let flatten = (f=>out=>input=>f(f)(out)(input))
(f=>
  (out)=>
    input=>
      input.length === 0 ?
      out:
      f(f)
      (
        // join function*/
        ((arr)=>(val)=>[...arr, ...val])
        (out)
        (input[0])
      )
      (
        // slice function
        (([,...a])=>a)
        (input)
      )
)
([])
連flatMap都可以