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# Thinking with types: building a Finite State Machine with TypeScript to drive an app's UI

Jorge Romero
·Apr 13, 2022·

• First, why?
• The Tagged Union type
• Building the PokeDex transition FSM in a Next.js app
• Check the GitHub repo!
• So what?
• Epilogue.

In my previous post I mentioned that I wanted to model my pokedex's interface interaction as a state machine.

I wanted implement a state machine with TypeScript to handle that for me. So I figured that I would share what I learned!

# First, why?

The functionality of my interface is completely determined at any point in time by the state the interface is in. Or rather, both the interface and the functionality are determined by the state.

So the obvious solution is to use a state machine to drive the functionality with some assurances towards correctness. <- This is important.

One of my desigin goals is to make it a seamless experience. So having correctness assurances helps with that.

First lets describe what is a state machine.

### A mathematical tangent (skippable)

A finite state machine, also called a finite automata. Has a finite set $$S$$ of possible internal states along with a finite set of allowed inputs from some alphabet $$A$$ of symbols.

We can represent the behaviour of the automata as a function: $$\delta : A \times S \rightarrow S$$ Which just means that the machine will take an input $$a_i \in A$$ and given an internal state $$s_i \in S$$ will return a new state: $$\delta(a_i, s_i) = s_n$$ Thats it. Does it seem familiar? This is just a reducer!

### End of tangent (ok, you can stop skipping now)

Let's reword what I said so that y'all skippers get the gist of it. A state machine is a mathematical entity that can "have" some defined state at some point in time. And can receive an input.

The next state of the machine is deterministically given by both the current state and the input:

$$S_2 = input+S_1$$

Again. That's just a reducer.

You may be wondering. Why go through the trouble? Why not just use redux or something like that?

The answer is that if we implement just the machine we need, we can prove (as in a mathematical proof) that it will behave correctly. No testing necessary. (even though writing unit tests for it is still recommended)

And if we are careful with how we implement it, using types, we can also make it so the we cannot break it.

Even if we tried!

The obligatory disclaimer: While the world of maths is crisp and perfect, the world of computers is anything but. We can make assurances about the model. But there is always the possibility that, given the internal workings of TypeScript, the JavaScript interpreter, or my own silliness, the whole thing can just break.

If anything, this can be a very good excuse for learning about programming with types!

## So what's the plan?

We can take two broad approaches to modeling a state machine with code.

1. Check the value of the input variable and use it to differentially choose one preexisting function for the behaviour (represented as the state). With a switch statement, for example.
2. Hold a reference to the behaviour. And let other behaviour change the reference.

With option 2 we cannot run behaviour that is not allowed for the current state. As the behaviour reference is the state. While that is possible with option 1.

We can implement option 2 either the OOP way, using the strategy pattern with objects encapsulating behaviour. Or the FP way, using that same pattern, but with typed functions. I would choose typed functions, of course :D

However. There is a third way of modeling the state machine. Using algebraic data types (ADTs) to constrain the behaviour of an object to a fininte set of possible behaviors. And then using a switch statement.

Unfortunately you cannot do this with TypeScript out of the box. So we are gonna be implementing a tagged union ADT to achieve this.

Then, I will share an implementation in TypeScript.

# The Tagged Union type

I will need to implement a type that is not quite supported by default in typescript. The sum type, also known as tagged union and discriminated union. Or just union type. I will use those interchangeably.

You may be wondering that there is already a sum type in TypeScript!

Well, yes. But also not quite.

In TypeScript, you have both union and intersection types. But these don't work as one may expect (from type theory).

The first thing to note is that TypeScript uses structural typing. Which means that ultimately, the only thing that matters is the shape of the object which you are typing (for non-primitive types)

In contrast to *nominal* typing in class-based object oriented languages, like C++

So in the following example, both types are interchangeable.

type Dog = {name:string, breed: string}
type PedigreePet = {name:string, breed:string}
// Dog === PedigreePet


Those two types would not be equivalent in C++, because of nominal typing.

How the union type works in typescript is constrained by this requirement. Let's see some examples. Consider the following union type Pet.

type Dog = { name: string, breed: string}
type Cat = { name: string, favoriteFood: string}

type Pet = Dog | Cat


You may think of the type Pet as "either a Dog or a Cat". But, do you think pet.breed is a valid property?

A function that takes a Pet cannot know if the argument is a cat or a dog:

const rename = (pet: Pet) => {
// This will make TypeScript complain!!!
pet.breed = "expensive breed you can't afford"
// But this is ok
pet.name = "bob"
}


So a more accurate way of thinking would be. "Pet is something that could pass as either a Dog or a Cat, but must be general enough to be both". So the union type is the type of values that are valid enough as any of either types.

