NonEmptyList with shapeless

[Andrey Kuznetsov]

Wiki says that sized besides fixed size "can also verify more complex relationships".

Is it possible to use shapeless features for checking list on non-emptiness like scalaz's NonEmptyList?

[Edmondo Porcu]

A typical way to solve this scenarios in shapeless is to provide compile-time proof.

- You create a class which acts as a witness that the proof is verified

- You provide the implicits only for certain cases.

@implicitNotFound("Cannot prove that ${L} is not empty")

trait NonEmptyConstraint[L <: HList]

object NonEmptyConstraint {

  implicit def oneElementNonEmptyHListConstraint[T]: NonEmptyConstraint[T::HNil] = new NonEmptyConstraint[T::HNil]{}

  implicit def hlistNonEmptyConstraint[T,H<:HList](implicit nec:NonEmptyConstraint[H]): NonEmptyConstraint[T::H] = new NonEmptyConstraint[T::H]{}

}

def a[A<:Liist](a:A)(implicit nec:NonEmptyConstraint[A]) 

2015-03-07 0:11 GMT+00:00 Andrey Kuznetsov <fe...@loathing.in>:

Wiki says that sized besides fixed size "can also verify more complex relationships".

Is it possible to use shapeless features for checking list on non-emptiness like scalaz's NonEmptyList?

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