scala - Shapeless HList type checking -

i using shapeless , have following method compute difference between 2 hlists:

  def diff[h <: hlist](lst1: h, lst2:h):list[string] = (lst1, lst2) match {     case (hnil, hnil)                 => list()     case (h1::t1, h2::t2) if h1 != h2 => s"$h1 -> $h2" :: diff(t1, t2)     case (h1::t1, h2::t2)             => diff(t1, t2)     case _                            => throw new runtimeexception("something went wrong")   } 

since both parameters method take h, expect hlists of different types not compile here. example:

diff("a" :: hnil, 1 :: 2 :: hnil) 

shouldn't compile does, , produces runtime error: java.lang.runtimeexception: went wrong. there can type parameters make method accept 2 sides identical types?

unfortunately, base hlist trait unparameterized, , in method call h resolved hlist (which indeed supertype of hlist irrespective of concrete element types). fix have change definition somewhat, , rely instead on generalized type constraints:

def diff[h1 <: hlist, h2 <: hlist](lst1: h1, lst2: h2)(implicit e: h1 =:= h2): list[string] = (lst1, lst2) match {   case (hnil, hnil)                 => list()   case (h1::t1, h2::t2) if h1 != h2 => s"$h1 -> $h2" :: diff(t1, t2)   case (h1::t1, h2::t2)             => diff(t1, t2)   case _                            => throw new runtimeexception("something went wrong") } 

let's check:

scala> diff("a" :: hnil, 1 :: 2 :: hnil) <console>:12: error: cannot prove shapeless.::[string,shapeless.hnil] =:= shapeless.::[int,shapeless.::[int,shapele               diff("a" :: hnil, 1 :: 2 :: hnil)                   ^  scala> diff("a" :: hnil, "b" :: hnil) res5: list[string] = list(a -> b)  scala> diff("a" :: 1 :: hnil, "b" :: 2 :: hnil) res6: list[string] = list(a -> b, 1 -> 2) 

now still "cheat" , explicitly set h1 , h2 hlist, , we're square one.

scala> diff[hlist, hlist]("a" :: hnil, 1 :: 2 :: hnil) java.lang.runtimeexception: went wrong   @ .diff(<console>:15)   @ .diff(<console>:13) 

unfortunately don't think solvable (it though, don't have quick solution).