Can Map be performed on a Scala HList

Question

I have done a few implementations of HList now. One based on Daniel Spiewak\'s High Wizardry in the Land of Scala talk and another based on a post in Apocalisp blog. The goal

Answer 1

what you need is a Klist with type constructor Request, and a natural transformation execute: Request ~> Id. All of this is detailed in the marvelous type-level programming series of posts at Apocalisp, in particular:

1. Natural transformation literals
2. Klist basics

you can checkout the code for the whole series from Mark Harrah's up repo

In your case, you'll need something like

val reqList = new Request[Int](1) :^: new Request[String]("1") :^: KNil    
val exec = new (Request ~> Id) { def apply[T](reqs: Request[T]): T = reqs.execute }    
val results = reqList down exec

the down method above is conceptually the same as map for a nat transf M ~> Id; you also have more general map which from a nat transf M ~> N and a Klist of kind M yields a KList of kind N.