package expAbstractData; /** A base class consisting of * - a root trait (i.e. abstract class) `Exp' with an `eval' function * - an abstract type `exp' bounded by `Exp' * - a concrete instance class `Num' of `Exp' for numeric literals */ trait Base { type exp <: Exp; trait Exp { def eval: int } class Num(v: int): exp extends Exp { val value = v; def eval = value } } object testBase extends Base with Executable { type exp = Exp; val term = new Num(2); System.out.println(term.eval); } /** Data extension: An extension of `Base' with `Plus' expressions */ trait BasePlus extends Base { class Plus(l: exp, r: exp): exp extends Exp { val left = l; val right = r; def eval = left.eval + right.eval } } /** Operation extension: An extension of `Base' with 'show' methods. */ trait Show extends Base { type exp <: Exp1; trait Exp1 extends Exp { def show: String; } class Num1(v: int): exp extends Num(v) with Exp1 { def show = value.toString(); } } /** Operation extension: An extension of `BasePlus' with 'show' methods. */ trait ShowPlus extends BasePlus with Show { class Plus1(l: exp, r: exp): exp extends Plus(l, r) with Exp1 { def show = left.show + " + " + right.show } }