type Rule = string -> bool * string

​

    let rules : Rule list = [

        fun s -> s.Split(' ').Length = 3, "Must be three words"

        fun s -> s.Length <= 30, "Max lengh is 30 characters"

        fun s -> s |> Seq.filter System.Char.IsLetter |> Seq.forall System.Char.IsUpper, "All letters must be caps" 

        ]

​

    let buildValidator (rules : Rule list) =

        rules

        |> List.reduce(fun firstRule secondRule -> 

            fun word -> 

                let passed, error = firstRule word

                if passed then

                    let passed, error = secondRule word

                    if passed then true, "" else false, error

                else false, error)

    let validate = buildValidator rules

    let word = "HELLO FrOM F#" 

​

    validate word