But if we take the intersection type of cat and dog we get a different behaviour.

type PetInt = Dog & Cat
// which is the same as
type PetInt = {name: string, breed: string, favoriteFood:string}


Which is quite uncomfortable to process right? It seems like the names "union" and "intersection" should be flipped! But the good people at microsoft have very good reasons to have them behave like this.

The union type as it works on TypeScript means that if you take the set of all possible values for all the types you are considering (that is, the set theoretical union), then you can only talk about the common properties.

This is because if you have a function with the signature

type namePet = (pet: Pet) => Pet


As in the example above, that would mean the same as

type namePet = (pet: {name:string}) => {name:string}


That is, you have to be able to pass any object that could be either a Dog or a Cat. So you have to keep only the common elements. You loose some information about the original type when you use Pet. The function namePet cannot know whether the pet you pass is a cat or a dog.

But all that is just a JavaScript quirk! Because of how its object system works. And an artifact of TypeScript using structural typing.

But the type theoretical union types are a bit different in how they behave. Although the concept is the same.

In order to explore the union type, we will need to switch to Haskell. Don't worry, the syntax is quite easy, so you can follow.

In Haskell, a discriminated (tagged) union type looks like this.

data MyBool = Yes | No


Here we are defining a new type MyBool. And we are saying that any value of type MyBool must be either a Yes or a No. Those two are types, not values. (Although the way they are expressed in Haskell here means that they are single-value types...)

But nothing stops us from doing this:

data MyBoolZ = Yes | No | Z


And any value of type MyBoolZ must be either a Yes, a No , or a Z. And any function that accepts a MyBoolZ must handle all possible cases.

checkIfBool :: MyBoolZ -> Bool
checkIfBool x = case x of
Yes -> True
No -> True
Z -> False


A note: this kind of check is kind of pointless in Haskell. And seems iffy to me. You really shouldn't need checking types in Haskell.

Here, the function checkIfBool takes a value x of type BoolZ and returns true if it is either Yes or No. But returns false otherwise.

So, for the pet example, we can do something like this:

data Pet = Cat | Dog

checkIfCat :: Pet -> Bool
checkIfCat pet = case pet of
Cat -> True
Dog -> False


Which in haskell means that any value of type Pet must be either a dog or a cat. But if it is a Dog, then it is a fully fledged Dog. And so for Cat. That is, after receiving Pet, a function can know if it is a Dog or a Cat.

You cannot do this in TypeScript. As Pet looses information. But in Haskell, the union is tagged, in the sense that any Pet carries information about whether it is a Dog or a Cat.

Let's fix that!

## Implementing a Tagged Union in TypeScript

The good news is that we can, without much effort, implement the union type from Haskell in TypeScript.

What we are implementing is an algebraic data type which means that our type should behave accordingly to some particular algebraic rules or properties. Right now the only property we care about is that each type is tagged. So the sum type doesn't loose information when passing into a function.

If you are curious, we are implementing something called the *coproduct* in the category of types. But it is not super important to understand that. As not even Haskell is mathematically rigorous enough for that. The only thing you probably should know is that it means more or less the same as the set theoretical discriminated union. If two sets $$A$$ and $$B$$ both have the element $$x$$, then the discriminated union $$A \coprod B$$ has the elements $$(x,a)$$ and $$(x,b)$$.

We just need to make sure that we can always ask Pet for its underlying type. That means, we tag a Pet as either a Dog or a Cat.

type Dog = {type: "dog", name: string, breed: string}
type Cat = {type: "cat", name: string, favoriteFood: string}


Now Pet would look like this:

type Pet = Dog | Cat
// means the same as
type Pet = {type: "dog"|"cat", name: string}


Really, it's that simple...

The words "dog" and "cat" are not strings. They are just the names for two singleton types (which can only have one single value).

Please take a moment to think about what this means. A union type in type theory is just a type that can have multiple representations. Not one that can pass as either one of them, as is the default in TypeScript. Haskell by default uses this type theoretical union type.

Hence, the equivalence with the natural language "or" (and the logical or). A pet is a cat or a dog.

If you are curious again, I can make this claim thanks to an important result in logic called the Curry-Howard correspondence.

