rust_printf.rs

use frunk::hlist::*;
use std::marker::PhantomData;

pub struct FString(&'static str);
pub struct FVar;

trait Format<ArgList> {
  fn format(&self, args: ArgList) -> String;
}

impl Format<HNil> for HNil {
  fn format(&self, _args: HNil) -> String {
    "".to_string()
  }
}

impl<ArgList, FmtList> Format<ArgList> for HCons<FString, FmtList>
where
  FmtList: Format<ArgList>,
{
  fn format(&self, args: ArgList) -> String {
    self.head.0.to_owned() + &self.tail.format(args)
  }
}

impl<T, ArgList, FmtList> Format<HCons<T, ArgList>> for HCons<FVar, FmtList>
where
  FmtList: Format<ArgList>,
  T: ToString,
{
  fn format(&self, args: HCons<T, ArgList>) -> String {
    args.head.to_string() + &self.tail.format(args.tail)
  }
}

#[test]
fn test() {
  use frunk::hlist;
  let fmt = hlist![FString("Hello "), FVar];
  assert_eq!(fmt.format(hlist!["world"]), "Hello world");
}