Add Let to staged interpreter test

Author: odersky

tests/pos/stagedInterpreter.scala

@@ -4,19 +4,30 @@ enum Exp {
   case Num(n: Int)
   case Plus(e1: Exp, e2: Exp)
   case Var(x: String)
+  case Let(x: String, e: Exp, in: Exp)
 }
 
 object Test {
   import Exp._
 
+  val keepLets = true
+
   val exp = Plus(Plus(Num(2), Var("x")), Num(4))
 
+  val letExp = Let("x", Num(3), exp)
+
   def compile(e: Exp, env: Map[String, Expr[Int]]): Expr[Int] = e match {
     case Num(n) => n
     case Plus(e1, e2) => '(~compile(e1, env) + ~compile(e2, env))
     case Var(x) => env(x)
+    case Let(x, e, body) =>
+      if (keepLets)
+        '{ val y = ~compile(e, env); ~compile(body, env + (x -> '(y))) }
+      else
+        compile(body, env + (x -> compile(e, env)))
   }
 
-  val res = (x: Int) => ~compile(exp, Map("x" -> '(x)))
+  val res1 = (x: Int) => ~compile(exp, Map("x" -> '(x)))
 
+  val res2 = compile(letExp, Map())
 }