open System

let tests = ["helloWorld";"Helloworld";"HelloWorld";"Hello";"hello"]

​

(

let contains (s:string) (v:string) (c:StringComparison) = s.IndexOf(v,c) >= 0

let optFromBool s = if s then Some() else None

​

let (|InvariantContainsI|_|) (value:string) (input:string) = 

    contains input value StringComparison.InvariantCultureIgnoreCase

    |> optFromBool

​

let boolFromAp (|Pattern|_|) v = match v with |Pattern -> true |_ -> false

​

let invariantContainsI input = boolFromAp <| (|InvariantContainsI|_|) input

​

​

tests

|> Seq.filter (invariantContainsI "helloworld")

|> printfn "%A"

)

(* another option *)

​

let contains v (c:StringComparison) (s:string)  = s.IndexOf(v,c) >= 0

let optTFromBool s f = if f s then Some s else None

let (|InvariantContainsI|_|) value (input:string) = optTFromBool input <| contains value StringComparison.InvariantCultureIgnoreCase

tests

|> Seq.choose ((|InvariantContainsI|_|) "helloworld")

|> printfn "%A"