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>

I was reading Domain Driven Design (A GREAT BOOK!) by Scott Wlaschin, one of the best F# sites!. On page 106 he shows how to jump through some hoops to add a "create" function that checks for constraints on a type. How it was shown felt very hacky (but effective). However, there was a kind of throw away line about there being "other tecniques, such as using signature files, but we won't discuss them here." WHY? Well, I decided it was time I learn about these "signature files" and how they can be used for constraints. And the above is quick hack that seems to work! And it doesn't feel nearly as dirty as the method shown in the book... Thoughts?

I use this approach a lot and found it very useful! @jfrank-summit

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