What's wrong with my simple compiler?

EDIT; I solved it, the reason it wasn't working was that it was in StarterGui, and I didn't spawn a player, I'm an idiot. it's still pretty broken though, so if you want to help me iron out some of the problems, I'd appreciate it.

I've been working on a simple compiler, but it just won't do anything, what's wrong with it? the value of input is '5+7/2-1' none of the print functions are called, and the script isn't disabled.

The OS Modulescript it imports from;

function Write(s) -- Writes a string
	if s ~= nil then
		print(s)
	else
		print("")
	end
end

function WriteLn(s) -- Writes a string to a new line
	if s ~= nil then
		print(s)
	else
		print("")
	end
end

function Halt() -- Halts the program
	print("HALT")
end

function In(x,t) -- checks to see if an item is in a table
	for i = 1,#t do
		if t[i] == x then
			return true
		end
	end
	return false
end

Alphas = -- All Letters
	{'A','B','C','D','E','F','G','H','I','J','K','L','M','N',
	'O','P','Q','R','S','T','U','V','W','X','Y','Z'}
	
Digits =  -- All Digits
	{'1','2','3','4','5','6','7','8','9','0'}

D = {nil,nil,nil,nil,nil,nil,nil,nil}

S = {}

return {Write,WriteLn,Halt,In,Alphas,Digits,'ready',D,S}

The Broken Script

print("start")

DEBUG = true -- set to false to disable debug printing

input = script.Parent.Parent.input -- fake input

print("1")
-- Necessary Functions
OS = require(script.Parent.OS)
print(OS[7])
D = OS[8]
SP = OS[9]
print('2')

Write = OS[1]
WriteLn = OS[2]
Halt = OS[3]
In = OS[4]

rd = 0
function Read()
	rd = rd + 1
	local re = string.sub(input.Value,rd,rd+1)
	PrintD(re)
	return re
end

function SP_push(x)
	PrintD(x)
	table.insert(SP, x)
end

function SP_pop()
	local rt = SP[-1]
	PrintD(rt)
	table.remove(SP)
	return rt
end
-- Debug Function
function PrintD(s)
	if DEBUG == true then
	if s ~= nil then
		WriteLn("DEBUG; "..s)
	else
		WriteLn("DEBUG MODE")
	end
	end
end	
-- Necessary Tables
Alphas = OS[5]
Digits = OS[6]
-- Compiler
local TAB = "	" -- Tab
local Look = "" -- lookup a char
	
local function GetChar() -- get Look
	Look = Read()
end

local function Error(s) -- Write an error
	warn("Error: "..s..".")
end	

local function Abort(s) -- Abort the program
	Error(s)
	Halt()
end

local function Expected(s) -- Write a "Char Expected" Error
	Abort(s.. " Expected")
end

local function Match(x) -- Match Characters
	if Look == x then GetChar() else
		Expected('"'..x..'"')
	end
end
	
local function IsAlpha(c) -- Check if a character is an Alpha Letter
	return In(string.upper(c),Alphas)
end


local function IsDigit(c) -- Check if a character is a Digit
	return In(c,Digits)
end

local function GetName() -- Get a Name
	if not IsAlpha(Look) then Expected('Name') end
	PrintD(string.upper(Look))
	return string.upper(Look)
end

local function GetNum() -- Get a Number
	if not IsDigit(Look) then Expected('Integer') end
	PrintD(Look)
	return Look
end

local function Emit(s) -- Print a String
	Write(TAB..s)
end

local function EmitLn(s) -- Print a String on a new line
	Emit(s)
	WriteLn()
end

--local function Term() -- Term
--	--Look = '7'
--	D[1] = GetNum()
--end

local function Factor()
	D[1] = GetNum()
end

local function Multiply()
	Match('*')
	Factor()
	EmitLn(SP_pop() * D[1])
end

local function Divide()
	Match('/')
	Factor()
	D[2] = SP_pop
	EmitLn(D[2]/D[1])
end

local function Term()
	Factor()
	while Look == '*' or Look == '?' do
		SP_push(D[1]) D[1] = nil
		
		if Look == '*' then
			Multiply()
		elseif Look == '/' then
			Divide()
		else Expected("Mulop") end
	end
end

local function Add() -- add
	Match('+')
	Term()
	EmitLn(SP_pop() + D[1])
end

local function Subtract() -- subtract
	Match('-')
	Term()
	EmitLn(SP_pop()-D[1])
end

local function Expression() -- Expressions
	Term()
	while Look == '+' or Look == '-' do
		SP_push(D[1]) D[1] = nil
		
		if Look == '+' then
			Add()
		elseif Look == '-' then
			Subtract()
		else Expected("Addop") end
			
	end
end

local function Init() -- Initiate
	GetChar()
end

local function begin() -- Start Function
	WriteLn("ZerOS")
	PrintD()
	Init()
	Expression()
end

begin() -- Run