Example of a constraints problem using Maybe and function composition

Constraints.fs

let mustBeFourDigits x = 
    if x > 999 && x < 10000 
    then Some x
    else None

let mustBeEven x = 
    if x % 2 = 0
    then Some x
    else None

let firstDigitMustEqualLast x =
    let str = x.ToString()
    if (str |> Seq.head) = (str |> Seq.last)
    then Some x
    else None

let constraints =
    mustBeFourDigits 
    >> Option.bind mustBeEven
    >> Option.bind firstDigitMustEqualLast

let result = [1000 .. 9999] |> Seq.choose constraints

Constraints.hs

module Constriants where
  import Data.Maybe
  
  mustBeFourDigits x =
    if x > 999 && x < 10000
    then Just x
    else Nothing

  mustBeEven x =
    if x `mod` 2 == 0
    then Just x
    else Nothing

  constraints x = mustBeFourDigits x >>= mustBeEven

  matches = mapMaybe constraints [1000..9999]