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5# Data model: `noun`

7A value in Hoon is called a `noun`. A noun is either a unsigned

8integer of any size (`atom`), or an ordered pair of any two

9nouns (`cell`). In other words, a noun is a binary tree whose

10leaves are unsigned integers.

12Nouns are a Nock concept; there is no Nock syntax; so there is no

13general noun syntax. But nouns are usually written with square

14brackets associating right; `[a b c]` means `[a [b c]]`. In Hoon

15we also see `{a b c}`, a constructor (`mold`) for `[a b c]`.

17Nouns are almost like Lisp's S-expressions, but S-expressions

18have dynamic type tags in atoms. Nouns can lose this feature

19because they're designed to sit under a static type system. For

20the same reason, nouns tend to be tuple-centric where Lisp is

21list-centric. Lisp `(a b c)` as a noun is `[a b c 0]`; Nock `[a

22b c]` is Lisp `(a . b . c)`.

24Nouns are immutable and acyclic. A naive implementation uses

25reference counting and needs no tracing GC. There is no test for

26pointer equality as in Lisp. But indirect nouns have a lazily

27computed short hash, so nonduplicates have O(1) comparison cost.

29All kinds of atomic values can be and are stored as atoms:

30unsigned integers, signed integers (with sign bit low), dates,

31floats, UTF-8 strings (LSB first), even whole files. Again,

32a static type system is essential; an atom is not usable or even

33printable if we don't know what it means.