### Example: A recursive binary tree.

Let's see an example on how to use tagged unions in Haskell. Consider the following MyTree type

data MyTree a = MyEmptyNode
| MyTree a (MyTree a) (MyTree a)


Which is a (recursive) binary tree data structure! And here may be the same implementation in "pseudo-typescript"

// NOT REAL TYPESCRIPT!
type MyEmptyNode = {}
type MyFilledNode<T> = {value: T, left?: Tree<T>, right?: Tree<T> }

type MyTree<T> = MyEmptyNode | MyFilledNode


But note that the "|" i am using in this hypothetical example is the tagged union from Haskell which TypeScript doesn't have. If you code this in your app, you would have a very empty binary tree.

So lets implement the binary tree in real TypeScript now

type TaggedNode<T extends string> = {tag: T}

type MyEmptyNode = TaggedNode<"Empty"> & {}
type MyFilledNode<V> = TaggedNode<"Filled"> & {
value: V,
left: MyTree<V> | MyEmptyNode,
right: MyTree<V> | MyEmptyNode
}

type MyTree<V> = MyEmptyNode | MyFilledNode<V>


Now the "|" will behave as in Haskell. Here's an example:

const jsonifiableTree : MyTree<number> = {
tag: "Filled",
value: 10,
left: {
tag: "Empty"
},
right : {
tag: "Filled",
value: 5,
left: {
tag:"Empty"
},
right: {
tag: "Empty"
}
}
}


Now you can send fully typed binary trees over HTTP! XD

## Representing states with types

Let's use tagged unions to represent the type of states, Actions and build a simple finite state machine. Let's consider an example. The AI for an enemy NPC in a game like Zelda.

### State Types

It has several possible states:

• Wander
• Attack
• RunAway

And we would like to have it so that each state can carry some info that we can use to program the NPC's behaviour. I'll use intersection types for that.

type TaggedState<T extends string> = {tag:T}

type Wander = TaggedState<"Wander"> & {
isAsleep: boolean
}
type Attack = TaggedState<"Attack"> & {
enraged: boolean,
attackPoints: number
}
type RunAway = TaggedState<"RunAway"> & {
isTired: boolean,
}
// We can attack information to each state to drive the behaviour in the NPC's script

type NPCState = Wander | Attack | RunAway


### Action Types

And several "actions":

• SeePlayer: Wander -> Attack
• GetHit: Attack -> RunAway, Wander -> RunAway
• LoosePlayer: RunAway -> Wander
type TaggedAction<A extends string> = {tag:A}

type SeePlayer = TaggedAction<"SeePlayer"> & {
distance: "far" | "mid" | "near"
}
type GetHit = TaggedAction<"GetHit"> & {
dmgAmount: number,
canBeBlocked: boolean
}
type LoosePlayer = TaggedAction<"LoosePlayer">

type Action = SeePlayer | GetHit | LoosePlayer


### Transition function

Each action is a rule that takes you from one state to another, if you are in a valid state:

• SeePlayer: Wander -> Attack
• GetHit: Attack -> RunAway, Wander -> RunAway
• LoosePlayer: RunAway -> Wander

For example. Sending the action "LoosePlayer" when the NPC is in the "Wander" state doesn't make sense and should be ignored. The time and effort we spent thinking about Types will pay off when we make it so that it is impossible to get any weird runtime error because of some invalid action!

We can create a function with this signature:

type Reducer = (a: Action, s: State) => State

const reducer:Reducer = (action, state) => {
switch (state.tag) {
case "Wander":
return reduceWander(action, state)
case "Attack":
return reduceAttack(action, state)
case "RunAway":
return reduceRunAway(action, state)
default:
throw new TypeError ("Nope, you can't do that!")
}
}


We cannot have an invalid state. We can then handle the actions state-wise:

import {Wander, Attack, RunAway} from "./states.ts"

const reduceWander = (action, state) => {
switch(action.tag) {
case "SeePlayer" :
... // Logic for handling the resulting state
return <Attack>{...} // Whatever extra info we want to add
case "GetHit" :
...
return <RunAway>{...}
default:
return state // Ignore invalid actions
}
}

const reduceAttack = (action, state) => {
switch (action.tag) {
case "GetHit" :
...
return <RunAway>{...}
default :
return state
}
}

const reduceRunAway = (action, state) => {
switch (action.tag) {
case "LoosePlayer" :
...
return <Wander>{...}
default:
return state
}
}


Now the behaviour of the state machine cannot recieve invalid actions, nor can it have invalid transitions.

In other parts of the code we can "dispatch" actions to the reducer, to change the state of the NPC. There may be some issues that arise then, but not because of the state machine.

The reducer gives us guarantees that invalid transitions cannot happen.

To use the machine, you just need to initialize a State and have whatever function will send inputs dispatch an Action to the reducer, along with the current state.

