class Prism::ParseResult::Comments

When we’ve parsed the source, we have both the syntax tree and the list of comments that we found in the source. This class is responsible for walking the tree and finding the nearest location to attach each comment.

It does this by first finding the nearest locations to each comment. Locations can either come from nodes directly or from location fields on nodes. For example, a ‘ClassNode` has an overall location encompassing the entire class, but it also has a location for the `class` keyword.

Once the nearest locations are found, it determines which one to attach to. If it’s a trailing comment (a comment on the same line as other source code), it will favor attaching to the nearest location that occurs before the comment. Otherwise it will favor attaching to the nearest location that is after the comment.



The parse result that we are attaching comments to.

Public Class Methods

new(parse_result) click to toggle source

Create a new Comments object that will attach comments to the given parse result.

# File prism/parse_result/comments.rb, line 86
def initialize(parse_result)
  @parse_result = parse_result

Public Instance Methods

attach!() click to toggle source

Attach the comments to their respective locations in the tree by mutating the parse result.

# File prism/parse_result/comments.rb, line 92
def attach!
  parse_result.comments.each do |comment|
    preceding, enclosing, following = nearest_targets(parse_result.value, comment)

    if comment.trailing?
      if preceding
        (following || enclosing ||
      # If a comment exists on its own line, prefer a leading comment.
      if following
      elsif preceding
        (enclosing ||

Private Instance Methods

nearest_targets(node, comment) click to toggle source

Responsible for finding the nearest targets to the given comment within the context of the given encapsulating node.

# File prism/parse_result/comments.rb, line 119
def nearest_targets(node, comment)
  comment_start = comment.location.start_offset
  comment_end = comment.location.end_offset

  targets = [] #: Array[_Target] do |value|
    case value
    when StatementsNode
      targets.concat( { |node| })
    when Node
      targets <<
    when Location
      targets <<

  preceding = nil #: _Target?
  following = nil #: _Target?

  left = 0
  right = targets.length

  # This is a custom binary search that finds the nearest nodes to the
  # given comment. When it finds a node that completely encapsulates the
  # comment, it recurses downward into the tree.
  while left < right
    middle = (left + right) / 2
    target = targets[middle]

    target_start = target.start_offset
    target_end = target.end_offset

    if target.encloses?(comment)
      # @type var target: NodeTarget
      # The comment is completely contained by this target. Abandon the
      # binary search at this level.
      return nearest_targets(target.node, comment)

    if target_end <= comment_start
      # This target falls completely before the comment. Because we will
      # never consider this target or any targets before it again, this
      # target must be the closest preceding target we have encountered so
      # far.
      preceding = target
      left = middle + 1

    if comment_end <= target_start
      # This target falls completely after the comment. Because we will
      # never consider this target or any targets after it again, this
      # target must be the closest following target we have encountered so
      # far.
      following = target
      right = middle

    # This should only happen if there is a bug in this parser.
    raise "Comment location overlaps with a target location"

  [preceding,, following]