An incomplete implementation of JSONPath in Typst

example.typ

#let data = (
  books: (
    (title: "The Final Empire", author: "Brandon Sanderson"),
    (title: "The Sword of Kaigen", author: "M.L. Wang"),
  ),
  query: 0,
)

#faux-jsonpath(data, "$.books[0].author")
#faux-jsonpath(data, "$.books[$.query].title")

jsonpath.typ

/// A limited version of JSONPath.
/// It supports the following syntaxes:
/// - `$`: The root object
/// - `@`: The current object
/// - `.field`: Field access
/// - `[0]`: Index by number
/// - `['foo']`: Index by string
#let faux-jsonpath(obj, path) = {
  let expr(root, obj, i, path) = {
    let current = obj
    let i = i
    while i < path.len() {
      let c = path.at(i)
      if c == "$" {
        // The root object.
        current = root
        i += 1
      } else if c == "@" {
        // The current object.
        current = obj
        i += 1
      } else if c == "." {
        // Parse identifier.
        i += 1
        let match = path.slice(i).match(regex("\p{XID_Start}\p{XID_Continue}*"))
        if match == none {
          panic("syntax error: invalid field access syntax")
        }
        current = current.at(match.text)
        i += match.end
      } else if c == "[" {
        i += 1
        let (value, i_) = expr(root, current, i, path)
        current = current.at(value)
        i = i_
        if path.at(i) != "]" {
          panic("syntax error: missing closing bracket `]`")
        }
        i += 1
      } else if c == "'" {
        // Parse string.
        i += 1
        let start = i
        while i < path.len() and path.at(i) != "'" {
          i += 1
        }
        if i == path.len() {
          panic("syntax error: missing closing mark `'`")
        }
        current = path.slice(start, i)
        i += 1
        break
      } else {
        // Parse integer.
        let match = path.slice(i).match(regex("[0-9]+"))
        if match != none {
          current = int(match.text)
          i += match.end
        }
        break
      }
    }
    (current, i)
  }
  
  let (current, i) = expr(obj, obj, 0, path)
  if i != path.len() {
    panic("syntax error: unexpected character `" + c + "`")
  }
  current
}