Recall function interpExpr function in

• Handling Scopes in Interpreters
• Semantic Analysis as Simplified Interpretation

Forget about uninitialized values for now, worry only about types

We evaluate type analogously to evaluating expressions

Principles:

• all variables are declared with their types
• scoping rules determine how to update environment (innermost declarations take precedence)
• lookup in environment gives types for variable uses
• each operator has type evaluation rules

Relevant piece of code from Semantic Analysis as Simplified Interpretation:

  def interpExpr(e : Env)(expr : Expression) : Value = {
    expr match {
      case Var(n) => {
        getVar(e, n).v
      }
      case IntConst(i) => IntValue
      case Plus(expr1,expr2) => {
	 (interpExpr(e)(expr1),interpExpr(e)(expr2)) match {
	   case (IntValue,IntValue) => IntValue
	   case (_,_) => error("type error")
	 }
       }
       case BoolConst(b) => BoolValue
       case Leq(expr1, expr2) => {
	 (interpExpr(e)(expr1),interpExpr(e)(expr2)) match {
	   case (IntValue,IntValue) => BoolValue
	   case (_,_) => error("type error")
	 }
       }
       case And(expr1, expr2) => {
	 (interpExpr(e)(expr1),interpExpr(e)(expr2)) match {
	   case (BoolValue,BoolValue) => BoolValue
	   case (_,_) => error("type error")
	 }
       }
     }
  }