Example of how you could use SiB to do dynamic tab completion

0_example_of_dynamic_completion.rb

# Based on an issue someone opened on a gem I'm mirroring
# https://github.com/JoshCheek/rcodetools/issues/1#issuecomment-278332781
require_relative 'completion'

Completion.new(source: DATA.read, line: 20, column: 12).completions
# => [["Foo", "mouse"],
#     ["Foo", "monster"],
#     ["Foo", "mongoose"],
#     ["Object", "mocking_bird"]]

__END__
class Foo
  def mouse
    :squeek
  end

  def monster
    :rar
  end

  def mongoose
    :blep
  end
end

class Object
  def mocking_bird
    :oooh_look_at_me_SQUAWK_whoooo_im_an_object_SQUAWK!
  end
end

1+Foo.new.mo+2

completion.rb

require 'seeing_is_believing'
require 'parser/ruby24'
require 'json'

class Completion
  def initialize(source:, line:, column:)
    self.code   = SeeingIsBelieving::Code.new source
    self.line   = line
    self.column = column
    self.index  = column + code.linenum_to_index(line)
    self.ast    = find_ast code.root, index
  end

  def completions
    # This is for a :send ast on an explicit receiver, you'd have to figure out
    # what it should be for each syntactic context you might be completing in
    receiver, to_complete = ast.children
    handler = SeeingIsBelieving.call code.raw, rewrite_code: -> * do
      code.rewriter.replace ast.location.expression,
                            competions_for_send(receiver)
      code.rewriter.process
    end
    handler.result[line]
           .map(&JSON.method(:parse))
           .select { |klass, method| method.start_with? to_complete.to_s }
  end

  private

  attr_accessor :code, :ast, :line, :column, :index

  def contains?(ast, index)
    return false unless ast.kind_of? AST::Node
    expr = ast.location.expression
    expr.begin_pos <= index && index <= expr.end_pos
  end

  def find_ast(ast, index)
    return nil unless contains?(ast, index)
    ast.children.map { |child| find_ast child, index }.compact.first || ast
  end

  def competions_for_send(ast)
    receiver_code = ast.location.expression.source
    <<-RUBY
    ( receiver = (#{receiver_code})
      klass = receiver.class
      begin klass = receiver.singleton_class
      rescue TypeError
        # you can't get the singleton class of every object
      end
      require 'json'
      klass.ancestors.each do |ancestor|
        ancestor.instance_methods(false).each do |method|
          $SiB.record_result :inspect, #{line}, [ancestor.name, method], &:to_json
        end
      end
      exit 0
    )
    RUBY
  end
end