1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
|
sealed trait HList
case class HCons[+HD, +TL](hd: HD, tl: TL) extends HList
case object HNil extends HList
sealed trait Num
case object Zero extends Num
case class Succ[N <: Num](pred: N) extends Num
object Test {
type HNil = HNil.type
type Zero = Zero.type
trait Concat[Xs <: HList, Ys <: HList, Zs <: HList] {
def concat(xs: Xs, ys: Ys): Zs
}
implicit def concatNil[Xs <: HList]: Concat[HNil, Xs, Xs] =
new Concat[HNil, Xs, Xs] {
def concat(fst: HNil, snd: Xs) = snd
}
implicit def concatCons[X, Xs <: HList, Ys <: HList, Zs <: HList](
implicit ev: Concat[Xs, Ys, Zs]
): Concat[HCons[X, Xs], Ys, HCons[X, Zs]] =
new Concat[HCons[X, Xs], Ys, HCons[X, Zs]] {
def concat(xs: HCons[X, Xs], ys: Ys): HCons[X, Zs] =
HCons(xs.hd, ev.concat(xs.tl, ys))
}
def concat[Xs <: HList, Ys <: HList, Zs <: HList](xs: Xs, ys: Ys)(implicit ev: Concat[Xs, Ys, Zs]): Zs =
ev.concat(xs, ys)
val xs = HCons(1, HCons("A", HNil))
val ys = HCons(true, HNil)
val zs = concat(xs, ys)
val zs1: HCons[Int, HCons[String, HCons[Boolean, HNil]]] = zs
trait At[Xs <: HList, N <: Num] {
type Out
def at(xs: Xs, n: N): Out
}
implicit def atZero[XZ, Xs <: HList]: At[HCons[XZ, Xs], Zero] { type Out = XZ } =
new At[HCons[XZ, Xs], Zero] {
type Out = XZ
def at(xs: HCons[XZ, Xs], n: Zero) = xs.hd
}
implicit def atSucc[XX, Xs <: HList, N <: Num](
implicit ev: At[Xs, N]
): At[HCons[XX, Xs], Succ[N]] { type Out = ev.Out } = new At[HCons[XX, Xs], Succ[N]] {
type Out = ev.Out
def at(xs: HCons[XX, Xs], n: Succ[N]): Out = ev.at(xs.tl, n.pred)
}
def at[Xs <: HList, N <: Num](xs: Xs, n: N)(
implicit ev: At[Xs, N]
): ev.Out = ev.at(xs, n)
def main(args: Array[String]) = {
val ys1 = HCons(1, HNil)
println(at(ys1, Zero))
val ys2 = HCons(1, HCons("A", HNil))
val y2 = at(ys2, Succ(Zero))
println(at(ys2, Succ(Zero)))
val ys3 = HCons(1, HCons("A", HCons(true, HNil)))
println(at(ys3, Succ(Succ(Zero))))
val ys4 = HCons(1, HCons("A", HCons(true, HCons(1.0, HNil))))
println(at(ys4, Succ(Succ(Zero))))
println(zs1)
println(at(zs1, Zero))
println(at(zs1, Succ(Zero)))
println(at(zs1, Succ(Succ(Zero))))
}
}
|
