Inferred type of function that zips HLists

hlist, scala, shapeless

Solution

You can wrap everything up in one method using the `Zip` (or in this case `Zip.Aux`) type class:

import shapeless._, shapeless.ops.hlist._

object mix extends Poly1 {
  implicit def caseTuple[T] = at[(T, T)](t =>
    if (util.Random.nextBoolean) t._2 else t._1)
}

def zipAndMix[L <: HList, Z <: HList](h1: L, h2: L)(implicit
  zipper: Zip.Aux[L :: L :: HNil, Z],
  mapper: Mapper[mix.type, Z]
) = (h1 zip h2) map mix

Now assuming you have `h1` and `h2` defined as in the question, you can write this:

scala> zipAndMix(h1, h2)
res0: shapeless.::[Int,shapeless.::[String,shapeless.HNil]] = 5 :: b :: HNil

scala> zipAndMix(h1, h2)
res1: shapeless.::[Int,shapeless.::[String,shapeless.HNil]] = 6 :: a :: HNil

scala> zipAndMix(h1, h2)
res2: shapeless.::[Int,shapeless.::[String,shapeless.HNil]] = 5 :: a :: HNil

And so on. This will work in either 2.0.0-M1 or the latest snapshot, although (as I've noted in a comment above) you may run into confusing issues on the way before this bug was fixed.

Problem

Thanks to https://github.com/milessabin/shapeless/wiki/Feature-overview:-shapeless-2.0.0 I understand how to zip shapeless HLists: Import some stuff from Shapeless 2.0.0-M1: ``` import shapeless._ import shapeless.ops.hlist._ import syntax.std.tuple._ import Zipper._ ``` Create two HLists: ``` scala> val h1 = 5 :: "a" :: HNil h1: shapeless.::[Int,shapeless.::[String,shapeless.HNil]] = 5 :: a :: HNil scala> val h2 = 6 :: "b" :: HNil h2: shapeless.::[Int,shapeless.::[String,shapeless.HNil]] = 6 :: b :: HNil ``` Zip them: ``` scala> (h1, h2).zip res52: ((Int, Int), (String, String)) = ((5,6),(a,b)) ``` Now try to define a function that does the same thing: ``` scala> def f[HL <: HList](h1: HL, h2: HL) = (h1, h2).zip f: [HL <: shapeless.HList](h1: HL, h2: HL)Unit ``` The inferred return type is Unit, and indeed applying f to h1 and h2 does just that: ``` scala> f(h1, h2) scala> ``` Is there a way to define f such that I get ((5,6),(a,b)) back in this case? Ultimately what I'm trying to do is define a function that zips the two HLists and then maps over them, choosing either _1 or _2 based a coin toss, which would yield another HL. ``` object mix extends Poly1 { implicit def caseTuple[T] = at[(T, T)](t => if (util.Random.nextBoolean) t._2 else t._1) } ``` Which works fine in the REPL: ``` scala> (h1, h2).zip.map(mix) res2: (Int, String) = (5,b) ``` But I'm getting tripped up on the above issue when trying to pull this into a function. Thanks!

Original source