scala - Composing two functions to get a function that returns HList -

this naive question shapeless:

suppose have functions a => m[b] , b => m[c]. how can compose them new function a => m[b::c::hnil] ?

if want generically can use scalaz's arrow:

import scalaz._, scalaz._  def andthenbutkeep[arr[_, _]: arrow, a, b, c](   f: arr[a, b],   g: arr[b, c] ): arr[a, (b, c)] = f >>> (category[arr].id &&& g) 

or if want hlist instead of tuple:

import scalaz._, scalaz._ import shapeless._, shapeless.syntax.std.tuple._  def andthenbutkeep[arr[_, _], a, b, c](   f: arr[a, b],   g: arr[b, c] )(implicit arr: arrow[arr]): arr[a, b :: c :: hnil] =   f >>> (arr.id &&& g) >>> arr.arr((_: (b, c)).productelements) 

now you'd wrap functions in kleisli arrow:

type optionfunc[a, b] = kleisli[option, a, b]  val f: optionfunc[int, string] = kleisli(i => some("a" * i)) val g: optionfunc[string, int] = kleisli(s => some(s.length))  val c = andthenbutkeep(f, g) 

and then:

scala> println(c.run(10)) some(aaaaaaaaaa :: 10 :: hnil) 

you make little less fussy type inference (but less generic) restricting arrow kleisli arrow on m.