Does it seem familiar? Of course! Because it is just our good friend, the finite automata's transition function:

$$\delta: A\times S \rightarrow S$$

The input set $$A$$ and state set $$S$$ sets are the Action and State types we were careful to define with tagged union types.

Then the reducer function is the state transition function $$\delta$$. Which internally just asks about the input s and checks if given s, the action a does anything. If not, it does nothing.

If you pass an invalid state, it won't break. It just won't compile. An invalid action will do nothing.

So, we ended up coding the mathematical definition of the finite automata!

# Building the PokeDex transition FSM in a Next.js app

If you are still thinking that this is just an overcomplicated way to use the redux pattern... Yes, yes it is. But I am learning a lot by writing this. And I hope you can share some of the learniness with me!

But also, no. I am using Next.js so I don't need redux for handling state while data fetching. I will make use of Next's own means of fetching. So that leaves me with the question of how to handle states.

One of my priorities with this app is having a seamless experience. I want to minimize the chances of weird or unintended behaviour. And having a purpose built state machine will allow me to provide some assurances of correctness.

The way we will implement it will make sure that we cannot break it. Even if we try. And going through the effort of using types will make sure that only the intended behaviour can happen. Because any non-intended behaviour will yield a compiler error!

## Defining types for States and Actions

First, lets code the types for the states of the FSM diagram we had in the previous post:

type TaggedState<S extends string> = { tag: S } & StateInfo;

type StateInfo = {
mapId: number | null,
pkmId: number | null,
sectionId: number | null,
}

// States
export type Search = TaggedState<"Search">
export type MapToSearch = TaggedState<"MapToSearch">
export type MapScreen = TaggedState<"MapScreen">
export type PkmInfoScreen = TaggedState<"PkmInfoScreen">
export type PkmInfoToSearch = TaggedState<"PkmInfoToSearch">
export type MapSectionScreen = TaggedState<"MapSectionScreen">

// State type
export type State =
| Search
| MapToSearch
| MapScreen
| PkmInfoScreen
| PkmInfoToSearch
| MapSectionScreen;


I also attached some extra information to each state. This way, a component that needs to know about the PkmInfoScreen state knows which pokemon to fetch from the API.

We will do the same for the actions.

type TaggedAction<A extends string> = { tag: A };

// Actions
export type InputSearch = TaggedAction<"InputSearch">;
export type ClickPkm = TaggedAction<"ClickPkm"> & { pkmId: number };
export type ClickMap = TaggedAction<"ClickMap"> & { mapId: number };
export type ClickSearch = TaggedAction<"ClickSearch">;
export type ClickSection = TaggedAction<"ClickSection"> & {
mapId: number;
sectionId: number;
};

// Action type
export type Action =
| InputSearch
| ClickPkm
| ClickMap
| ClickSearch
| ClickSection;


## Using a Test Driven Development approach to code the transition logic

Before we start with the reducers, lets write some test to capture the intended behaviour of the FSM. I'm using Jest. Here I'll just share the test for one of the reducers, so you can appreciate that I am using the tests to codify the intended behaviour of the FSM. In a TDD approach of sorts.

If you want to see how I coded the rest of the tests, check the github repo :D

describe("Search State Transitions", () => {
const initialState: Search = {
type: "Search",
pkmId: null,
mapId: null,
sectionId: null,
};

it("action 'ClickPkm' should transition to PkmInfoScreen with pkmId", () => {
const expectedState: PkmInfoScreen = {
type: "PkmInfoScreen",
pkmId: 1,
mapId: null,
sectionId: null,
};

const action: Action = {
type: "ClickPkm",
pkmId: 1,
};

expect(fsmReducer(action, initialState)).toEqual(expectedState);
});

it("action 'ClickMap' should transition to MapScreen with mapId", () => {
const expectedState: MapScreen = {
type: "MapScreen",
pkmId: null,
mapId: 3,
sectionId: null,
};

const action: ClickMap = {
type: "ClickMap",
mapId: 3,
};
expect(fsmReducer(action, initialState)).toEqual(expectedState);

});

it("should do nothing when given an invalid action", () => {
const invalidAction = {
type: "IamInvalid",
pkmId: "nope",
nonExistentId: 5,
};

// This test will run in JavaScript. But TypeScript will complain,
// which is exatly what I want. I shouldn't be able to run this in the app
expect(fsmReducer(invalidAction, initialState))
.toEqual(initialState);

});

});


