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6# Expression: `twig`

8A `twig` is an AST node: the noun that the Hoon compiler makes

9when it parses a source expression.

11The definition of `twig` is the definition of Hoon. So let's

12define the syntax and semantics of Hoon, a twig at a time. But

13first, basic twig anatomy.

15## Twig anatomy and jargon

17A twig is a tagged union (`book`) with the form `{stem bulb}`,

18where the `stem` is an atomic symbol. The bulb for each stem has

19its own type, which varies by stem. (Compare with Lisp, where

20every AST node is a tag and a list of child nodes.)

22There are no user-level macros. All stems are built into Hoon.

23Most stems are implemented as internal macros, however.

25The bulb is usually a tuple, sometimes a list or map. An element

26of this structure is a *subexpression*.

28Usually a subexpression is itself a `twig`; sometimes it's an

29`@tas` (symbol) or a `wing` (reference path).

31## Moss versus wood

33Solely for clarity, we use the type alias `moss` for twigs whose

34product is used as a mold. A `moss` is just a twig, both in

35syntax and semantics; again, the difference is how we use it.

37Twigs are described as *mossy* and *woody*; an example of X is

38the woody form of X, a mpld producing X is its woody form. For

39instance, `$foo` (mossy) is a mold whose product is always `%foo`

40(woody). (Obviously, wood is *yang* and moss is *yin*, so we

41can also say "yang foo" or "yin foo".)

43## Twig categories

45Fortunately, the twigs fit neatly into clear categories which

46match their rune prefix. Read forward by category:

48<div><list/></div>

50## Twig structure

52The full Core Hoon `twig`. Note that this is simplified, but

53everything useful can still be done in Core.