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5+title: Glossary of Urbit Terms
6+---
7+
8+# Glossary of Urbit Terms
9+
10+Here is a simple overview of the most common Urbit terms. Once you understand this, you know almost everything you need to start using Urbit.
11+
12+## The Universe in a Table
13+
14+ Size Name Parent Object Example
15+ ----- ------ ------ ------ -------
16+ 2^8 galaxy ~ datacenter ~zod
17+ 2^16 star galaxy sysadmin ~doznec
18+ 2^32 planet star user ~tasfyn-partyv
19+ 2^64 moon planet device ~sigsam-nimbot-tasfyn-partyv
20+ 2^128 comet ~ bot ~racmus-mollen-fallyt-linpex--watres-sibbur-modlux-rinmex
21+ any urbit * * ~somnym-anynym
22+
23+> The below summary is also available [here](http://urbit.org/posts/address-space/).
24+
25+An Urbit identity is a string like `~firbyr-napbes`. It
26+means nothing, but it's easy to remember and say out loud.
27+`~firbyr-napbes` is actually just a 32-bit number, like an IP address,
28+that we turn into a human-memorable string.
29+
30+Technically, an urbit is a secure digital identity that you own and
31+control with a cryptographic key, like a Bitcoin wallet. As in
32+Bitcoin, the supply of urbits is mathematically limited. This keeps
33+the network friendly, by making spam and abuse expensive.
34+
35+An urbit name is just a number; smaller numbers make shorter names.
36+Shorter names are easier to remember, so they're more valuable. So
37+urbits are classified by the number of bits in their name. (A ship
38+name is just a scrambled base-256 representation of the number.)
39+
40+A 32-bit urbit (like `~firbyr-napbes`) is called a "planet." A 16-bit
41+urbit (like `~pollev)` is a "star." An 8-bit ship (like `~mun`) is a
42+"galaxy." A planet is an identity for an independent, adult human.
43+Stars and galaxies are network infrastructure.
44+
45+Each planet or star is launched by its "parent," the star or galaxy
46+whose number is its bottom half. So the planet `~firbyr-napbes`,
47+`0xdead.beef` or `3.735.928.559`, is the child of `~pollev`, `0xbeef`
48+or `48.879`, whose parent is `~mun`, `0xef`, `239`. The parent of
49+`~mun` and all galaxies is `~zod`, `0`.
50+
51+### Nock
52+
53+> Nock is a Turing-complete, non-lambda combinator function.
54+
55+- A `noun` is an `atom` or a `cell`.
56+- An `atom` is any natural number.
57+- A `cell` is an ordered pair of `noun`s.
58+- `subject`: a `noun` - the argument against which a `formula` is evaluated
59+- `formula`: a `noun` - any of the Hoon formula numbers (0-10), followed by *(TODO: ?)*
60+- `product`: a `noun` - the result of evaluating a `formula` against a `subject`
61+
62+*See [Nock definition](/nock/definition/)*
63+
64+### Hoon
65+
66+> Hoon is a strict, higher-order typed functional language which compiles itself to Nock.
67+
68+###### `span`: an inferred type
69+
70+A `span` defines a set (finite or infinite) of `noun`s and ascribes some semantics to it. There is no Hoon syntax for a `span`; it is always produced as the inferred range of an expression (`twig`).
71+
72+*See [basic types](/hoon/basic)*
73+
74+###### `mold`: a type constructor / validator
75+
76+A mold is an idempotent `gate` (function), accepting any `noun` and normalizing it to a range.
77+
78+- `icon`: the `span` of a `mold`
79+- `bunt`: *(TODO: ??)*
80+
81+*See [`mold`](hoon/twig/buc-mold/)*
82+
83+###### `gate`: a function, lambda or closure
84+
85+*TODO: simplify description*
86+
87+A `gate` is a specialized `core` with only one arm (whose name is the empty string (the symbol `$`). To call a `gate` on an argument, replace the `sample` (at tree address `6` in the `core`) with the argument, and then compute the `arm`.
