Dynamically create extensible record in shapeless 2.0 - scala

Question

I need to produce an extensible record given an HList of keys and a map of values, here's a MWE of what I'm trying to achieve (you can copy/paste this in any REPL with shapeless 2.0 available, in order to reproduce the issue)
import shapeless._; import syntax.singleton._; import record._
case class Foo[T](column: Symbol)
val cols = Foo[String]('column1) :: HNil
val values = Map("column1" -> "value1")
object toRecord extends Poly1 {
implicit def Foo[T] = at[Foo[T]] { foo =>
val k = foo.column.name
val v = values.get(k)
(k ->> v)
}
}
val r = cols.map(toRecord)
// r: shapeless.::[Option[String] with shapeless.record.KeyTag[k.type,Option[String]] forSome { val k: String },shapeless.HNil] = Some(value1) :: HNil
val value = r("column1")
// error: No field String("column1") in record shapeless.::[Option[String] with shapeless.record.KeyTag[k.type,Option[String]] forSome { val k: String },shapeless.HNil]
val value = r("column1")
If I try defining the record manually everything works as expected
val q = ("column1" ->> Some("value1")) :: HNil
// q: shapeless.::[Some[String] with shapeless.record.KeyTag[String("column1"),Some[String]],shapeless.HNil] = Some(value1) :: HNil
q("column1")
// Some[String] = Some(value1)
Clearly the difference is that in one case the KeyTag has type
KeyTag[String("column1"), Some[String]]
and in the (non-working) other
KeyTag[k.type,Option[String]] forSome { val k: String }
I sense the issue is with the string k not being statically known, but I have no clue on how to fix this.
Generally speaking, is there a way of dynamically generating an extensible record from a list of keys?
I fear the answer is to use a macro, but I'd be glad if another solution existed.

Answer 1

This isn't too bad if you can change your Foo definition a bit to allow it to keep track of the singleton type of the column key (note that I've removed the unused T type parameter):
import shapeless._; import syntax.singleton._; import record._
case class Foo[K <: Symbol](column: Witness.Aux[K])
val cols = Foo('column1) :: HNil
val values = Map("column1" -> "value1")
object toRecord extends Poly1 {
implicit def atFoo[K <: Symbol] = at[Foo[K]] { foo =>
field[K](values.get(foo.column.value.name))
}
}
val r = cols.map(toRecord)
And then:
scala> val value = r('column1)
value: Option[String] = Some(value1)
Note that I've changed your string key ("column1") to a symbol, since that's what we've put into the record.