Cool Monday - HList and Shapeless

Posted on October 29, 2012

HList as in heterogenous lists. This means every element is of different type. Yeah sure, just list List in Java, but that is in no way typesafe. I want compiler to know the type of every element and stop me if I try to do something silly.

Linked lists to the rescue

So what’s a linked list anyway? A sequence of nodes with pointers to next. And a nice implementation(still talking Java here) would be generic to allow type-safety for homogeneous lists. It turns out generics are solution for HLists too. Just introduce additional type parameter. Apocalisp has a great post on implementing them in Java. Java requires A LOT of type annotation. It works but it’s just painful and it doesn’t pay off.

Type inference to the rescue

Type inference gets rid of this problem entirely. Let’s implement whole working HList in scala.

abstract class HList[H,T<:HList[_,_]] {
    def head: H
    def tail: T  def ::[A](a: A) = Hcons(a, this)
}

object HNil extends HList[Nothing, Nothing]{
    def head = throw new IllegalAccessException("head of empty hlist")
    def tail = throw new IllegalAccessException("tail of empty hlist")
}

case class Hcons[H,T<:HList[_,_]](
    private val head: H, private val tail: T) extends HList[H, T]

So this list can be instantiated like this

scala> val myHList = 1 :: "hi" :: 2.0 :: HNil
myHList: Hcons[Int,HList[java.lang.String,HList[Double,HList[Nothing,Nothing]]]] =
    Hcons(1,Hcons(hi,Hcons(2.0,[email protected])))

And it just works. Scala compiler does all the heavy lifting with type annotations. This implementation bare bones and doesn’t provide any useful methods(even random access!). Check out Miles Sabin’s shapeless project for a useful implementation and much more. I provides indexing, map, fold, concatenation, type-safe casts, conversions to tuples(and abstracting over arities!) and back. And even conversions with case classes. Just click the link above and read the readme. It’s awesome.

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