Alex BespoyasovAuthor's photo

Domain Modelling Made Functional. Part 2

Let's continue reading “Domain Modelling Made Functional” by Scott Wlaschin. Last time we read the first part of the book. We discussed what a domain is, why it is needed, and how to decompose large domains into small components that can develop independently of each other.

This time we will design one of the processes in a functional style. We'll look at the functional decomposition of the domain model and learn how to use types to reflect business requirements. By the end of the chapter, we will have written code that is both documentation and a compilable basis for implementing the system.

Chapter 4. Understanding Types

In this chapter, we will try to reflect business requirements with the help of the type system. We will learn what types are, how to declare and use them, and how they can represent the domain model.

Understanding Functions

Each function has a signature—a description of its behavior in the form of types. In most cases F# will be able to define types by itself. For example:

let add1 x = x + 1   // Signature: int -> int
let add x y = x + y  // Signature: int -> int -> int

If a function can handle different types, it is called a generic function. Generic types in the signature start with quotes:

// areEqual : 'a -> 'a -> bool
let areEqual x y =
  (x = y)

In typeScript we would express it like this:

function areEqual<T>(a: T, b: T): boolean {
  return a === b;

Types and Functions

Type is a name for some set of possible values.

Set of                              Set of
valid      →     Function     →     valid
inputs                              outputs

We can denote this type as a conversion of input data to output data:

input -> output

For example, if the function takes as input a number between -32768 and 32767, then it takes int16 as input. And if it returns some string, such as "abc", "cool", it returns string. We can write such a function and its signature as follows:

Inputs                    Function                Outputs
-32768, -32767,                                   "abc"
…, -1, 0, 1, …,     →     int -> string     →     "cool"
32766, 32767                                      "something"

Types do not necessarily have to contain primitives. They can reflect “complex things” as well:

Inputs             Function                  Outputs
😊 😃 😄     →     Person -> Fruit     →     🍉 🍎 🍌

Functions are “things” too, so we can use function sets as types too! In the signature, such a type will be enclosed in brackets:

Inputs            Function                              Outputs
"abc"                                                   😊 → 🍉
"cool"      →     string -> (Person -> Fruit)     →     😃 → 🍎
"yeah"                                                  😄 → 🍌

A value is something that can be used as an argument or a result. All values in FP are immutable by default and have no “methods” that do anything. Values are only data.

Composition of Types

Composition is the creation of something from something smaller. It applies to types, too. You can compose types with logical “AND” and logical “OR”.

For example, if we describe a type for a fruit salad, where we want bananas, apples and cherries, we will use a record type:

type FruitSalad = {
  Apple: AppleVariety
  Banana: BananaVariety
  Cherries: CherryVariety

And if we describe a snack in which you can choose an apple, a banana or a cherry, then we describe it as a discriminated union:

type FruitSnack =
  | Apple of AppleVariety
  | Banana of BananaVariety
  | Cherries of CherryVariety

This is the simplest composition of types. A system in which complex types are combined from simple types using AND and OR operations is called algebraic type system.

Building a Domain Model by Composing Types

Type composition can help when modeling systems. Suppose we want to model payment in an online store. Let's start with wrappers over primitive types:

type CheckNumber = CheckNumber of int
type CardNumber = CardNumber of string

Then we'll write out the types of accepted cards as a union, and all the information about the card as a record:

type CardType = Visa | Mastercard
type CreditCardInfo = {
  CardType: CardType
  CardNumber: CardNumber

If the store accepts multiple payment methods, we can again use union:

type PaymentMethod =
  | Cash
  | Check of CheckNumber
  | Card of CreditCardInfo

The amount and currency can also be described by basic types:

type PaymentAmount = PaymentAmount of decimal
type Currency = EUR | USD

And the payment type might look like this:

type Payment = {
  Amount: PaymentAmount
  Currency: Currency
  Method: PaymentMethod

Moreover, we can also describe how payments will be made or how currencies will be converted—in the form of function types:

type PayInvoice = UnpaidInvoice -> Payment -> PaidInvoice
type ConvertPaymentCurrency = Payment -> Currency -> Payment

Modeling Optional Values, Errors, and Collections

F# uses Option<'a> to handle optional values, Result<'Success,'Failure> to handle errors, unit is used instead of void, and several types are used to handle collections. To designate collections, the author suggests always using list when modeling. The implementation may vary, but it is easier to use it in a model.

It is better to read about subtleties of F# directly in the book or in the language manual.

