Shapeless - turn a case class into another with fields in different order

Question

I\'m thinking of doing something similar to Safely copying fields between case classes of different types but with reordered fields, i.e.

case class A(foo: I         


        

Answer 1

As noticed @MilesSabin (godlike shapeless creator), there is an align operation, it is used like:

import ops.hlist.Align

val aGen = LabelledGeneric[A]
val bGen = LabelledGeneric[B]
val align = Align[aGen.Repr, bGen.Repr]
bGen.from(align(aGen.to(A(12, 13)))) //> res0: B = B(13,12)

P.S. Noticed that there is an example on GitHub.

Answer 2

I should leave this for Miles but it's happy hour where I'm from and I can't resist. As he points out in a comment above, the key is ops.hlist.Align, which will work just fine for records (which are just special hlists, after all).

If you want a nice syntax, you need to use a trick like the following for separating the type parameter list with the target (which you want to provide explicitly) from the type parameter list with all the other stuff (which you want to be inferred):

import shapeless._, ops.hlist.Align

class SameFieldsConverter[T] {
  def apply[S, SR <: HList, TR <: HList](s: S)(implicit
    genS: LabelledGeneric.Aux[S, SR],
    genT: LabelledGeneric.Aux[T, TR],
    align: Align[SR, TR]
  ) = genT.from(align(genS.to(s)))
}

def convertTo[T] = new SameFieldsConverter[T]

And then:

case class A(foo: Int, bar: Int)
case class B(bar: Int, foo: Int)

And then:

scala> convertTo[B](A(12, 13))
res0: B = B(13,12)

Note that finding alignment instances will get expensive at compile time for large case classes.