Tagless Final chat discord

tagless_final.rs

trait Num {
  type Repr;
  fn num(n : i32)->Self::Repr;
  fn plus(l : Self::Repr, r : Self::Repr)->Self::Repr;
}

struct ReprInt;
impl Num for ReprInt {
  type Repr = i32;
  fn num(n : i32)->Self::Repr { n }
  fn plus(l : Self::Repr, r : Self::Repr)->Self::Repr { l + r };
}
struct ReprString;
impl Num for ReprInt {
  type Repr = String;
  fn num(n : i32)->Self::Repr { format!{"{}"n} }
  fn plus(l : Self::Repr, r : Self::Repr)->Self::Repr { format!{"({l} + {r})"} };
}

fn one_plus_one<N : NUM>() -> N { N::plus(N::num(1), N::num(1)) } 

fn g() {
  let s = one_plus_one::<ReprString>();
  let i = one_plus_one::<ReprInt>();
}


trait Num {
  type Repr;
  fn num(&self, n : i32)->Self::Repr;
  fn plus(&self, l : Self::Repr, r : Self::Repr)->Self::Repr;
}
struct ReprInt;
impl Num for ReprInt {
  type Repr = i32;
  fn num(&self, n : i32)->Self::Repr { n }
  fn plus(&self, l : Self::Repr, r : Self::Repr)->Self::Repr { l + r };
}
struct ReprString;
impl Num for ReprInt {
  type Repr = String;
  fn num(&self, n : i32)->Self::Repr { format!{"{}"n} }
  fn plus(&self, l : Self::Repr, r : Self::Repr)->Self::Repr { format!{"({l} + {r})"} };
}

fn one_plus_x(n : &dyn Num, x : i32) -> N { 
  n.plus(n.num(1), n.num(x))
}


fn g() {
  let s = one_plus_one(&ReprString);
  let i = one_plus_one(&ReprInt);
}

TaglessFinal.sml

datatype expr = PLUS of expr * expr 
              | NUM  of int

fun eval_expr (NUM n)      = n
  | eval_expr (PLUS (l,r)) = eval_expr l + eval_expr r

fun pretty_expr (NUM n)      = Int.toString n
  | pretty_expr (PLUS (l,r)) = "( " ^ pretty_expr l ^ " + " ^ pretty_expr r ^ " )"


val one_plus_one = PLUS (NUM 1, NUM 1)


fun num  (repr, _        ) n   = repr n
fun plus (_   , plus_repr) l r = plus_repr l r

fun one_plus_one d = plus d (num d 1) (num d 1)

val repr_string = ( Int.toString, 
                    fn l => fn r => "( " ^ l ^ " + " ^ r ^ " )"
                  )

val repr_num = ( fn x => x , 
                 fn l => fn r => l + r
               )

val repr_ast = (NUM, fn l => fn r => PLUS(l,r) )

val _ = print (one_plus_one repr_string)
val _ = (print o Int.toString) (one_plus_one repr_num)
val _ = one_plus_one repr_ast