88+
89+The `payload` of a `gate` has a shape of `{sample context}`.
90+
91+- `sample`: the argument *(TODO: or plural?)* of a `gate`
92+- `context`: the subject in *(TODO: against?)* which a `gate` was defined
93+
94+*See [basic types](/hoon/basic)*
95+
96+###### `arm`: a named, functionally-computed attribute of a `core`
97+
98+The `twig` of each `arm` is compiled to a Nock formula, with the enclosing `core` itself as the subject.
99+
100+*See [basic types](/hoon/basic)*
101+
102+###### `core`: a code-data `cell`
103+
104+The code (`battery`) is the head, the data (`payload`) is the tail. All code-data structures in normal languages (functions, objects, modules, etc) become `core`s in Hoon.
105+
106+- `battery`: the code of a `core`, a tree of `arm`s
107+- `payload`: the data in a `core`
108+
109+`core`s are *polymorphic* structures; every function call (`gate`) in Hoon replaces the `payload` of a `core` with a different `noun`, and then invokes an `arm` from the `battery`.
110+
111+- `wet`: polymorphic by means of *genericity*
112+
113+*TODO: simplify*
114+
115+> When we call a `wet` `arm`, we're essentially using the `twig` as a macro. We are not generating new code for every call site; we are creating a new type analysis path, which works as if we expanded the callee with the caller's context.
116+
117+- `dry`: polymorphic by means of *variance*
118+
119+For a `dry` `arm`, we ask, is the new `payload` compatible with the old `payload` (against which the `core` was compiled)?
120+
121+Every `dry` `core` has a `metal` *(TODO: check)* which defines its *variance* model (ie, the properties of the `span` of a compatible `core`).
122+
123+*TODO: is the core dry, the arm dry, or both?*
124+
125+- `gold`: *invariant*
126+- `lead`: *bivariant*
127+- `zync`: *covariant*
128+- `iron`: *contravariant*
129+
130+*See [advanced types](/hoon/advanced)*
131+
132+###### `foot`: a `wet` or `dry` `twig`
133+
134+*TODO: any `twig`, or always an `arm` within a `battery`?*
135+
136+###### `atom`
137+
138+A Hoon `atom` is a Nock `atom`, with two additional pieces of metadata (TODO: ?): an `aura`, and a `unit`.
139+
140+- `unit`: a nullable pointer (TODO: ?), or a Maybe
141+
142+`atom`s are said to be `warm` or `cold` based on their `unit`
143+
144+- `warm`: if the `unit` is `~` (null), any `atom` is in the `span`
145+- `cold`: if the `unit` is `[~ x]`, where `x` is any `atom`, its only legal value is the constant `x`. *(TODO: what does this actually mean? What is the consequence of illegality?)*
146+
147+###### `aura`: a soft `atom` type
148+
149+*alternately `odor`*
150+
151+*TODO: write a short summary*
152+
153+*TODO: are `aura`s always `mold`s? just the built-in `aura`s?*
154+
155+> Auras are a lightweight, advisory representation of the units, semantics, and/or syntax of an `atom`. An `aura` is an atomic string; two auras are compatible if one is a prefix of the other.
156+>
157+> For instance, @t means UTF-8 text (LSB low), @ta means ASCII text, and @tas means an ASCII symbol. @u means an unsigned integer, @ud an unsigned decimal, @ux an unsigned hexadecimal. You can use a @ud atom as a @u or vice versa, but not as a @tas.
158+>
159+> Auras are truly soft; you can turn any `aura` into any other, statically, by casting through the empty `aura` `@`. Hoon is not dependently typed and can't statically enforce data constraints (for example, it can't enforce that a `@tas` is really a symbol).
160+>
161+
162+- `term` (`@tas`): a symbol - an atomic ASCII string which obeys symbol rules: lowercase and digit only, infix hyphen ("worm-case"), first character must be lowercase *(TODO: alphabetic?)*.
