diff --git a/src/main/scala/project2/Interpreter.scala b/src/main/scala/project2/Interpreter.scala
index 88338c6..5a60ea7 100644
--- a/src/main/scala/project2/Interpreter.scala
+++ b/src/main/scala/project2/Interpreter.scala
@@ -262,7 +262,7 @@ class StackInterpreter extends Interpreter with BugReporter {
     case Prim(op, lop, rop) => 
       eval(lop, sp)(env)
       eval(rop, sp + 1)(env)
-      evalBin(sp, sp + 1)
+      evalBin(op)(sp, sp + 1)
     case Let(x, a, b) => 
       eval(a, sp)(env)
       eval(b, sp + 1)(env.withVal(x, sp))
diff --git a/src/main/scala/project2/Parser.scala b/src/main/scala/project2/Parser.scala
index e189f23..1343cf3 100644
--- a/src/main/scala/project2/Parser.scala
+++ b/src/main/scala/project2/Parser.scala
@@ -128,7 +128,7 @@ class Scanner(in: Reader[Char]) extends Reader[Tokens.Token] with Reporter {
    * TODO: implement the method
    */
   def getNum() = {
-    val buf = new java.lang.StringBuilder
+    val buf = new StringBuilder
     while (in.hasNext(isDigit)) {
       buf += in.next()
     }
@@ -596,7 +596,7 @@ class VariableParser(in: Scanner) extends BranchParser(in) {
   // TODO: remove ??? and complete the implementation.
   override def parseSimpleExpression = (in.peek, in.peek1) match {
     case (Ident(x), Delim('=')) => 
-      val pos = in.pos 
+      val pos:Position = in.peek.pos 
       VarAssign(x, parseSimpleExpression).withPos(pos)
     case _ => super.parseExpression
   }
diff --git a/src/main/scala/project2/SemanticAnalyzer.scala b/src/main/scala/project2/SemanticAnalyzer.scala
index 1241697..fb75a4a 100644
--- a/src/main/scala/project2/SemanticAnalyzer.scala
+++ b/src/main/scala/project2/SemanticAnalyzer.scala
@@ -122,8 +122,8 @@ class SemanticAnalyzer(parser: Parser) extends Reporter {
     case Prim(op, lop, rop) => // Done
       if(!(isOperator(op) || isBOperator(op))) 
         error("undefined primitive operator")
-      analyze(lop)
-      analyze(rop)
+      analyze(lop)(env)
+      analyze(rop)(env)
     case Let(x, a, b) => // val x = a; b
       // Done: check variable reuse
       if(!env.vars.contains(x))
@@ -137,7 +137,6 @@ class SemanticAnalyzer(parser: Parser) extends Reporter {
     case Cond(op, l, r) =>
       if (!isBOperator(op))
         error(s"$op is not a boolean operator")
-      analyze(op)(env)
       analyze(l)(env)
       analyze(r)(env)
       // Done
@@ -152,7 +151,7 @@ class SemanticAnalyzer(parser: Parser) extends Reporter {
       analyze(body)(env)
       // TODO
     case VarAssign(x, rhs) => // x = rhs
-      if (!(env.vars.contains(x) && env.vars[x])) 
+      if (!(env.vars.contains(x) && env.vars(x))) 
         error(s"cannot assign $x: ${if(env.vars.contains(x))"immutable"else"undefined"}")
       analyze(rhs)(env)
       // TODO