F# Signature File - Constrained Types

Constrained.Types.fs

module StringConstraints 
    open System
    let stringPattern str pattern = 
        if String.IsNullOrEmpty(str) then
            let msg = sprintf "%s: Must not be null or empty" str 
            Error msg
        elif System.Text.RegularExpressions.Regex.IsMatch(str, pattern) then
            Ok (str)
        else
            let msg = sprintf "'%s' must match the pattern '%s'"  str pattern
            Error msg

    type EmailAddress (emailAddress:string) = 
        member x.Value = 
            let pattern = @"^\S+@\S+\.\S+$"            
            stringPattern emailAddress pattern

Constrained.Types.fsi

module StringConstraints 
    val stringPattern  : string -> string -> Result<string, string>

    //[<Sealed>]
    type EmailAddress = 
        new : string -> EmailAddress
        member Value : Result<string, string>

your approach seems to make most sense to me! @tamizhvendan