Extend module

(In learning from "Programming F# 3.0, 2nd Edition")

Extending existing module is just same syntax to creating new module. For example, add slice function like phosphorescence: The way to add both indexer and slice to F# sequence with extending both Seq and List modules.

> module Seq =
-     let slice (lower : int option, upper : int option) aSeq =
-         match lower, upper with
-         | Some(lower), Some(upper) -> aSeq |> Seq.skip lower |> Seq.take (upper - lower + 1)
-         | Some(lower), None -> aSeq |> Seq.skip lower
-         | None, Some(upper) -> aSeq |> Seq.take (upper + 1)
-         | None, None -> aSeq;;

module Seq = begin
  val slice : lower:int option * upper:int option -> aSeq:seq<'a> -> seq<'a>
end

> let seq1To5 = seq {1..5};;

val seq1To5 : seq<int>

> Seq.slice (Some(1), Some(3)) seq1To5;;
val it : seq<int> = seq [2; 3; 4]
> module List =
-     let slice (lower : int option, upper : int option) aList =
-         match lower, upper with
-         | Some(lower), Some(upper) -> aList |> Seq.skip lower |> Seq.take (upper - lower + 1) |> Seq.toList
-         | Some(lower), None -> aList |> Seq.skip lower |> Seq.toList
-         | None, Some(upper) -> aList |> Seq.take (upper + 1) |> Seq.toList
-         | None, None -> aList;;

module List = begin
  val slice : lower:int option * upper:int option -> aList:'a list -> 'a list
end

> let list1To5 = [1..5];;

val list1To5 : int list = [1; 2; 3; 4; 5]

> List.slice (Some(1), Some(3)) list1To5;;
val it : int list = [2; 3; 4]