5 Parsed Representation🔗ℹ

The parse of a shrubbery can be represented by an S-expression, which in Rhombus terms corresponds to symbols and other non-list values within pair lists:

• A shrubbery document as a sequence of group is represented as a list that starts multi, and the rest of the list is a sequence of groups.

• Each group is represented as a list that starts group, and the rest of the list is a sequence of terms.

• Atom terms are represented as “themselves” within a group, including identifiers a symbols, except that an operator is represented as a 2-element list that is op followed by the operator name as a symbol.

• A group sequence is represented as a list of group lists.

• A term created by (…) is represented by parens consed onto a group-sequence list.

• A term created by […] is represented by brackets consed onto a group-sequence list.

• A term created by {…} is represented by braces consed onto a group-sequence list.

• A term created by '…' is represented by quotes consed onto a group-sequence list.

• A block is represented as block consed onto a group-sequence list. A block list appears only at the end of a group list or just before an alts list that is at the end of the group list.

• A sequence of alternatives is represented as alts consed onto a list of block lists, where each block list represents a | alternative. An alts list appears only at the end of a group list.

• To support mixtures of shrubbery terms and others, 3-element list that starts with parsed is treated like an atom. The list’s second element is typically a keyword representing a parsing category, and the third element is payload data.

Note that there is no possibility of confusion between symbol atoms in the input and group, block, etc., at the start of a list in an S-expression representation, because symbol atoms will always appear as non-initial items in a group list.

Overall, the grammar of S-expression representations is as follows:

	‹document›	::=	(top ‹group› ...)
	‹group›	::=	(group ‹item› ... ‹item›)
		|	(group ‹item› ... ‹block›)
		|	(group ‹item› ... ‹alts›)
		|	(group ‹item› ... ‹block› ‹alts›)
	‹item›	::=	‹atom›
		|	(op ‹symbol›)
		|	(parens ‹group› ...)
		|	(brackets ‹group› ...)
		|	(braces ‹group› ...)
		|	(quotes ‹group› ...)
		|	(parsed ‹any› ‹any›)
	‹block›	::=	(block ‹group› ...)
	‹alts›	::=	(alts ‹block› ...)

Here’s the same grammar, but expressed using Rhombus constructors:

	‹document›	::=	PairList[#'multi, ‹group›, ...]
	‹group›	::=	PairList[#'group, ‹item›, ..., ‹item›]
		|	PairList[#'group, ‹item›, ..., ‹block›]
		|	PairList[#'group, ‹item›, ..., ‹alts›]
		|	PairList[#'group, ‹item›, ..., ‹block›, ‹alts›]
	‹item›	::=	‹atom›
		|	PairList[#'op, ‹symbol›]
		|	PairList[#'parens, ‹group›, ...]
		|	PairList[#'brackets, ‹group›, ...]
		|	PairList[#'braces, ‹group›, ...]
		|	PairList[#'quotes, ‹group›, ...]
		|	PairList[#'parsed, ‹any›, ‹any›]
	‹block›	::=	PairList[#'block, ‹group›, ...]
	‹alts›	::=	PairList[#'alts, ‹block›, ...]

Here are some example shrubberies with their S-expression parsed representations: