Upstar Profunctor instance in Gleam

profunctor.gleam

// dimap :: (a -> b) -> (c -> d) -> p b c -> p a d
// p b c -> c b
// p a d -> d a
pub type Profunctor(p, q, a, b, c, d) {
  Profunctor(dimap: fn(#(fn(a) -> b, fn(c) -> d, p)) -> q)
}

pub type Upstar(f, a) {
  Upstar(run: fn(a) -> f)
}

pub fn upstar_profunctor() -> Profunctor(Upstar(c, b), Upstar(d, a), a, b, c, d) {
  let dimap = fn(inp: #(fn(a) -> b, fn(c) -> d, Upstar(c, b))) {
    let f = inp.0
    let g = inp.1
    let Upstar(run) = inp.2
    Upstar(run: fn(x) { g(run(f(x))) })
  }
  Profunctor(dimap)
}