Making Illegal States Unrepresentable

Making Illegal States Unrepresentable

What is an illegal state?

From my perspective, an illegal state is such state of an application in which it demonstrates an unexpected behaviour, makes its data inconsistent, or cannot proceed with operation. Examples of illegal states:

  • Passing user’s name to a function expecting user’s email;
  • Calling a function expecting validated data with something raw & erroneous;
  • Allowing constructing objects which are incorrect from business perspective;
  • Executing a wrong chain of business functions — e.g., making double write-off;
  • Doing illegal state transition of a finite state machine.

In order to deal with such behaviour we can take an approach of making illegal states unrepresentable.

Making illegal states unrepresentable is a way of statically proving that all runtime values correspond to valid objects in the business domain.

A good starter example of this principle would be a NonEmptyArray — a static way of telling the compiler that functions we write require a non-empty arrays as arguments. In TypeScript this idea is expressed quite simply:

interface NonEmptyArray<A> extends Array<A> {
  0: A;

Using this type, we can statically ensure that our functions are not called with empty arrays:

// a function to extract a head (first element) of an array:
const head = <A>(arr: NonEmptyArray<A>): A => arr[0]; // statically known to be `A`, not `A | undefined`!

const first = head([1, 2, 3, 4]); // first :: number, first == 1
const error = head([]); // ❌ will fail to compile: Property '0' is missing in type '[]'

You may be curious: how do we create NonEmptyArray from a dynamically filled array, for example, when it’s read from the disk or obtained using the network? In this case one of the next topics comes into play — a so-called “smart constructor” for NonEmptyArray can return its instance iff the underlying array’s length is greater than zero. We use Option to express the idea of possibly missing result:

import { option } from 'fp-ts';
import type { Option } from 'fp-ts/Option';

// A constructor for a `NonEmptyArray`:
const fromArray = <A>(arr: A[]): Option<NonEmptyArray<A>> => arr.length > 0 ? option.some(arr) : option.none;

We will meet this approach of wrapping the results in either Option or Either later on. Meanwhile, let’s switch to another topic and talk about making primitive types distinguishable one from another.

Smart Validators and Opaque types

Opaque types

Let’s imaging we’re working in a e-commerce domain, and writing a function which does something with a user ID and an order ID — say, checks in the database whether given order belongs to the given user. Both user ID and order ID are modelled as UUIDs, so we write a function:

const doesOrderBelongsToUser = (userId: string) => (orderId: string): Promise<boolean> => ...;

Then a time passes, the system evolves, and we want to do a refactoring. For expressiveness reasons we decide to swap the parameters:..

const doesOrderBelongsToUser = (orderId: string) => (userId: string): Promise<boolean> => ...;

…and bam!, we have to manually search for each and every reference to this function (including even the most obscure), and check if its arguments are really swapped. In order to make this task a bit simpler, it’s better to differentiate various kinds of primitive types like strings, numbers, or booleans one from another.

And this is a job for opaque types! Opaque type is a type which wraps another type at the compile time, but has no runtime overhead. General ideal is somewhat reminiscent of a mixin — we “embellish” a primitive type with a compiler-level tag, which should be globally unique. Fortunately, TypeScript has a unique symbol type, which we will use for this task. Due to lack of nominal typing in TypeScript implementing opaque types will require some twiddling with namespaces — we’d want to hide symbol used for tags from an end-user.

namespace Tag {
  declare const OpaqueTagSymbol: unique symbol;
  declare class OpaqueTag<S extends symbol> {
      private [OpaqueTagSymbol]: S;
  export type OpaqueType<T, S extends symbol> = T & OpaqueTag<S>;

export type Opaque<T, S extends symbol> = Tag.OpaqueType<T, S>;

Final usage will look like this: first we define or declare a unique symbol for the opaque type, and then apply Opaque geenric to the base type and this tag:

// Either define:
const UserIdS = Symbol.for('UserId');
// ...or declare a tag symbol:
declare const UserIdS: unique symbol;

// UserId type will be unassignable to other opaque types, but will be assignable to strings:
type UserId = Opaque<string, typeof UserIdS>;

Now we can implement the doesOrderBelongsToUser in a type-safe manner:

const UserIdS = Symbol.for('UserId');
type UserId = Opaque<string, typeof UserIdS>;

const OrderIdS = Symbol.for('OrderId');
type OrderId = Opaque<string, typeof OrderIdS>;

const doesOrderBelongsToUser = (orderId: OrderId) => (userId: UserId): Promise<boolean> => ...;

Even if we swap the arguments once again, the compiler will warn us about all cases where e.g. UserId type is being assigned a plain string or some other opaque type like OrderId.

