Homework_1.sml

(*
	Homework 1: Simple Register Machine
	By: Ashley Sanders
*)

(*Used to store the values of the two registers: a and b*)
type MachineState = {a:int, b:int};

(*May contain the value of any defined instruction*)
datatype Instruction = SetA of int | SetB of int | Add | Sub | Disp;

(*Creates a new MachineState with the given initial values*)
fun newMachine (a, b) : MachineState = {a=a, b=b};

(*Gets the current value of a in the given MachineState*)
fun getA (m : MachineState) = #a m;

(*Gets the current value of b in the given MachineState*)
fun getB (m : MachineState) = #b m;

(*Sets the value of a in the given MachineState*)
fun setA (m : MachineState, newA) = newMachine(newA, #b m);

(*Sets the value of b in the given MachineState*)
fun setB (m : MachineState, newB) = newMachine(#a m, newB);

(*Converts an Instruction to a string for ease of printing values*)
fun i2s (SetA a)	= "SetA " ^ Int.toString(a)
	| i2s (SetB b)	= "SetB " ^ Int.toString(b)
	| i2s Add		= "Add"
	| i2s Sub		= "Sub"
	| i2s Disp		= "Disp";

(*Evaluates the given Instruction based on the given MachineState*)
fun eval (m: MachineState, SetA a)	: MachineState = 
		(setA (m, a))
	| eval (m: MachineState, SetB b): MachineState = 
		(setB (m, b))
	| eval (m: MachineState, Add)	: MachineState = 
		(newMachine(getA (m) + getB (m), getB (m)))
	| eval (m: MachineState, Sub)	: MachineState = 
		(newMachine(getA (m) - getB (m), getB (m)))
	| eval (m: MachineState, Disp)	: MachineState = 
		(print (Int.toString (#a m) ^ "\n"); m);

(*Runs and prints the given list of Instructions*)
fun run (m: MachineState, []: Instruction list) = m
	| run (m: MachineState, prog: Instruction list) =
		let val instr = hd prog
			val instrs = tl prog
			val _ = print (i2s instr ^ "\n");
			val m1 = eval (m, instr);
		in 
			run (m1: MachineState, tl prog : Instruction list)
		end;