern can telescope beyond doors, however, into the nested core pattern. In this article, we examine the nested core pattern through its most common instantiation, the `++abet` pattern.
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For instance, many applications store their data in `+$map`s. The basic nested core pattern uses an auxiliary core to manage building state changes. As a core contains `[battery payload]`, code and data, we can recursively place cores into the `battery` of code. By carefully maintaining encapsulated state (i.e. state that doesn't leak, state that is enclosed by its core), we can build a pattern of stacked cores that maintain their own mutable state locally. That is, the inner core becomes a sort of “scripting language” for the outer core, wherein local mutable state is finalized and propagated back to the outer core using the `++abet` arm. Even a gate in a core is actually a stacked core in this sense: it stacks a sample and `$` arm onto the core stack which formed its subject.
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The basic nested core pattern uses an auxiliary core to manage building state changes. As a core contains `[battery payload]`, code and data, we can recursively place cores into the `payload` of a core. By carefully maintaining encapsulated state (i.e. state that doesn't leak, state that is enclosed by its core), we can build a pattern of stacked cores that maintain their own mutable state locally. That is, the inner core becomes a sort of “scripting language” for the outer core, wherein local mutable state is finalized and propagated back to the outer core using the `++abet` arm. Even a gate in a core is actually a stacked core in this sense: it stacks a sample and `$` arm onto the core stack which formed its subject.
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A door (like a Gall agent) typically shares state between its daughter arms (most or all of which are gate cores). In many nontrivial agents, an auxiliary core is placed next to the primary agent door, to which agent state can be passed and by which it can be manipulated, but which does not maintain its own internal state. The nested core pattern rather has an outer core (like the auxiliary core) with one or more inner cores to script its behavior, but which are never part of the returned state.
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