Getting items from an HList

Question

I played with HList and worked as follows:

val hl = 1 :: "foo" :: HNil val i: Int = hl(_0) val s: String = hl(_1) 

However, I can’t get the following code snippet (let random access to the lists for a moment be a smart idea ;-)):

 class Container(hl: HList) { def get(n: Nat) = hl(n) } val container = new Container(1 :: "foo" :: HNil) val i: Int = container.get(_0) val s: String = container.get(_1) 

I would like get return Int and String according to this parameter. I assume that, if possible at all, I should use Aux or at , but I'm not sure how to do this.

Answer 1

Try something on these lines,

 scala> import shapeless._, nat._, ops.hlist._ import shapeless._ import nat._ import ops.hlist._ scala> class Container[L <: HList](hl: L) { | def get(n: Nat)(implicit at: At[L, nN]): at.Out = hl[nN] | } defined class Container scala> val container = new Container(1 :: "foo" :: HNil) container: Container[shapeless.::[Int,shapeless.::[String,shapeless.HNil]]] = ... scala> container.get(_0) res1: Int = 1 scala> container.get(_1) res2: String = foo 

The first key difference is that instead of typing hl like a regular HList that loses all the specific information about the types of elements, we parameterize the exact type of the argument and save its structure as L The second difference is that we use L to index an implicit instance of a class of type At , which is used for indexing in get .

Also note that since there is an implicit conversion from Int literals to Nat , you can write

 scala> container.get(0) res3: Int = 1 scala> container.get(1) res4: String = foo