Little Schemer group, chapter 1

.gitignore

*.pdf
*.dot

example.scm

(car (cons 1 (list 2 3)))
(list 4 5)

Makefile

.PHONY: clean all

all: diag.pdf

diag.dot: example.scm
	ruby ./vis.rb < example.scm > diag.dot

diag.pdf: diag.dot
	dot -Tpdf < diag.dot > diag.pdf

clean:
	rm -f diag.dot diag.pdf

parse.rb

require "./parser"

p Parser.parse_program($stdin.read)

parser.rb

require 'strscan'

module Parser
  module_function

  def parse_expression(scanner)
    if scanner.scan(/\(/)
      parse_list(scanner)
    elsif scanner.scan(/\s/)
      return :space
    elsif scanner.scan(/\)/)
      return :rparen
    elsif str = scanner.scan(/[^\s\(\)]+/)
      return parse_atom(str)
    elsif scanner.eos?
      return :eos
    else
      raise "dunno before #{scanner.rest}"
    end
  end

  def parse_atom(str)
    return str
  end

  def parse_list(scanner)
    list = []
    loop do
      element = parse_expression(scanner)
      case element
      when :rparen
        return list
      when :space
        # ignore me
      when :eos
        raise "unexpected end of input"
      else
        list << element
      end
    end
    raise "unbalanced parentheses before #{scanner.rest}"
  end

  def parse_program(string)
    scanner = StringScanner.new(string)
    list = []
    loop do
      element = parse_expression(scanner)
      case element
      when :eos
        return list
      when :space
        # ignore me
      else
        list << element
      end
    end
  end
end

vis.rb

require "./parser.rb"

def label_node(node)
  if node.respond_to?(:each)
    %(a#{node.object_id} [label="List" shape="rect"])
  else
    %(a#{node.object_id} [label="#{node.inspect.gsub(/"/, "'")}"];)
  end
end

def render_edges(structure, acc = [])
  structure.each do |e|
    acc << label_node(structure)
    acc << label_node(e)
    acc << "a#{structure.object_id} -> a#{e.object_id};"
    if e.respond_to?(:each)
      render_edges(e, acc)
    end
  end
  acc
end

structure = Parser.parse_program($stdin.read)

puts "digraph G {"
puts render_edges(structure).join("\n")
puts "}"