163+
164+- `cord` (`@t`): UTF-8 text, least-significant-byte first
165+
166+- `char` (`@tD`): a character, a single unicode byte (for multi-byte characters and codepoints, see `@c`)
167+
168+- `tape` (`(list char)`): a `tape` is not an `aura`, but a `mold` for a collection where every item has an `aura` of `char` *(TODO: is this for easy indexing vs `@t`?)*
169+
170+*See [basic types](/hoon/basic)*
171+
172+###### `loobean`: a Hoon boolean
173+
174+`0` (`%.y`) is *true*, `1` (`%.n`) is *false*.
175+
176+> Why? It's functional: good code has `0` side effects.
177+
178+###### `twig`: a Hoon expression
179+
180+A `twig` is the name for any Hoon expression, and for the AST node representing the expression as compiled to Nock formula *(TODO: seems to be used interchangeably in the docs)*. It is a `book` with the form `{stem bulb}`.
181+
182+- `book`: a tagged-union
183+- `page`: an item in a `book`
184+- `stem`: an atomic symbol (`@tas`) - uniquely identifies a `twig` *(TODO: ?)*
185+- `bulb`: The `bulb` for each `stem` has its own type, usually a tuple, sometimes a list or a map.
186+
187+A `twig` is either a `moss` ("mossy") or a `seed` ("woody"):
188+
189+- `moss`: a `twig` whose `product` is used a `mold`
190+- `seed`: a `twig` whose `product` could by anything
191+
192+The vast majority of `twig`s have a *regular form*, beginning with either a keyword `sigil` or a digraph `rune`. Some `twig`s also have a syntactic *irregular form*, a handful have only an *irregular form*.
193+
194+- `sigil`: a keyword used to begin a `twig`
195+
196+Hoon does not have reserved words, but `sigil`s (prefixed with `:`), which are effectively aliases for `rune`s. For example, [`:cons` is `:-`](/hoon/twig/col-cel/hep-cons).
197+
198+- `rune`: a pair of ASCII symbols used to begin a `twig` - the first symbol represents a family of related `twig`s. For example, the [`:` family](/hoon/twig/col-cell/) are all for cell-construction.
199+
200+*See [`twig` concept](/hoon/concepts) and [expressions](/hoon/twig/)*
201+
202+###### `limb`: attribute or variable reference
203+
204+To resolve a `limb` named "foo", the `subject` is searched depth-first, head-first for either a `face` named "foo" or a `core` with an `arm` of "foo". If a `face` is found, the result is a `leg`, if a `core` is found, the result is the `product` of the `arm`.
205+
206+- `leg`: a subexpression, or subtree of the `subject`. usually another `twig`, sometimes a symbol (`@tas`) or `wing`
207+- `wing`: a list of `limb`s
208+
209+*See [Limbs and wings](/hoon/twig/limb)*
210+
211+###### `face`: a named variable
212+
213+Hoon has no scope or symbol-table; there is only the `subject`. To "declare" a "variable" is to construct a new `subject`: `[variable old-subject]`.
214+
215+A `face` is a `span` *(TODO: ?)* with a label (`@tas`) wrapped around it.
216+
217+- `taco`: a `symbol` (for a *type-inferred* `face`), or a `symbol` *with* a `mold` (for a *type-checked* `face`)
218+
219+*TODO: check these*
220+
221+- `tune`: advanced `face`s:
222+ - `alias`: a `face` that is a lazy reference to another `face`
223+ - `bridge`: a `face` that references a `twig`, against which the `face` label is resolved
224+
225+*See [advanced types](/hoon/advanced)*
226+
227+###### `nest`: a type-casting operation
228+
229+*TODO:*
230+
231+### Arvo
232+
233+> Arvo is a nonpreemptive OS kernel, a single-threaded event interpreter.
234+
235+- `mark`
236+- `generator`
237+
238+*... TODO*