Tagless Final

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trait Expr[T] {
    def num(n: Int): T
    def add(l: T, r: T): T
}

implicit def eval: Expr[Int] = new Expr[Int] {
    def num(i: Int): Int = i
    def add(l: Int, r: Int): Int = l + r
}

implicit def show: Expr[String] = new Expr[String] {
    def num(i: Int): String = i.toString
    def add(l: String, r: String): String = l + "+" + r
}

def program[T](implicit expr: Expr[T]) = expr.add(expr.num(1), expr.num(2))

 

 

trait Expr[T[_]] {
    def num(n: Int): T[Int]
    def bool(b: Boolean): T[Boolean]
    def add(l: T[Int], r: T[Int]): T[Int]
    def eq(l: T[Int], r: T[Int]): T[Boolean]
}

type Id[T] = T

implicit def eval: Expr[Id] = new Expr[Id] {
    def num(i: Int): Id[Int] = i
    def bool(b: Boolean): Id[Boolean] = b
    def add(l: Id[Int], r: Id[Int]): Id[Int] = l + r
    def eq(l: Id[Int], r: Id[Int]): Id[Boolean] = l == r
}

def program[T[_]](implicit expr: Expr[T]) = expr.add(expr.num(1), expr.num(2))