11In the last section we built a few small apps that sent moves. These
12apps were entirely stateless, however. Most useful apps require some
13amount of state. Let's build a trivial stateful app. It'll keep a
14running the sum of all the atoms we poke it with. Here's
15`app/examples/sum.hoon`:
25 ~& [%so-far (add state arg)]
26 [~ +>.$(state (add state arg))]
29We can start it with `|start %examples-sum`, and then run it:
31 ~fintud-macrep:dojo> :examples-sum &atom 5
34 ~fintud-macrep:dojo> :examples-sum &atom 2
37 ~fintud-macrep:dojo> :examples-sum &atom 15
41We can see that app state is being saved, but when, where, and how?
43The state is stored as the second thing in the `|_` line. In our case,
44it's simply an atom named `state`. We change it by producing as our
45state not with `+>.$` (as before), but with
46`+>.$(state (add state arg))`. We've seen all these parts before, but
47you might not recognize them.
49Recall in the first chapter that we recursed with the expression
50`$(b (add 3 b))`. This meant "produce `$` with `b` changed to
51`(add 3 b)`. Similarly, `+>.$(state (add state arg))` means "produce
52`+>.$` (i.e. our context, which contains our state) with `state` changed
55At a high level, then, when we handle state, we do it explicitly. It's
56passed in and produced explicitly. In Unix systems, application state is
57just a block of memory, which you need to serialize to disk if you want
58to keep it around for very long.
60In Urbit, app state is a single (usually complex) value. In our example,
61we have very simple state, so we defined `state=@`, meaning that our
62state is an atom. Of course, `state` is just a name, and you're free to
63name your state whatever you like. But let's clarify a couple other
64things before we continue.
66First, `bowl` is a set of general global states. This set is managed by
67the system. It includes things like `now` (the current time), `our` (our
68urbit identity), and `eny` (256 bits of guaranteed-fresh entropy). For
69the full list of things in `++bowl`, search for `++ bowl` (note: two
70spaces) in `/arvo/zuse.hoon`.
72> This is, perhaps, the most common way to learn Hoon. The easiest way
73> to learn about an identifier you see in code is to search in
74> `/arvo/zuse.hoon` and `/arvo/hoon.hoon` for it.\
75> These "standard libraries" are usually simple to read. Since Urbit's
76> codebase is less than 15000 lines of code combined, including the hoon
77> parser, the compiler, and the `/arvo` microkernel, you can usually use
78> the code and its comments as a reference doc.
81> [zuse.hoon](https://github.com/urbit/arvo/blob/master/arvo/zuse.hoon)
83> [hoon.hoon](https://github.com/urbit/arvo/blob/master/arvo/hoon.hoon)
86The second thing we should clear up is this: Urbit needs no "serialize
87to disk" step. Everything you produce in the app state is persistent
88across calls to the app, restarts of the urbit, and even power failure.
89If you want to write to the Unix filesystem, you can, but it's not
90needed for persistence. Urbit has transactional events, which makes it
91an [ACID operating system](https://en.wikipedia.org/wiki/ACID). Thus,
92you don't have to worry about persistence when programming in Urbit, or
93ever go through the hassle of having to set up and write to a database.
97- Modify `:examples-sum` to reset the counter when you poke it with 0.
99- Write an app that prints out the previous value you poked it with.
102\`\`\` \~fintud-macrep:dojo\> :examples-last 7 [%last 0] \>=
103\~fintud-macrep:dojo\> :examples-last [1 2 3][%last 7] \>=
104\~fintud-macrep:dojo\> :examples-last 'howdy' [%last [1 2 3]]
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