Add better error messages

Author: JonathanBrouwer

src/main/scala/Chapter2Lib.scala

@@ -198,7 +198,7 @@ object Lvar {
     extends Lint.Interpretable {
     override def interpExpr(e: Expr, nv: List[(String,InterpreterValue)], inputs: ListBuffer[Long], outputs: ListBuffer[Long]): InterpreterValue = e match {
       case Var(x) =>
-        nv.find({ case (y, _) => x == y }).get._2
+        nv.find({ case (y, _) => x == y }).getOrElse(throw new RuntimeException("The interpreter to read variable " + x + " that had no value."))._2
       case Let(x, e, body) =>
         interpExpr(body, (x, interpExpr(e, nv, inputs, outputs)) :: nv, inputs, outputs)
       case Prim(Print(), List(e)) =>

src/main/scala/X86Lib.scala

@@ -132,16 +132,15 @@ object X86Interpreter {
                          val strict: Boolean) {
 
 
-
     def read(arg: Arg): Long = arg match {
       case Imm(v) => v
-      case Reg(reg) => this.registers(reg)
+      case Reg(reg) => this.registers.getOrElse(reg, throw new RuntimeException("The interpreter to read register " + reg + " that had no value."))
       case Deref(reg, offset) =>
-        val addr = this.registers(reg) + offset
+        val addr = this.registers.getOrElse(reg, throw new RuntimeException("The interpreter to read register " + reg + " that had no value.")) + offset
         assert(addr % 8 == 0, "Offset reads not supported")
         if (strict) assert(addr >= registers(RSP()), "Write outside of stack")
         this.memory(addr)
-      case XVar(name) => this.variables(name)
+      case XVar(name) => this.variables.getOrElse(name, throw new RuntimeException("The interpreter to read variable " + name + " that had no value."))
       case ByteRegister(reg) => reg match {
         case AL() => this.registers(RAX()).toByte
         case BL() => this.registers(RBX()).toByte
@@ -158,7 +157,7 @@ object X86Interpreter {
       case Imm(_) => assert(false, "Write to immediate value")
       case Reg(reg) => this.registers(reg) = w
       case Deref(reg, offset) =>
-        val addr = this.registers(reg) + offset
+        val addr = this.registers.getOrElse(reg, throw new RuntimeException("The interpreter to read register " + reg + " that had no value.")) + offset
         assert(addr % 8 == 0, "Offset reads not supported")
         if (strict) assert(addr >= registers(RSP()), "Write outside of stack")
         this.memory(addr) = w