Chapter 5. Domain Modeling with Types

In this chapter, we'll create a domain model using a type system so that it can be read not only by F# developers but also by domain experts.

Seeing Patterns in a Domain Model

Each area of knowledge is unique, but some things will still repeat from project to project. For example:

  • Simple values are basic building blocks, most often will be defined as primitives in wrappers.
  • Groups of values combined by “AND”, in these, the data is closely related.
  • Groups of values combined by “OR” represent some sort of selection;
  • Processes are types with input and output values.

Modeling Simple Values

Experts don't think of simple values as “strings” or “numbers”. They think of “product codes,” “quantities,” and “prices.” This means two things:

  • In the domain, primitives will always be limited somehow (numbers will have valid value ranges, strings will have format, etc.)
  • The different value types are not interchangeable; number in the item quantity is different from number in the item code.

There is a native way in F# to declare such types to be different:

type CustomerId = CustomerId of int
type OrderId = OrderId of int

Then you cannot pass OrderId to a function that expects CustomerId:

let processCustomerId (id:CustomerId) = ...

// If try and call with OrderId the error will appear:
processCustomerId orderId
//                ^ This expression was expected to have type
//                'CustomerId' but here has type 'OrderId'

In the summary, in TypeScript code samples, we will ignore this nuance. As I said above, you can use branding to achieve the same result. We will just use aliases to make the code easier to understand.

Modeling Complex Data

Part of the closely related data we can represent as record-types:

type Order = {
  CustomerInfo : CustomerInfo
  ShippingAddress : ShippingAddress
  BillingAddress : BillingAddress
  OrderLines : OrderLine list
  AmountToBill : // ...

At first stages, it may not be clear which types are the same and which are not. This is worth checking with the experts. If they talk about ShippingAddress and BillingAddress as different things, it is better to make them different types. They may evolve in different directions, and it will be harder to separate the types than to put them together.

At the beginning of the design, you may also lack knowledge about the constraints or structure of some types. This is not a problem, now you can replace unknown types with explicitly undefined types. This will allow you to continue designing without being distracted by compiler errors. (Clearly, after constraints are specified, the unknown structures will need to be updated.)

type Undefined = exn

type CustomerInfo = Undefined
type ShippingAddress = Undefined
type BillingAddress = Undefined
type OrderLine = Undefined
type BillingAmount = Undefined

We can represent the data that provide choices as union-types:

type OrderQuantity =
  | Unit of UnitQuantity
  | Kilogram of KilogramQuantity

Modeling Workflows with Functions

Unions and record-types play the role of nouns in a common language. The role of verbs would be played by function types. For example, we could represent the order validation process as:

type ValidateOrder = UnvalidatedOrder -> ValidatedOrder

If the process returns multiple events, we can use the record-type to show it:

type PlaceOrderEvents = {
  AcknowledgmentSent: AcknowledgmentSent
  OrderPlaced: OrderPlaced
  BillableOrderPlaced: BillableOrderPlaced

type PlaceOrder = UnvalidatedOrder -> PlaceOrderEvents

If the process accepts several groups of data as input, there are two ways to proceed: use several arguments or use a record with several fields:

type CalculatePrices = OrderForm -> ProductCatalog -> PricedOrder

// Or:

type CalculatePricesInput = {
  OrderForm: OrderForm
  ProductCatalog: ProductCatalog

type CalculatePrices = CalculatePricesInput -> PricedOrder

The second way is more suitable when the inputs are technically related and mandatory. But if one of the parameters is more of a dependency rather than a direct argument, it is better to use the first variant—it will help in the future with “functional dependency injection”.

We can use Result, Async types and their wrappers to describe effects:

type ValidationResponse<'a> = Async<Result<'a,ValidationError list>>
type ValidateOrder = UnvalidatedOrder -> ValidationResponse<ValidatedOrder>

A Question of Identity

The DDD divides things into “entities” and “value objects”. The former have a unique identity, while the latter are the same if they have the same contents.

For example, if we are talking about names of people, two different people can have the same names. The name may then represent a value object. In F#, two record values of the same type are the same if their field values are the same. This is called structural identity:

let name1 = {FirstName="Alex"; LastName="Adams"}
let name2 = {FirstName="Alex"; LastName="Adams"}
printfn "%b" (name1 = name2)  // "true"

But sometimes we need to model things that remain the same, even if their content changes. For example, a person moving to a new address is still the same person. Such things are called entities.

