--[[ Expression Language 1, Values Module
     CSci 450: Organization of Programming Languages, Fall 2016
     H. Conrad Cunningham, Professor
     Computer and Information Science
     University of Mississippi

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2016-09-13: Separated as module from Assignment #1 code
2016-09-21: Added valToString function
            Added global variables for module names

This module encapsulates implementations of the Values used
in the Expression Language interpreter.

This module will need modification for languages with richer sets 
of values than integers.

For example, the representation for the concepts false and true
differs from one language to another and likely differs from the
values used by Lua. The intentions of features FALSEVAL, TRUEVAL, 
isFalse, isTrue, and asBoolVal are to isolate the differences between Lua and how those are handled in the interpreted languages.

Note:  The features of this layer should be sufficient for
Assignment #1,

Public constants:            TRUEVAL, FALSEVAL
Public query functions:      isTrue, isFalse
Public conversion function:  asBoolVal, valToString

--]]

--[[ Module names now global
local UTILITIES   = "utilities"
--]]

-- Import Utilities module

local util = require(UTILITIES)
local treeConcat, show_data = util.treeConcat, util.show_data

-- BOOLEAN VALUES

-- Public constants for true and false in interpreted language

local FALSEVAL = 0
local TRUEVAL  = 1

-- Public query functions

local function isFalse(v)
   return v == FALSEVAL
end

local function isTrue(v)
  return v ~= FALSEVAL
end

-- Public conversion function "asBoolVal" takes a Lua falsey or truthy
-- value and returns the corresponding value used in the interpreted
-- language (FALSEVAL or TRUEVAL).

local function asBoolVal(b) 
  if b then return TRUEVAL else return FALSEVAL end 
end

-- Public function "valToString" converts value "v" to a string
-- and returns it.

local function valToString(v)
  local tv = type(v)
  if tv == "number" then
     return tostring(v)
  else
    return "Bad value:  " .. show_data(v)
  end
end

-- MODULE EXPORT LIST

return {
   TRUEVAL     = TRUEVAL,
   FALSEVAL    = FALSEVAL,
   isTrue      = isTrue,
   isFalse     = isFalse,
   asBoolVal   = asBoolVal,
   valToString = valToString
}