Applicative validation

applicative validation.fs

open FSharpx.Choice
open FSharpx.Validation
open FSharpx.Collections
open System

// generic validator which returns Success if given predicate succeeds, otherwise Failure of NonEmptyList of errors
let validator pred error value =
    if pred value then Choice1Of2 value
    else Choice2Of2 (NonEmptyList.singleton error)

// some simple validators
let notEqual x = validator ((<>) x) <| sprintf "should not equal to %d" x
let evenNumber = validator (fun x -> x % 2 = 0) "should be even"

// a more complex validator made by combining a couple of simpler ones
let validateNumber n = 
    returnM n 
    <* evenNumber n
    <* notEqual 3 n

let printRes res = 
    match res with
    | Success n -> printfn "%A" n
    | Failure errors -> errors |> String.concat ", " |> printfn "%s"

// test
[-5..5] |> List.iter (fun x -> (x, validateNumber x) ||> print)

results:

-5 | should be even
-4 | -4
-3 | should be even
-2 | -2
-1 | should be even
 0 | 0
 1 | should be even
 2 | 2
 3 | should be even, should not equal to 3
 4 | 4
 5 | should be even