Enforce Bounded Nat?

问题

How can I enforce a Nat that's <= N?

Example:

def lessThan5(x: NatLT5) = ???

where

lessThan5(Nat(4)) would compile, but lessThan5(Nat(6)) would not.

回答1:

You can use the type class LTEq (or LT if you want strictly less than).

import shapeless.nat._
import shapeless.ops.nat._

def lessThan5[N <: Nat](n: N)(implicit ev: LTEq[N, _5]) = ???

lessThan5(_4) // compiles
lessThan5(_5) // compiles
lessThan5(_6) // doesn't compile because LTEq[_6, _5] cannot be found

来源：https://stackoverflow.com/questions/39156628/enforce-bounded-nat