I need something like this:
def encryptBlock(arr: FixedArray[Size16]) = ???
val blocks = originalArray.splitFixed[Size16]
val encrypted = encryptBlock(blocks)
In order to be sure that I receive only 128-bit array as input.
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1 Answer
Shapeless can do that for seqs:
import shapeless._
import nat._
import syntax.sized._
scala> def func(l: Sized[List[Int], _3]) = l
func: (l: shapeless.Sized[List[Int],shapeless.nat._3])shapeless.Sized[List[Int],shapeless.nat._3]
scala> List(1,2,3,4,5,6).grouped(3).map(_.sized(3).get).map(func)
res26: Iterator[shapeless.Sized[List[Int],shapeless.nat._3]] = non-empty iterator
scala> List(1,2,3,4,5,6).grouped(2).map(_.sized(2).get).map(func)
<console>:25: error: type mismatch;
found : shapeless.Sized[List[Int],shapeless.nat._3] => shapeless.Sized[List[Int],shapeless.nat._3]
required: shapeless.Sized[List[Int],shapeless.Succ[shapeless.Succ[shapeless._0]]] => ?
List(1,2,3,4,5,6).grouped(2).map(_.sized(2).get).map(func)
Parameter passed to .size should be Literal(Constant(n: Int)), so you can't pass some variable or expression.
It's also possible convert an Array to some IndexedSeq (.toSeq), like Vector (.toVector)
You can also specify some set of accepatable sizes using type disjunction:
def func[A <: Nat](l: Sized[List[Int], A])(implicit ev: (_2 with _3) <:< A) = l
func(List(1,2).sized(2).get)
res17: shapeless.Sized[List[Int],shapeless.Succ[shapeless.Succ[shapeless._0]]] = shapeless.Sized@3ac1111f
scala> func(List(1,2,3).sized(3).get)
res18: shapeless.Sized[List[Int],shapeless.Succ[shapeless.Succ[shapeless.Succ[shapeless._0]]]] = shapeless.Sized@17191095
scala> func(List(1,2,3,4).sized(4).get)
<console>:24: error: Cannot prove that shapeless.nat._2 with shapeless.nat._3 <:< nat_1.N.
func(List(1,2,3,4).sized(4).get)
^
Maximum-N restriction (from @DougC and @Miles Sabin):
import ops.nat._
import LT._
scala> def func[N <: Nat](l: Sized[List[Int], N])(implicit ev: N < _3) = l
func: [N <: shapeless.Nat](l: shapeless.Sized[List[Int],N])(implicit ev: shapeless.ops.nat.LT.<[N,shapeless.nat._3])shapeless.Sized[List[Int],N]
scala> func(List(1,2).sized(2).get)
res25: shapeless.Sized[List[Int],shapeless.Succ[shapeless.Succ[shapeless._0]]] = shapeless.Sized@3ac1111f
scala> func(List(1).sized(1).get)
res26: shapeless.Sized[List[Int],shapeless.Succ[shapeless._0]] = shapeless.Sized@73f49b57
scala> func(List(1,2,3).sized(3).get)
<console>:30: error: could not find implicit value for parameter ev: shapeless.ops.nat.LT[nat_1.N,shapeless.nat._3]
func(List(1,2,3).sized(3).get)
^
4 Comments
DougC
Is it possible to use shapeless to specify a seq that has a maximum size, rather than a fixed size?
DougC
Thanks. in fact, on gitter, Miles provided this as a building block
def frob[N <: Nat](l: Sized[N])(implicit ev: LT[N, _3]) ... that, I think, points to a way to do what I'm after. The compiler will generate the implicit value for ev at the call sites. I'm pretty sure I can use that to build up a a FiniteSeq[T, N :< Nat] class or similar.
dk14
LessThan, cool, i'll add it to my answer. I suppose it's in
ops.nat packageDougC
Yep. Along with some other very interesting type classes.
