func-hs

$ rustc test.rs
error[E0308]: mismatched types
 --> test.rs:2:35
  |
2 |     let r: Result<(), &str> = Err(format!("hello!"));
  |                                   ^^^^^^^^^^^^^^^^^
  |                                   |
  |                                   expected &str, found struct `std::string::String`
  |                                   help: consider borrowing here: `&format!("hello!")`
  |

func-hs

$rustc --version
rustc 1.23.0 (766bd11c8 2018-01-01)

$ rustc test.rs
...
error[E0308]: mismatched types
  --> test.rs:12:31
   |
12 |         self.m.insert("hoge", S2);
   |                               ^^ expected type parameter, found struct `S2`

func-hs

{-#LANGUAGE GADTs, KindSignatures, DataKinds, TypeOperators, TypeFamilies#-}

import GHC.TypeLits

data DList :: Nat -> * -> * where
    DNil :: DList 0 a
    DCons :: a -> DList n a -> DList (n + 1) a

append :: DList n a -> DList m a -> DList (n + m) a
append = undefined