diff --git a/src/lambda/AST.scala b/src/lambda/AST.scala
index 42ae209..6b3af2b 100644
--- a/src/lambda/AST.scala
+++ b/src/lambda/AST.scala
@@ -1,6 +1,8 @@
 package lambda
 
-sealed trait Expr
+import scala.util.parsing.input.Positional
+
+sealed trait Expr extends Positional
 
 case class Lambda(arg: Var, body: Expr) extends Expr
 
diff --git a/src/lambda/LambdaParser.scala b/src/lambda/LambdaParser.scala
index 5344656..08d1e7f 100644
--- a/src/lambda/LambdaParser.scala
+++ b/src/lambda/LambdaParser.scala
@@ -12,11 +12,11 @@ class LambdaParser extends StdTokenParsers with PackratParsers {
   type P[+T] = PackratParser[T]
   lazy val expr: P[Expr]         = application | notApp
   lazy val notApp                = variable | parens | lambda
-  lazy val lambda: P[Lambda]     = ("λ" | "\\") ~> variable ~ "." ~ expr ^^
-                                   { case v ~ "." ~ e  => Lambda(v, e) }
-  lazy val application: P[Apply] = expr ~ notApp ^^
-                                   { case left ~ right => Apply(left, right) }
-  lazy val variable: P[Var]      = ident ^^ Var.apply
+  lazy val lambda: P[Lambda]     = positioned(("λ" | "\\") ~> variable ~ "." ~ expr ^^
+                                   { case v ~ "." ~ e  => Lambda(v, e) })
+  lazy val application: P[Apply] = positioned(expr ~ notApp ^^
+                                   { case left ~ right => Apply(left, right) })
+  lazy val variable: P[Var]      = positioned(ident ^^ Var.apply)
   lazy val parens: P[Expr]       = "(" ~> expr <~ ")"
 
   def parse(str: String): ParseResult[Expr] = {
diff --git a/src/lambda/LambdaREPL.scala b/src/lambda/LambdaREPL.scala
index abb7400..e6f94cc 100644
--- a/src/lambda/LambdaREPL.scala
+++ b/src/lambda/LambdaREPL.scala
@@ -13,7 +13,15 @@ object LambdaREPL {
       val exprSrc = readLine("λ> ")
       import parser.{Success, NoSuccess}
       parser.parse(exprSrc) match {
-        case Success(expr, _) => println(pretty(eval(bind(expr))))
+        case Success(expr, _) =>
+          val bound = bind(expr)
+          if (bind.messages.isEmpty)
+            println(pretty(eval(bound)))
+          else {
+            for (m <- bind.messages)
+              println(m.pos.longString + m.msg)
+            bind.messages.clear()
+          }
         case err: NoSuccess   => println(err)
       }
     }
diff --git a/src/lambda/Message.scala b/src/lambda/Message.scala
new file mode 100644
index 0000000..ac746a2
--- /dev/null
+++ b/src/lambda/Message.scala
@@ -0,0 +1,5 @@
+package lambda
+
+import scala.util.parsing.input.Position
+
+case class Message(pos: Position, msg: String)
\ No newline at end of file
diff --git a/src/lambda/Scope.scala b/src/lambda/Scope.scala
index 12167ad..c2d1714 100644
--- a/src/lambda/Scope.scala
+++ b/src/lambda/Scope.scala
@@ -1,5 +1,7 @@
 package lambda
 
+import scala.collection.mutable.ListBuffer
+
 object Scope {
   var id = 0
   def nextId = { val i = id; id += 1; i }
@@ -19,6 +21,7 @@ class Scope(val parent: Option[Scope], val boundNames: Set[String]) {
 }
 
 class Binder() {
+  val messages = ListBuffer[Message]()
 
   def apply(term: Expr) = bind(term, Scope.TOP)
 
@@ -29,7 +32,9 @@ class Binder() {
     case v @ Var(name, _) =>
       (parent closestBinding name) match {
         case Some(scope)             => v.copy(scope = scope)
-        case None                    => sys.error("Undefined variable: " + name)
+        case None                    =>
+          messages += Message(v.pos, "Unbound variable: " + name)
+          v
       }
     case Apply(fun, arg) =>
       Apply(bind(fun, parent), bind(arg, parent))