docs/programming/core/twig.md · urbit.org

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5# Expression: `twig`

7A `twig` is an AST node: the noun that the Hoon compiler makes

8when it parses a source expression.

10The definition of `twig` is the definition of Hoon. So let's

11define the syntax and semantics of Hoon, a twig at a time. But

12first, basic twig anatomy.

14## Twig anatomy and jargon

16A twig is a tagged union (`book`) with the form `{stem bulb}`,

17where the `stem` is an atomic symbol. The bulb for each stem has

18its own type, which varies by stem. (Compare with Lisp, where

19every AST node is a tag and a list of child nodes.)

21There are no user-level macros. All stems are built into Hoon.

22Most stems are implemented as internal macros, however.

24The bulb is usually a tuple, sometimes a list or map. An element

25of this structure is a *subexpression*.

27Usually a subexpression is itself a `twig`; sometimes it's an

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

30## Moss versus wood

32Solely for clarity, we use the type alias `moss` for twigs whose

33product is used as a mold. A `moss` is just a twig, both in

34syntax and semantics; again, the difference is how we use it.

36Twigs are described as *mossy* and *woody*; an example of X is

37the woody form of X, a mpld producing X is its woody form. For

38instance, `$foo` (mossy) is a mold whose product is always `%foo`

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

40can also say "yang foo" or "yin foo".)

42## Twig categories

44Fortunately, the twigs fit neatly into clear categories which

45match their rune prefix. Read forward by category:

47<div><list/></div>

49## Twig structure

51The full Core Hoon `twig`. Note that this is simplified, but

52everything useful can still be done in Core.