This trick works well for all logic which is located inside the bounded context of a program. But for the data coming outside the boundary, usage of opaque types offers little to no help:

import * as fs from 'fs/promises';

interface Order {
  readonly id: OrderId;
  readonly owner: UserId;

const order: Order = await fs.readFile('order.json', 'utf8'); // ❌ readFile returns an `any`...
// now we don't know — is `order` a well-formed order? Does it contain `id` and `owner` at all?

As a rule of thumb, all data coming into the boundary, should be validated. And to illustrate this, I’d like to present a pattern called “smart constructors”.

Smart Constructors and Validators

Smart constructor is a function returning either error(s) of input validation, or input as a validated type:

type SmartCtor<T> = (input: unknown) => Either<ValidationError, T>;

In general, a developer shouldn’t be able to construct an instance of ‘smart’ type without using a constructor.

N.B. I think it should be obvious, but still would like to note that type casts via any or unknown should be publicly condemned 🙃

An awesome example of library for builing runtime validators (or codecs) is io-ts. Its API is reminiscent of TypeScript’s types themselves, which makes its usage really ergonomic:

// TS type:
interface User {
  readonly id: number;
  readonly name: string;
  readonly comments: string[];
// which could be rewritten as:
type User = Readonly<{
  id: number;
  name: string;
  comments: Array<string>

// Now compare to io-ts validators and types:
import * as t from 'io-ts';

const User = t.readonly(
    id: t.number,
    name: t.string,
    comments: t.array(t.string)
type User = t.TypeOf<typeof User>; // will be inferred exactly as if we were to write it by hand

Now, we can use io-ts and its satellite libraries to define type codecs. For example, here’s how we can define an opaque type wrapper for email type:

import * as t from 'io-ts';
import { withMessage } from 'io-ts-types';

const EmailS = Symbol.for('Email');
const Email = withMessage(
  t.brand(t.string, (str): str is Email => /.+@.+\..+/gu.test(str), 'Email'),
  input => `Email should contain at-sign and a dot, got instead: ${JSON.stringify(input)}`
type Email = t.Branded<string, { readonly Email: typeof EmailS }>;

This is an example of a branded type — an opaque type embellished with a runtime check in form of a type guard. Passing an email-containing string through Email codec will result in a Right Either containing an Email type. In the runtime, though, it’ll be just a string, so no overhead related to packing/unpacking will be caused.

Trying to assign a string to a variable of type Email will cause a compilation error, while using Email.decode will force the end-user to handle both case of validation fail & success:

// Type 'string' is not assignable to type 'Brand<{ readonly Email: unique symbol; }>':
const x: Email = 'foo@bar';
const y = Email.decode('foo@bar'); // y: Validation<Email>

As a conclusion:

  • all data coming into program’s boundary should be validated using types with smart constructors;
  • each public key of all domain entities should be modelled as a separate opaque type;
  • inside the program’s boundary use simpler opaque types with only compile-time representation to avoid performance hit caused by unwrapping an Either.

Type-Level Programming

The second big topic which helps in making illegal states unrepresentable is type-level programming — an approach of utilising different capabilities of TypeScript’s type system in order to make incorrect branches of business logic to cause compilation errors, which is opposed to an approach when the code compiles, but fails in runtime.

N.B. In current state of TypeScript development not every cases are possible to express in the type system. In order to express many checks a different type system is required — for example, dependent types or linear/affine types. Still, TypeScript’s type system is expressive enough to cover a lot of cases.

Morph types by chainging their parts

Core idea I always try to explain: treat your non-generic types as values, and generics as type-level functions or combinators. For example, consider the following example:

interface User {
  readonly name: string;
  readonly isActive: boolean;

Now we want to make some business logic which should work with only active users (User.isActive === true). Instead of letting such function accept a User type, we will use a type combinator ReplaceDeep to create a specialisation of a UserActiveUser:

type ReplaceDeep<Type, Path extends string, With> =
  Path extends `${infer Key}.${infer Rest}`
    ? Key extends keyof Type
      ? Omit<Type, Key> & { [K in Key]: ReplaceDeep<Type[Key], Rest, With> }
      : Omit<Type, Key> & { [K in Key]: With }
    : Omit<Type, Path> & { [K in Path]: With };

type ActiveUser = ReplaceDeep<User, 'isActive', true>;

const user: ActiveUser = {
  name: 'John',
  isActive: false // ❌ Type 'false' is not assignable to type 'true'

The ReplaceDeep combinator allows passing a path for which the replacement should be done:

interface Foo {
  readonly foo: {
    readonly bar: {
      readonly baz: number;

type FooString = ReplaceDeep<Foo, '', string>;

const x: FooString = {
  foo: {
    bar: {
      baz: 'hello!'

The core idea is to force the compiler to fail the compilation if the end-user tries to call our functions with incorrect data.

Model domain entity updates with AtLeastOne

Another great combinator I’d like to talk about is called AtLeastOne. Suppose we’re writing an e-commerce service, and our domain model is Order:

interface Order {
  readonly id: OrderId;
  readonly issuer: User;
  readonly date: Date;
  readonly comment: string;
  readonly state: OrderState;

We want to write a CRUD API for this entity, specifically the updateOrder function. How could we model a payload to that function?

const updateOrder = (id: OrderId, update: ???): Promise<void> => ...;

The first — naïve — approach would be to mark the payload as Partial<Order>. That could work for almost every cases, but it will pass as a valid case an empty object:

const updateOrder = (id: OrderId, update: Partial<Order>): Promise<void> => ...;

updateOrder(orderId, {}); // ❌ typechecks while it really should't
updateOrder(orderId, { date: new Date() }) // ✅

But we cannot make the updateOrder function require a full object of Order type as well — it won’t allow us to partially update an entity:

const updateOrder = (id: OrderId, update: Order): Promise<void> => ...;

updateOrder(orderId, {}); // ✅ empty objects are prohibited...
updateOrder(orderId, { date: new Date() }) // ❌ ...but partial updates as well

Enter AtLeastOne type combinator! It will turn an interface into a giant sum type, each constituent of which has only one key required, and all others are optional:

type AtLeastOne<T, Keys extends keyof T = keyof T>
  = Partial<T> & { [K in Keys]: Required<Pick<T, K>> }[Keys];

const updateOrder = (id: OrderId, update: AtLeastOne<Order>): Promise<void> => ...;

updateOrder(orderId, {}); // ✅ doesn't compile: "Property 'state' is missing in type '{}'"
updateOrder(orderId, { comment: 'Ship, please' }); // ✅
updateOrder(orderId, { date: new Date() }); // ✅
updateOrder(orderId, { state: 'deleted', date: new Date() }); // ✅

To be honest, an even better approach would be the following: exclude order ID from the update payload, and turn the rest into a sum type using AtLeastOne. This will disallow updating order IDs, which could be dangerous:

type OrderUpdate = AtLeastOne<Omit<Order, 'id'>>;

const updateOrder = (id: OrderId, update: OrderUpdate): Promise<void> => ...;

// ✅ Won't compile with error:
// Object literal may only specify known properties, and 'id' does not exist in type 'OrderUpdate'
updateOrder(orderId, { id: anotherOrderId }); 

Use never to your advantage

One of the most underestimated features of TypeScript’s type system is never type. This is a so-called bottom type (often written as ⊥), which cannot be instantiated in the runtime. Each and every imaginable type — primitives, standard library, each one of your own types — is a supertype of never. But why it’s so important?

