diff --git a/src/main/wacc/Frontend/microWacc.scala b/src/main/wacc/Frontend/microWacc.scala
index 1faf3a8..88e43f2 100644
--- a/src/main/wacc/Frontend/microWacc.scala
+++ b/src/main/wacc/Frontend/microWacc.scala
@@ -5,16 +5,19 @@ import cats.data.NonEmptyList
 object microWacc {
   import wacc.types._
 
-  sealed trait CallTarget
+  sealed trait CallTarget(val retTy: SemType)
   sealed trait Expr(val ty: SemType)
-  sealed trait LValue
+  sealed trait LValue extends Expr
 
   // Atomic expressions
   case class IntLiter(v: Int) extends Expr(KnownType.Int)
   case class BoolLiter(v: Boolean) extends Expr(KnownType.Bool)
   case class CharLiter(v: Char) extends Expr(KnownType.Char)
   case class ArrayLiter(elems: List[Expr])(ty: SemType) extends Expr(ty)
-  case class Ident(name: String)(identTy: SemType) extends Expr(identTy) with CallTarget with LValue
+  case class Ident(name: String)(identTy: SemType)
+      extends Expr(identTy)
+      with CallTarget(identTy)
+      with LValue
   case class ArrayElem(name: Ident, indices: NonEmptyList[Expr])(ty: SemType)
       extends Expr(ty)
       with LValue
@@ -48,17 +51,18 @@ object microWacc {
   // Statements
   sealed trait Stmt
 
-  enum Builtin extends CallTarget {
-    case Read
-    case Free
-    case Exit
-    case Print
+  object Builtin {
+    case object ReadInt extends CallTarget(KnownType.Int)
+    case object ReadChar extends CallTarget(KnownType.Char)
+    case object Print extends CallTarget(?)
+    case object Exit extends CallTarget(?)
+    case object Free extends CallTarget(?)
   }
 
   case class Assign(lhs: LValue, rhs: Expr) extends Stmt
   case class If(cond: Expr, thenBranch: List[Stmt], elseBranch: List[Stmt]) extends Stmt
   case class While(cond: Expr, body: List[Stmt]) extends Stmt
-  case class Call(target: CallTarget, args: List[Expr]) extends Stmt
+  case class Call(target: CallTarget, args: List[Expr]) extends Stmt with Expr(target.retTy)
   case class Return(expr: Expr) extends Stmt
 
   // Program