reader sample for medium article

reader.kt

class Reader<C, out A>(val run: (C) -> A) {

  inline fun <B> map(crossinline fa: (A) -> B): Reader<C, B> = Reader {
    c -> fa(run(c)) 
  }

  inline fun <B> flatMap(crossinline fa: (A) -> Reader<C, B>): Reader<C, B> = Reader {
    c -> fa(run(c)).run(c)
  }

  fun <B> zip(other: Reader<C, B>): Reader<C, Pair<A, B>> = this.flatMap { a ->
    other.map { b -> Pair(a, b) }
  }
  
  inline fun <D> local(crossinline fd: (D) -> C): Reader<D, A> = Reader { d ->
    run(fd(d))
  }

  companion object Factory {
    fun <C, A> pure(a: A): Reader<C, A> = Reader { _ -> a }

    fun <C> ask(): Reader<C, C> = Reader { it }
  }
}

fun <A, B> ((A) -> B).reader(): Reader<A, B> = Reader(this)

fun <C, A> Reader<C, Reader<C, A>>.flatten(): Reader<C, A> = flatMap { it }