Entities need an identifier to distinguish one entity from another. Sometimes such identifiers are provided by the domain area (serial number, social security number, etc.), sometimes you have to create them yourself (GUID, UUID).

It is more convenient to store identifiers “inside” the entity itself than “outside”, because it makes it more convenient to pattern-matching.

// ID outside:

type UnpaidInvoiceInfo = ...
type PaidInvoiceInfo = ...
type InvoiceInfo =
  | Unpaid of UnpaidInvoiceInfo
  | Paid of PaidInvoiceInfo

type InvoiceId = ...
type Invoice = {
  InvoiceId : InvoiceId
  InvoiceInfo : InvoiceInfo

// ID inside:

type UnpaidInvoice = {
	InvoiceId : InvoiceId
  // …

type PaidInvoice = {
  InvoiceId : InvoiceId
  // …

type Invoice =
  | Unpaid of UnpaidInvoice
  | Paid of PaidInvoice

Value objects must be immutable because when some field is changed, the object becomes another object, i.e. it cannot “just change”. Entities can change, but we won't just “change” them; instead, we will create copies with changes, keeping the identifier. This makes all changes to the entity explicit:

// You can that the function will change the Person entity:
type UpdateName = Person -> Name -> Person


If the user changes an order item, should the entire order change? Answer: yes, it should, for three reasons:

  • To preserve consistency of the data;
  • To avoid violation of invariance;
  • Immutability makes it necessary ¯\_(ツ)_/¯

By consistency we mean that if even one item changes, we will need to recalculate the total order amount. If this is not done, the data will become inconsistent.

By invariance we mean that if there should be at least one item in the order, we need to check that after removing several items, there will still be at least one item there.

Immutability actually triggers a chain of changes from internal structures to the “root” structure, that is, from the item to the order to which it belongs. In DDD terms, a list of items would be called aggregate, and an order would be called aggregate root.

Aggregates are separate, independent units of persistence. We should draw boundaries so that immutability does not trigger unnecessary updates to structures. For example, if an order contains a reference to the user who placed that order, what's the best way to reference it: using the whole user or just his ID?

type Order = {
  OrderId: OrderId
  Customer: Customer
  OrderLines: OrderLine list
  // …

// Or:

type Order = {
  OrderId: OrderId
  CustomerId: CustomerId
  OrderLines: OrderLine list
  // …

In the first case, if you change the user name, you will have to change the orders, and in the second case, you won't have to. The second way is preferable, because it does not entail unnecessary changes.

Chapter 6. Integrity and Consistency in the Domain

In this chapter, we will look at the concepts of integrity and consistency. We want to make sure that the data within a bounded context can be trusted, that it is valid, and that the different parts of the domain are consistent with each other and that there is no data that contradict each other.

The Integrity of Simple Values

The domain rarely has unrestricted strings, numbers, etc. More often than not, businesses have some valid ranges for values. We want to use these ranges so that a non-valid value simply cannot be created.

In F#, we can use a smart constructor to do this. We first make the constructor private, and then we add our own constructor that will validate the data:

type UnitQuantity = private UnitQuantity of int

module UnitQuantity =
  let create qty =
    if qty < 1 then
      Error "UnitQuantity can not be negative"
    else if qty > 1000 then
      Error "UnitQuantity can not be more than 1000"
      Ok (UnitQuantity qty)

	// To make it possible to use pattern-matching:
	let value (UnitQuantity qty) = qty

// And use it like this:
let unitQtyResult = UnitQuantity.create 1

In TypeScript, it is more difficult to automate this, but you can agree not to create types by hand. Instead, use mechanisms with prevalidation, such as factories:

type UnitQuantity = number;

function createQuantity(raw: number): UnitQuantity {
  if (raw < 1) throw new Error("UnitQuantity can not be negative");
  if (raw > 1000) throw new Error("UnitQuantity can not be more than 1000");
  return raw as UnitQuantity;

Units of Measure

In F# you can use measures to tag types:

type kg

type m

type KilogramQuantity = KilogramQuantity of decimal<kg>

In TypeScript you can also tag types, but again, it would not be as reliable or convenient as in F#.

Capturing Business Rules in the Type System

One of the main rules of domain modeling sounds like:

Make illegal [data] states unrepresentable

That is, try to build the type model in such a way that it is impossible to represent invalid data in these types.

The easiest way to understand this is to use an example. Suppose we want to write a module for confirming and restoring accounts via email. We have two versions of user mail:

  • Confirmed, such addresses have already been sent a confirmation link and users have clicked on it;
  • And unconfirmed.

