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56 lines
2.3 KiB
Markdown
56 lines
2.3 KiB
Markdown
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---
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hide: true
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next: false
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sort: 3
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title: Advanced Applications
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---
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XXX PLACEHOLDER
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But what is our app state, exactly? In Unix systems, application
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state is just a block of memory, which you need to serialize to
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disk if you want to keep it around for very long.
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In urbit, app state is a single (usually complex) value. In our
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example, we don't have any special state, so we defined
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`state=~`, meaning that our state is null. Of course, `state` is
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just a name we're assigning to it, and you're free to use
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whatever name you want.
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Since urbit is purely functional, we can't just implicitly "have"
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and "change" our state. Rather, it's explicitly passed to us, in
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the `|_ [bowl state=~]` line, and we produce the new state with
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`+>.$` in the `[~ +>.$]` line.
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Two points you may be wondering about. Firstly, `bowl` is a set
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of general global state that is managed by the system. It
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includes things like `now` (current time), `our` (our urbit
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identity), and `eny` (256 bits of guaranteed-fresh entropy). For
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the full list of things in `++bowl`, search for `++ bowl` (note
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the double space) in `/arvo/zuse.hoon`.
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> This is a very common technique in learning hoon. While some
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> documentation exists, often the easiest way to learn about an
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> identifier you see in code is to search in `/arvo/zuse.hoon`
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> and `/arvo/hoon.hoon` for it. These are our two "standard
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> libraries", and they're usually not hard to read. Since
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> urbit's codebase is relatively small (those two files are less
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> than 15000 lines of code combined, and besides the standard
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> library they include the hoon parser and compiler, plus the
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> /arvo microkernel), you can usually use the code and the
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> comments as reference doc.
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Second point is that urbit needs no "serialize to disk" step.
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Everything you produce in the app state is persistent across
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calls to the app, restarts of the urbit, and even power failure.
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If you want to write to the filesystem, you can, but it's not
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needed for persistence. Urbit has transactional events, which
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makes it an ACID operating system. Persistence is just another
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one of those things you don't have to worry about when
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programming in urbit.
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As fascinating as state is, we don't actually need any state to
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accomplish our immediate goal, which is to get apps on two urbits
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talking to each other. We'll discuss state more in a later
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chapter.
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