As I’ve stated at the beginning, making illegal states unrepresentable implies a compilation error when a user tries to express an incorrect case of business logic. By having those cases infer to never, we can make the compilation of such program fail. Consider an example — we’re writing a framework around Express.js, and need to write a function which will parse the passed route:

const parseRoute = (route: string): string[] => ...;

Without type-level machinery, we have to add a runtime check to test whether our route is a well-types Express.js route:

const parseRoute = (route: string): string[] => {
  if (!route.startWith('/')) {
    throw new Error('Route should start with a slash');
  // ... and so on

Instead, we will force this check during th ecompilation phase by writing a type combinator PrefixedWithSlash, which will make our job easier:

type PrefixedWIthSlash<S extends string> = S extends `/${string}` ? S : never;

const parseRoute = <S extends string>(route: PrefixedWithSlash<S>): string[] => ...;

parseRoute('/hello'); // ✅
parseRoute('hello!'); // ❌ Argument of type 'string' is not assignable to parameter of type 'never'.

Another example where never shines is related to exhaustiveness checks in switch branches. Let’s imagine that we have some ADT:

type ADT =
  | { readonly tag: 'First'; readonly foo: string }
  | { readonly tag: 'Second'; readonly bar: number[] }
  | { readonly tag: 'Third'; readonly baz: boolean };

const matchOnADT = (x: ADT): string => {
  switch (x.tag) {
    case 'First': return;
    case 'Second': return;
    case 'Third': return `${x.baz}`;

When I encounter code like this, I always recommend adding another branch — default — which will call the absurd function:

switch (x.tag) {
  case 'First': return;
  case 'Second': return;
  case 'Third': return `${x.baz}`;
  default: return absurd(x);

Remember from one of my previous articles — the absurd is a function which cannot be called, as its signature is the following:

const absurd = <A>(x: never): A => {
  throw new Error('Should not be here');

So when we add another case to our ADT type, we will get a nice compilation error, and if we suppress it, the absurd will ensure that the code will blow up in the runtime, and the developer who caused this will have to go and fix it.

Finally Tagless, Partially Evaluated

Another big topic which helps in making illegal states unrepresentable is a Tagless Final design pattern.

Tagless Final style was introduced by Oleg Kiselyov et. al. in the paper called “Finally Tagless, Partially Evaluated: Tagless Staged Interpreters for Simpler Typed Languages”. This is a way of builting embedded domain-specific languages (eDSLs) and interpreters for them, which allows abstracting away the type of computational effect of each operation.

Using Tagless Final style, we create a set of interfaces (called F-algebras), and use them to express dependencies of each piece of business logic. So you may think about Tagless Final as a way of doing inversion of control in a functional program.

In an imaginary TypeScript-with-kinds syntax it looks like this:

interface BusinessEntity<F<_>> {
  readonly getSomethingById: (id: EntityId) => F<Option<Entity>>;
  readonly doSomething: (entity: Entity) => F<void>;

By making BusinessEntity kind-polymorphic in F, we abstract the details of an interpreter — for example, some of possible interpreters may substitute F with a Promise, Task or Future, and run the actual computations asynchronously. Another interpreter may substitute F with a Writer, and log something alongsite the synchronously executed computation, and so on.

In order to be able to write real programs it’s useful to declare F as a descendant of a monad type class, so we could use it to chain computations. Using defunctionalization technique of lightweight higher-kinded polymorphism, we rewrite these ideas in the real TypeScript:

interface BusinessEntity<F extends URIS> {
  readonly getSomethingById: (id: EntityId) => Kind<F, Option<Entity>>;
  readonly doSomething: (entity: Entity) => Kind<F, void>;

type Program<F extends URIS> = BusinessEntity<F> & Monad1<F>;

N.B. Recently Michael Arnaldi from Matechs has written a good article about another way of encoding HKTs in modern TypeScript — you can find it in Matechs blog.

