--[[ Data Tagging Module
     H. Conrad Cunningham, Professor
     Computer and Information Science
     University of Mississippi
Developed for CSci 658, Software Language Engineering, Fall 2013

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2013-09-22: Separated module from Complex Number Tagged Data module

--]]

-- Load complex number utilities module
local util = require "complexUtilities"

-- local definitions for convenience
local show_data = util.show_data


-- From SICP Section 2.4.2: Tagged Data

-- Tagging

local function attach_tag(type_tag,contents)
  return { tag = type_tag, contents = contents }
end

-- Replaces Scheme function "pair?"  on cons list representation
local function is_tagged(datum)
  return type(datum) == "table" and datum.tag and datum.contents
end

local function type_tag(datum)
  if is_tagged(datum) then
    return datum.tag
  else
    error("Bad tagged datum -- TYPE_TAG" .. show_data(datum), 2)
  end
end

local function contents(datum)
  if is_tagged(datum) then
    return datum.contents
  else
    error("Bad tagged datum -- CONTENTS" .. show_data(datum), 2)
  end
end


-- MODULE EXPORT

return {
    attach_tag  = attach_tag,
    is_tagged   = is_tagged,
    type_tag    = type_tag,
    contents    = contents
  }