We want to send account recovery links to the verified email addresses, and we don't want to send validation links to the unverified ones. And vice versa, we don't want to send recovery links to unconfirmed addresses, but only email confirmation links.

One of the solutions could be this type:

type CustomerEmail = {
  EmailAddress: EmailAddress
  IsVerified: bool

But the IsVerified flag is a very unreliable solution. Mainly because it is not clear from the description when it should be on and when it should be off. But also because the rules to toggle it will be described in runtime, which means they can be skipped, forgotten, overlooked and written code that will invalidate the data.

Instead it is better to describe the constraint directly in the type:

type VerifiedEmailAddress = private VerifiedEmailAddress of EmailAddress
type CustomerEmail =
  | Unverified of EmailAddress
  | Verified of VerifiedEmailAddress

Now you can see that you can't “just create” a confirmed email address. If we create a new address, it will be by default unconfirmed. Such a type system could replace some of the runtime unit tests. (Well... at least in F#.)


Consistency is more of a business term than a technical one, because how and what kind of data should be consistent depends on the needs of the business.

Consistency within a single aggregate is easiest to achieve, just count all the dependent data from the original source. (As in the example with the total amount, it is enough to calculate it from the list of orders.) If additional data has to be saved, then, of course, before saving you should additionally make sure that the data is consistent.

On the example of changing an item in an order it might look like this:

/// Pass 3 parameters:
/// * top-level order;
/// * the ID of the order item to be changed;
/// * new price.
let changeOrderLinePrice order orderLineId newPrice =

  // Find the item among the order.OrderLines with orderLineId:
  let orderLine = order.OrderLines |> findOrderLine orderLineId

  // Make a copy of OrderLine with updated price:
  let newOrderLine = {orderLine with Price = newPrice}

  // Create a new order list with replacing the old line with the new one:
  let newOrderLines =
    order.OrderLines |> replaceOrderLine orderLineId newOrderLine

  // Recalculate AmountToBill:
  let newAmountToBill = newOrderLines |> List.sumBy (fun line -> line.Price)

  // Create an order copy with updated data:
  let newOrder = {
      order with
        OrderLines = newOrderLines
        AmountToBill = newAmountToBill

  // Return the created order:

Here the order is the aggregate. We intentionally count and update the data at the order (aggregate) level, because only it knows how to reconcile the total price from the list of items. If we store this data, then the order and list items need to be stored in the same transaction.

Consistency between different contexts is harder to enforce, but it's not always necessary. In most cases, it's enough to have eventual consistency and to set up communication through messages.

If a message gets lost, there are three chairs:

  • Do nothing, it might cost money;
  • Determine which message didn't get through and resend it;
  • Roll back the previous actions.

If immediate consistency is a requirement, you could look at 2 Phase Commit and other fun stuff.

Consistency between different aggregates within the same context is highly dependent on requirements. A general rule of thumb is to update one aggregate per transaction, but ensure final consistency between aggregates.

Sometimes, though, you need to update two aggregates in the same transaction—for example, to transfer money from one bank account to another. But more often than not, this can be redesigned to go from:

Start transaction
  Add X amount to accountA
  Remove X amount from accountB
  Commit transaction

...To a separate transaction in which the different units would no longer be directly responsible for updating the data:

type MoneyTransfer = {
  Id: MoneyTransferId
  ToAccount: AccountId
  FromAccount: AccountId
  Amount: Money

Chapter 7. Modeling Workflows as Pipelines

In this chapter we will try to apply the type system to describe the order-taking process. The process now consists of several steps:

  • Validation
  • Placement
  • Confirmation
  • Generation of output events

We will present these steps as parts of a larger pipeline—the process as a whole. Each step will somehow transform the input data. We will try to make each step stateless.

The Workflow Input

The input data must be a domain object. We'll assume that we get such objects from deserialized DTOs and just keep that in mind.

type UnvalidatedOrder = {
  OrderId: string
  CustomerInfo: UnvalidatedCustomerInfo
  ShippingAddress: UnvalidatedAddress
  // …

But we remember that it is not the object itself that really starts the process, but the command. As a rule, we often add additional data like user or timestamp to commands. In our case we can think of it this way:

type PlaceOrder = {
  OrderForm: UnvalidatedOrder
  Timestamp: DateTime
  UserId: string
  // …

There can be many commands, so let's make a generic generic-type command that takes care of describing the additional data:

type Command<'data> = {
  Data: 'data
  Timestamp: DateTime
  UserId: string
  // …

And we can create a process-specific command by passing a type-parameter:

type PlaceOrder = Command<UnvalidatedOrder>

Modeling an Order as a Set of States

From the interviews with the experts, it becomes clear to us that “Order” is not a static document, but rather a set of data that go through different transformations, i.e. are in different states.

