F# Smart Constructor

Feb 10, 2020 in DOMAIN-DRIVEN-DESIGN • FSHARP • VALUE OBJECT
3 min read

The “Smart Constructor” pattern allows us to create a type with F#. With validation.

TL/DR

Here is the F# boiler plate needed to implement the Value Object pattern.

type UserName = private UserName of string

module UserName =
    let isValid s = // ...

    // smart ctor
    let create (str: string) =
        if isValid str then
            Some (UserName str)
        else
            None

    // helper function extracting value
    let value (UserName s) = s

Usage:

let workingWithSmartCtor name =
    let maybeValidUserName = UserName.create name
    match maybeValidUserName with
    | Some validName -> validName |> UserName.value
    | None -> "invalid UserName"

workingWithSmartCtor ""     // -> "invalid UserName"
workingWithSmartCtor "lisa" // -> "lisa"

Example

Step 1

Let’s start with a type Person:

type Person = {
    FirstName: string
    LastName: string
    UserName: string
}

let homer = {
    FirstName = "Homer"
    LastName = "Simpson"
    UserName = "Homer Simpson"
}

Note that F#’s record types are immutable by default. homer can’t be modified.

Imagine a FormatName function expecting FirstName and LastName. A naive C# method signature would look like this:

public string FormatName(string firstName, string LastName) => // ...

A naive F# function signature would look the same way (with less boiler plate):

let FormatName firstName lastName = // ...

What will happen? Consumers of this C# method/F# function will mix up the order of the input parameters, because both are strings.

Step 2

Since F# provides a type system, let’s use it:

type FirstName = FirstName of string
type LastName = LastName of string
type UserNameSimple = UserNameSimple of string

type PersonWithSimpleTypes = {
    FirstName: FirstName
    LastName: LastName
    UserName: UserNameSimple
}

let bart = {
    FirstName = FirstName "Bart"
    LastName = LastName "Simpson"
    UserName = UserNameSimple "Bart Simpson"
}

This step removes the “Primitive Obsession Smell”.

Step 3

Now we suddenly have new requirements:

FirstName is optional
LastName is optional
UserName is required, must not be empty, and must have less than 50 characters

Making FirstName and LastName optional is easy in F# (using the option type).

type ValidPerson = {
    FirstName: FirstName option
    LastName: LastName option
    UserName: // ??
}

But how to guarantee a valid UserName?

This is where the Smart Constructor pattern comes into play:

// type is private
type UserName = private UserName of string

module UserName =
    let isValid s = not (String.IsNullOrEmpty(s)) && s.Length < 50

    // smart constructor
    let create (str: string) =
        if isValid str then
            Some (UserName str)
        else
            None

    // helper function to extract the string
    let value (UserName str) = str

Side note: The Smart Constructor’s create function doesn’t have to return an option type (like in this example): It could also return a Result type, or throw an exception, or do something completely different.

Here, the UserName.create function returns None for invalid user names. We can pattern match the option to a string:

match UserName.create someString with
| Some validName -> validName |> UserName.value
| None -> "invalid UserName"

We can now decide if we want to create another Smart Constructor for the ValidPerson type or some other type of pipeline (f.ex. “applicative validation”) or a combination of these strategies.

But we have ensured that the type UserName is always valid, so we don’t ever have to check it again.

UPDATE 2022-05-13: Fix syntax error (replace let with type).Thanks Alex!