Add new collection predicates: Contains, Index, and Unique

[fthomas]

Contains is just a type alias (Contains[A] = Exists[Equal[A]]), Index[N, P] checks if the value at index N satisfies the predicate P, and Unique checks if all elements in a collection are unique.

[jgpacker]

For ordered collections, we could also have a Sorted[A] predicate.
Though I'm not sure how the user could choose between ascendent and descendent order.

[fthomas]

Good idea, @jgpacker ! We could have SortedAsc and SortedDesc or provide an extra type for the sort order: trait SortOrder; trait Ascending extends SortOrder; trait Descending extends SortOrder. Another possibility is to have something like SortedWith[P] where P is some kind of binary predicate (which we don't have yet). For example: SortedWith[GreaterEqual2] where GreaterEqual2 gives you a BinaryPredicate instance with isValid(t1: T, t2: T): Boolean = t1 >= t2.

[fthomas]

Another collection predicate: IndexWhere[PA, PI]. It could be used like this IndexWhere[Digit, LessEqual[_2]] which validates any collection of Char where one of the first three chars is a digit.

[fthomas]

• Contains[A]
• Index[N, P]
• Unique
• Head[P]
• Last[P]
• Tail[P]
• Init[P]

[fthomas]

SortedWith as outlined in #6 (comment) is a misnomer. It should better be called Pairwise[P] since it checks if consecutive pairs of elements in a sequence satisfy the binary predicate P.

[sh0hei]

Hi, @fthomas :)

I want to Implement Unique Predicate.
I want you to tell me Signature of Unique and Usage of Unique.

[fthomas]

@sh0hei The idea of Unique is that it checks that a collection has no duplicated elements. It has no parameters, so would be defined as empty case class: case class Unique().

Usage would look something like this:

type UniqueList[A] = List[A] Refined Unique 

scala> RefType.applyRef[UniqueList[Int]](List(1, 2, 3))
res0: Either[String, UniqueList[Int]]= Right(Refined(List(1, 2, 3)))

scala>RefType.applyRef[UniqueList[Int]](List(1, 2, 2))
res1: Either[String, UniqueList[Int]]= Left("Predicate failed: List(1, 2, 2) contains duplicated elements: 2")

[sh0hei]

okay :)
I try it !

[fthomas]

Awesome! :-)

[sh0hei]

@fthomas Excuse me, I'd like also to know about Usage Tail, Init. I'll try to implement it.

[fthomas]

@sh0hei Tail and Init would be similar to the already existing Head and Last predicates (see collection.scala). They check if all but the first/last element of a sequence satisfy a given predicate.

[fthomas]

Closing since most of the predicates in this ticket are now implemented.

[kubukoz]

Hey, what's the status on Sorted predicates?

[fthomas]

@kubukoz It hasn't been implemented yet. :) But I'm open to adding Sorted or Sorted{Asc, Desc} or something similar to the librarary.