I'm not really sure of Rust's semantics, but this sort of thing is achieved using GADTs in Haskell. They look like this data T a where TI :: Int -

I'm not really sure of Rust's semantics, but this sort of thing is achieved using GADTs in Haskell. They look like this data T a where TI :: Int -> T Int TS :: String -> T String which lets you reflect a value up into its type. Something similar but more implicit seems to be going on with that Rust example. Another method is kind of a hack: unify the type variable with an unused argument. This looks like data T a b = T b -- we want to find a way to choose a setA :: a -> T a b -> T a b setA _ t = t -- Just returns the second argument -- BUT tells the compiler the `a` type unifies -- with the type of the first argument > :t setA Int (T "foo") setA Int (T "foo") :: T Int String