A DSL for safely building file paths

AbsoluteDirectory.rb

class AbsoluteDirectory < AbstractPath
  def self.root
    AbsoluteDirectory.new(nodes: [])
  end

  def initialize(nodes:)
    raise "Last node cannot be a file, got (#{nodes.last.class})" if nodes.last&.file?
    if nodes.first&.root?
      super(nodes: nodes)
    elsif nodes.first&.current?
      super(nodes: nodes.drop(1).unshift(RootNode.new))
    else
      super(nodes: nodes.unshift(RootNode.new))
    end
  end

  def initialize_named(name)
    @nodes = [RootNode.new, DirectoryNode.new(name)]
  end

  def initialize_converting(path)
    initialize(nodes: path.nodes)
  end

  def absolute?
    true
  end

  def file?
    false
  end
end

AbsoluteFile.rb

class AbsoluteFile < AbstractPath
  def initialize(nodes:)
    raise "Last node must be a file, got (#{nodes.last.class})" unless nodes.empty? || nodes.last.file?
    if nodes.first&.root?
      super(nodes: nodes)
    elsif nodes.first&.current?
      super(nodes: nodes.drop(1).unshift(RootNode.new))
    else
      super(nodes: nodes.unshift(RootNode.new))
    end
  end

  def initialize_named(name)
    @nodes = [RootNode.new, FileNode.new(name)]
  end

  def initialize_converting(path)
    initialize(nodes: path.nodes)
  end

  def absolute?
    true
  end

  def file?
    true
  end
end

AbstractPath.rb

class AbstractPath
  attr_reader :nodes

  def initialize(nodes:)
    @nodes = nodes
  end

  def self.named(name)
    obj = allocate
    obj.initialize_named(name)
    obj
  end

  def self.converting(path)
    obj = allocate
    obj.initialize_converting(path)
    obj
  end

  def relative?
    !absolute?
  end

  def directory?
    !file?
  end

  def to_relative
    return self if relative?
    if directory?
      RelativeDirectory.converting(self)
    elsif file?
      RelativeFile.converting(self)
    else
      raise "Impossible! How am I not a file or directory?"
    end
  end

  def to_absolute
    return self if absolute?
    if directory?
      AbsoluteDirectory.converting(self)
    elsif file?
      AbsoluteFile.converting(self)
    else
      raise "Impossible! How am I not a file or directory?"
    end
  end

  def + (other)
    raise "Cannot append an absolute path (#{self} + #{other})" if other.absolute?
    raise "Cannot append to a file path (#{self} + #{other})" if file?

    combined = nodes + other.nodes.drop(1)
    if absolute? && other.directory?
      AbsoluteDirectory.new(nodes: combined)
    elsif absolute? && other.file?
      AbsoluteFile.new(nodes: combined)
    elsif relative? && other.directory?
      RelativeDirectory.new(nodes: combined)
    elsif relative? && other.file?
      RelativeFile.new(nodes: combined)
    end
  end

  def to_s
    nodes.join('/') + (directory? ? '/' : '')
  end

  def inspect
    nodes.inspect
  end
end

main.rb

def current
  RelativeDirectory.current
end

def root(name = nil)
  if name
    AbsoluteDirectory.root + dir(name)
  else
    AbsoluteDirectory.root
  end
end

def dir(name)
  RelativeDirectory.named(name)
end

def file(name)
  RelativeFile.named(name)
end

bin = root("usr") + dir("local") + dir("bin")
ruby = bin + file("ruby")
# => ["/", "usr", "local", "bin", "ruby"]

ruby.to_s
# => "/usr/local/bin/ruby"

file("ruby") + bin
# ~> RuntimeError
# ~> Cannot append an absolute path (./ruby + /usr/local/bin/)

file("ruby") + dir("bad")
# ~> RuntimeError
# ~> Cannot append to a file path (./ruby + ./bad/)

root + current + current + current
# => ["/"]

PathNode.rb

class PathNode
  def root?
    false
  end

  def current?
    false
  end

  def directory?
    false
  end

  def file?
    false
  end

  private

  def escape(name)
    name # TODO: something smart like escaping spaces and slashes
  end
end

class RootNode < PathNode
  def root?
    true
  end

  def to_s
    '' # when joined, this will look like '/'
  end

  def inspect
    '/'.inspect
  end
end

class CurrentNode < PathNode
  def current?
    true
  end

  def to_s
    '.' # when joined, this will look like './'
  end

  def inspect
    to_s.inspect
  end
end

class DirectoryNode < PathNode
  attr_reader :name

  def initialize(name)
    @name = escape(name)
  end

  def directory?
    true
  end

  alias to_s name

  def inspect
    name.inspect
  end
end

class FileNode < PathNode
  attr_reader :name

  def initialize(name)
    @name = escape(name)
  end

  def file?
    true
  end

  alias to_s name

  def inspect
    name.inspect
  end
end

RelativeDirectory.rb

class RelativeDirectory < AbstractPath
  def self.current
    RelativeDirectory.new(nodes: [])
  end

  def initialize(nodes:)
    raise "Last node cannot be a file, got (#{nodes.last.class})" if nodes.last&.file?
    if nodes.first&.root?
      super(nodes: nodes.drop(1).unshift(CurrentNode.new))
    elsif nodes.first&.current?
      super(nodes: nodes)
    else
      super(nodes: nodes.unshift(CurrentNode.new))
    end
  end

  def initialize_named(name)
    @nodes = [CurrentNode.new, DirectoryNode.new(name)]
  end

  def initialize_converting(path)
    initialize(nodes: path.nodes)
  end

  def absolute?
    false
  end

  def file?
    false
  end
end

RelativeFile.rb

class RelativeFile < AbstractPath
  def initialize(nodes:)
    raise "Last node must be a file, got (#{nodes.last.class})" unless nodes.empty? || nodes.last.file?
    if nodes.first&.root?
      super(nodes: nodes.drop(1).unshift(CurrentNode.new))
    elsif nodes.first&.current?
      super(nodes: nodes)
    else
      super(nodes: nodes.unshift(CurrentNode.new))
    end
  end

  def initialize_named(name)
    @nodes = [CurrentNode.new, FileNode.new(name)]
  end

  def initialize_converting(path)
    initialize(nodes: path.nodes)
  end

  def absolute?
    false
  end

  def file?
    true
  end
end