All these states can be modeled in different ways. The naive way is to simply create a bunch of flags for different states:

type Order = {
  OrderId: OrderId
  // …
  IsValidated: bool
  IsPriced: bool

But it has a lot of disadvantages:

  • These states are implicit, so they will require a bunch of checks right in the code;
  • Some states contain data that other states don't need, we don't need to drag everything everywhere;
  • It's not clear which data belongs to which flags, and when we'll need it.

Another way is to use separate types for each state.

type Order =
  | Unvalidated of UnvalidatedOrder
  | Validated of ValidatedOrder
  | Priced of PricedOrder
  // …

The advantage of this approach is that you can add another state without much effort. It is enough to add a new type to the union.

The top-level Order type reflects an order at any point in its lifecycle, and individual types reflect it in individual states. The Order type can be used for storage or transfer between contexts.

State Machines

The union from the example is similar to a state machine, a model of transitions between different states. Programming in terms of state machines is actually a good thing, because it encourages:

  • Defining valid transitions for each of the states (where you can and cannot go from that state);
  • Documenting all possible states in advance;
  • Thinking about all the possible states the application might be in;
  • Thinking about errors.

Modeling Each Step in the Workflow with Types

The first thing to describe is validation. We remember that it accepts an unvalidated order and also refers to two “dependencies”. We can also describe these dependencies with types first, which will become an “interface” for the implementation later. Then we get something like:

type CheckProductCodeExists = ProductCode -> bool

type CheckedAddress = CheckedAddress of UnvalidatedAddress
type AddressValidationError = AddressValidationError of string
type CheckAddressExists = UnvalidatedAddress -> Result<CheckedAddress,AddressValidationError>

We will then describe the entire validation step as such a signature:

type ValidateOrder =
  CheckProductCodeExists    // dependency
    -> CheckAddressExists   // dependency
    -> UnvalidatedOrder     // input
    -> Result<ValidatedOrder,ValidationError>  // output

Note that we put dependencies at the beginning, before arguments. We need this for partial application of arguments in the future. This will be the functional equivalent of dependency injection.

I suggest you read the description of the other steps in the book yourselves. We will go straight to the return events :–)

We need events OrderPlaced for sending and BillableOrderPlaced for billing. For the first one, we can use an alias over an existing type, and create the second one from scratch:

type OrderPlaced = PricedOrder
type BillableOrderPlaced = {
  OrderId: OrderId
  BillingAddress: Address
  AmountToBill: BillingAmount

We may need to add some more events, so let the process return a list of events, which type will be:

type PlaceOrderEvent =
  | OrderPlaced of OrderPlaced
  | BillableOrderPlaced of BillableOrderPlaced
  | AcknowledgmentSent  of OrderAcknowledgmentSent

type CreateEvents = PricedOrder -> PlaceOrderEvent list

This way we make the code extensible, if a new event occurs, we don't have to change the whole process code.

Documenting Effects

Parts of the process can cause effects. For example, in validation we have a call to a third-party service, this should also be specified in the types:

type AsyncResult<'success,'failure> = Async<Result<'success,'failure>>
type CheckAddressExists =
  UnvalidatedAddress -> AsyncResult<CheckedAddress,AddressValidationError>

Async, like Result, “infects” everything it touches, so we have to change the validator type as well:

type ValidateOrder =
  CheckProductCodeExists    // dependency
    -> CheckAddressExists   // AsyncResult dependency
    -> UnvalidatedOrder     // input
    -> AsyncResult<ValidatedOrder,ValidationError list>  // output

Now we see that not only can the validation step gives an error, but it will also work asynchronously. We can use this knowledge when designing and working with requirements: we can decide if we trust third-party services enough to use them, or if we should write the validation from scratch.

The question may arise, do we need to show dependencies in types? There is no right answer, but the author suggests following these rules:

  • For public functions, we will hide dependencies;
  • For functions within a context, we will keep dependencies explicit.

This helps document what each step really requires for its work and what it really will return.

What Next

This time we discussed how to design flow of programs in a functional style, what the difference is from OOP, and how types can help document and reflect business requirements. In the next post we'll run through the steps of implementing a domain model, learning about functional composition, partial application, and monads.


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