Map over a List of hlist'able things, yield a List of their first elements

gistfile1.scala

scala> import shapeless._
import shapeless._

scala> import Tuples._
import Tuples._

scala> def fst[P <: Product, H, T <: HList](p : P)(implicit hl : HListerAux[P, H :: T]) = hl(p).head
fst: [P <: Product, H, T <: shapeless.HList](p: P)(implicit hl: shapeless.HListerAux[P,shapeless.::[H,T]])H

scala> val l = List(("foo", "bar"), ("baz", "wibble"))
l: List[(String, String)] = List((foo,bar), (baz,wibble))

scala> for(x <- l) yield fst(x)  // l map fst interacts badly with type inference ... must do better!
res0: List[String] = List(foo, baz)