Generic field accessor powered by Shapeless - scala

Question

I'm trying to implement a convenient generic field accessor based on LabelledGeneric.
The usage should look like:
case class Foo(aha: String, uhu: Double, ehe: Int)
case class Bar(uhu: Double, ahu: Boolean)
val foo: Foo = ???
val bar: Bar = ???
val uhuGenField = new GenField('uhu)
val uhuFooAccess = uhuGenField.from[Foo]
val uhuBarAccess = uhuGenField.from[Bar]
def someFunWithUhu[X](xs: Seq[X], access: uhuGenField.Access[X]) = ???
I spent some time trying to figure out how to achieve such behaviour.
Eventually I came up with this approach:
import shapeless._
import shapeless.ops.record.Selector
final class GenField[V](val fieldName: Symbol) {
val fieldWitness = Witness(fieldName)
type FieldNameType = fieldWitness.T
trait Access[C] {
def get(c: C): V
}
def from[C](implicit lg2hl: LGtoHL[C]): Access[C] = new Access[C] {
override def get(c: C): V = {
val labelledGeneric = lg2hl.labelledGeneric
val selector = Selector.mkSelector[labelledGeneric.Repr, FieldNameType, V]
selector(labelledGeneric.to(c))
}
}
}
// I need something like this to enable syntax like
// genField.from[DesiredClass]
// i.e. to "latch" Repr inside a single instance
// and to don't pass it explicitly to `from` method.
sealed trait LGtoHL[A] {
type Repr <: HList
val labelledGeneric: LabelledGeneric.Aux[A, Repr]
}
object LGtoHL {
implicit def mkLGtoHL[A, ARepr <: HList](implicit lg: LabelledGeneric.Aux[A, ARepr]): LGtoHL[A] = {
new LGtoHL[A] {
override type Repr = ARepr
override val labelledGeneric: LabelledGeneric.Aux[A, Repr] = lg
}
}
}
From my prospective this solution should be OK, but it still doesn't work.
The compilation fails with the following error message:
Error:(17, 41) lg2hl.Repr is not an HList type
val selector = Selector.mkSelector[labelledGeneric.Repr, FieldNameType, V]
Why does it complain lg2hl.Repr is not an HList type?
Repr is explicitly defined in LGtoHL as type Repr <: HList.
What is wrong with my code?
Very appreciate your help.

Answer 1

Why are lenses not enough?
import shapeless.{Lens, lens}
case class Foo(aha: String, uhu: Double, ehe: Int)
case class Bar(uhu: Double, ahu: Boolean)
val foo: Foo = Foo("a", 1.0, 2)
val bar: Bar = Bar(3.0, true)
val fooUhu: Lens[Foo, Double] = lens[Foo] >> 'uhu
val barUhu: Lens[Bar, Double] = lens[Bar] >> 'uhu
fooUhu.get(foo) // 1.0
barUhu.get(bar) // 3.0
The error message
lg2hl.Repr is not an HList type
comes from here: https://github.com/milessabin/shapeless/blob/master/core/src/main/scala/shapeless/generic.scala#L511
u.baseType(HConsSym) is now NoType.
I guess GenField[V](val fieldName: Symbol) will not work since fieldName in Witness(fieldName) must be known at compile time. For example
lens[Foo] >> 'uhu
works but
val uhu: Witness.`'uhu`.T = 'uhu.narrow
lens[Foo] >> uhu
doesn't. This is the reason why lenses, Witness, LabelledGeneric are implemented via macros.