Bond

A bond connects two adjacent atoms in a molecule’s nodes array. The kind enum covers IUPAC bond types plus ASCII-friendly variants for stereo.

Kind enum

Kind	ASCII	MathML	Use
`:single`	`-`	`-`	Single bond
`:double`	`=`	`=`	Double bond
`:triple`	`#`	`≡`	Triple bond
`:quadruple`	`##`	`≣`	Quadruple bond (organometallic)
`:wedge`	`>-`	`↑`	Stereo wedge (out of page)
`:hash`	`-<`	`↓`	Stereo hash (into page)
`:dative`	`~>`	`→`	Coordinate / dative bond
`:wavy`	`~~`	`∼`	Resonance / delocalised

Diagram

Bonds don’t have explicit endpoints — they’re positional. The molecule’s nodes array interleaves atoms and bonds:

   Atom(H) ── Bond(:single) ── Atom(O) ── Bond(:single) ── Atom(H)
                                                  ^^^^
                                      each Bond is a separator,
                                      not a standalone node with
                                      explicit endpoints

Grammar production

bond := "##"  (quadruple)
      | ">-"  (wedge)
      | "-<"  (hash)
      | "~>"  (dative)
      | "~~"  (wavy)
      | "#"   (triple)
      | "="   (double)
      | "-"   (single)

Alternation order is longest-first: ## must be tried before #, >- before -, ~> before ~. Parslet’s | is first-match-wins, so the order is the contract.

Why dative is `~>`, not `->`

-> is the reaction arrow. Reusing it for the dative bond would create an ambiguous grammar: A->B could be a dative bond or a reaction depending on context, which is fragile.

~> (wave + arrow) was chosen to convey electron-pair flow without the conflict. It’s not standard, but it’s unambiguous.

Why wedge/hash are `>-` and `-<`

The closing - keeps the bond separator family (-, =, #) intact. The leading > or trailing < evokes the wedge direction.

Alternative spellings considered:

/> and </ — slash-based, but / is overloaded in URLs/paths.
>- and <- — <- conflicts with the reverse reaction arrow.

>- and -< were the least-conflicting choice.

Formatter behaviour

Every formatter consults bond.ascii (Text/HTML/SVG) or bond.entity (MathML). Adding a new bond kind is one entry in the KINDS constant — no formatter edits.

Round-trip

AsciiChem.parse("H-O-H").to_text      # => "H-O-H"
AsciiChem.parse("HC#CH").to_text      # => "HC#CH"
AsciiChem.parse("Re##Re").to_text     # => "Re##Re"
AsciiChem.parse("C>-H").to_text       # => "C>-H"
AsciiChem.parse("NH_3~>BF_3").to_text # => "NH_3~>BF_3"
AsciiChem.parse("A~~B").to_text       # => "A~~B"