Now we can make use of Program type to express a dependency and use it to express business logic — say, we want to get an entity and doSomething with it, if it’s present:

const someLogic = <F extends URIS>(P: Program<F>) => (id: EntityId): Kind<F, void> => pipe(
  // we use monad here to chain computations — but we don't know at this moment which monad it will ever be:
  P.chain(maybeEntity => pipe(
      () => P.of(void 0),
      entity => P.doSomething(entity)

Finally, we need to write an interpreter for our type Program, so we could actually execute someLogic program. Let’s take Task as a base monad for our interpreter:

import { task, option, console } from 'fp-ts';

const entityRepository = new Map<EntityId, Entity>( ... ); // fake repo with entities for illustrative purpose

const taskInterpreter: Program<task.URI> = {
  getSomethingById: id => task.of(option.fromNullable(entityRepository.get(id))),
  doSomething: entity => task.fromIO(console.log(entity.toString()))

// Somewhere at the edge of the world:

const entityId: EntityId = 42;
await someLogic(taskInterpreter)(entityId)(); // will interpret the program and actually run computations

Such decoupling of business logic and tied to the runtime interpreter makes Tagless Final a solid choice for cases where bits of business logic are designed to be reused across several modules or even projects, but implementation of their dependencies may vary in those module. For example, using Tagless Final you can abstract away pieces of business logic in a library, and write an interpreter for this library in each project dependent on it. Due to the fact that Program<F> reuiqres an instance of a monad, by writing corresponding interpreter you can even run the business logic synchronously or asynchronously — depending on the interpreter you provide! To make a quick example, let’s write a synchronous interpreter for our someLogic:

import { io, option, console } from 'fp-ts';
import { constant } from 'fp-ts/function';

const ioInterpreter: Program<io.URI> = {,
  getSomethingById: id => io.of(constant({ id, name: 'My entity' } as Entity)),
  doSomething: entity => console.log(`Entity #${} has name "${}"`)

const entityId: EntityId = 42;
someLogic(ioInterpreter)(entityId)(); // Note: we do not need to `await` here!

You can find a more complex example in the repository with examples accompanying my talk.

Up till now we have seen examples of making the parameters of business logic functions more and more strict, but we still don’t have means of fixing the flow of business logic. In order to get a new tool at our hands, let’s continue with the final topic I’d like to present in this post — a concept of an Indexed Monad.

Indexed Monads

As you probably know, a heart of a monad for a higher-kinded type F is a function called chain:

interface Monad<F extends URIS> {
  readonly chain: <A, B>(next: (a: A) => Kind<F, A>) => (fa: Kind<F, A>) => Kind<F, B>;

This function states the following: “if you give me a function of type A => F<B>, and a variable of type F<A>, I can produce a value of type F<B> by “resolving” F<A> and providing it to next”. For a TypeScript developer, the closest intuition will be callbacks or Promise.then (as a mental model, not as an example of a monad per se!).

Now, an indexed monad expands on a regular monad by adding three additional type parameters to chain — types Input, Middle, and Output:

interface IxMonad<F extends URIS3> {
  readonly ixchain: <A, B, Middle, Output>(next: (a: A) => Kind3<F, Middle, Output, B>) =>
    <Input>(fa: Kind3<F, Input, Middle, A>) => Kind3<F, Input, Output, B>

Wow, that’s a lot of type parameters! But fear not, the ixchain really just states the following: “I can do the same as chain, but iff holds the flow of types Input -> Middle -> Output”. So if you have a variable of type F<Foo, Bar, number>, you cannot call ixchain with a function (n: number) => F<Quux, Baz, string>, as types Bar and Quux are not the same!

The topic of indexed monads is interesting, but quite large, and this article is already too big. If you’re interested in a more thorough explanation of indexed monads, I highly recommend reading “Using IxMonad to enforce good hamburger building in TypeScript”.


Let’s wrap up. “Making illegal states unrepresentable” principle is a really powerful modelling approach, which puts TypeScript type system to its full extent. You can use it to battle various development mistakes — ranging from accidentally swapping two string parameters to fixing the flow of business logic and ensuring that transactional properties hold. Here’s a table with possible mistakes on the left, and corresponding approaches of mitigating then on the right:

Table of Making Illegal States Unrepresentable approaches presented in this article