Once we fill out the tests, and write the reducers the result of running the tests should be a pass:

jorchrl@Jorges-MacBook-Pro pokedex-next % npm run test

> pokedex-next@0.1.0 test
> jest

PASS  modules/pokeFSM/Reducers.test.ts
Search State Transitions
✓ action 'ClickPkm' should transition to PkmInfoScreen with pkmId (2 ms)
✓ action 'ClickMap' should transition to MapScreen with mapId (1 ms)
✓ should do nothing when given an invalid action (1 ms)
MapScreen State Transitions
✓ action 'ClickSearch' should transition to MapToSearch (1 ms)
✓ action 'ClickSection' should transition to MapSectionScreen with mapId and sectionId (1 ms)
✓ should do nothing when given an invalid action (1 ms)
MapToSearch State Transitions
✓ action 'InputSearch' should transition to Search (1 ms)
✓ action 'ClickMap' should transition to MapScreen with mapId (1 ms)
✓ should do nothing when given an invalid action (1 ms)
PkmInfoScreen State Transitions
✓ action 'ClickSearch' should transition to PkmInfoToSearch (1 ms)
✓ action 'ClickMap' should transition to MapScreen with mapId  (2 ms)
✓ action 'ClickSection' should transition to MapSectionScreen with mapId and sectionId (1 ms)
✓ should do nothing when given an invalid action (1 ms)
PkmInfoToSearch State Transitions
✓ action 'InputSearch' should transition to Search (1 ms)
✓ action 'ClickPkm' should transitio to PkmInfoScreen with pkmId (1 ms)
✓ action 'ClickMap' should transition to MapScreen with mapId
✓ action 'ClickSection' should transition to MapSectionScreen with mapId and sectionId (1 ms)
✓ should do nothing when given an invalid action (1 ms)
MapSectionScreen State Transitions
✓ action 'ClickSection' should transition to MapSectionScreen with mapId and sectionId (1 ms)
✓ action 'ClickPkm' should transition to PkmInfoScreen with pkmId (1 ms)
✓ action 'ClickSearch' should transition to MapToSearch (2 ms)
✓ should do nothing when given an invalid action (1 ms)
Invalid States
✓ the reducer should throw a TypeError when an invalid state is passed (31 ms)

Test Suites: 1 passed, 1 total
Tests:       23 passed, 23 total
Snapshots:   0 total
Time:        1.59 s
Ran all test suites.


## Writing the Reducers

The reducer function will first filter through the input state's type. And pass the input to the adequate state reducer.

export type Reducer = (a: Action, s: State) => State;

export const fsmReducer: Reducer = (action, currentState) => {
switch (currentState.type) {
// First, filter by state. So you cannot send actions to an invalid state.
// But note that this is a pure function. So currentState is extenal.
case "Search":
return reduceSearch(action, currentState);
case "MapToSearch":
return reduceMapToSearch(action, currentState);
case "MapScreen":
return reduceMapScreen(action, currentState);
case "PkmInfoScreen":
return reducePkmInfoScreen(action, currentState);
case "PkmInfoToSearch":
return reducePkmInfoToSearch(action, currentState);
case "MapSectionScreen":
return reduceMapSectionScreen(action, currentState);
default:
throw new TypeError("You cannot pass an invalid state!");
}
};


Here's how one of those looks like:

// Each reducer handles the valid actions per state. So you cannot send an
// invalid action, ever. These are also pure functions.
export const reduceSearch: Reducer = (action, state) => {
// Filter by action type
switch (action.type) {
case "ClickMap":
// The action carries info about the map
return { ...state, type: "MapScreen", mapId: action.mapId };
case "ClickPkm":
// The action carries info about the pokemon
return { ...state, type: "PkmInfoScreen", pkmId: action.pkmId };
default:
return state;
}
};


Check the GitHub repo to see all of them.

Note that we are handling invalid input states by throwing a TypeError. And handling invalid action inputs, per state, by ignoring them. So we cannot trigger unexpected behaviour. TypeScript will not compile if we try.

Thus we cannot break this machine. <- this was the whole point of this exercise.

We can still break the app in other ways, but not the machine.

### No bussines logic here

I just want to also point out that what we have done here is intended to be detached from the actual logic that dictates how to decide when to change states. That logic will be implemented inside our components.

The whole point of doing things this way is that I can put some constrains in the business logic in a decoupled manner. We could use different apps that use the same state machine.

For example, I mentioned that I was considering doing a mobile version. In that case, the interaction may be different, from the point of view of the user, but I could use the same FSM. Or make some slight modifications.

That's no problem because I wrote it as a separate module. I coded just the barebones machine. Nothing else!

In the next post I will start building the actual interface!

## Wiring it up to the app

I will try out Zustand, a unidirectional state management solution for React. It's smaller and simpler than Redux, and way less 'boilerplatey' than React's Context API.

The way it works is, in my opinion, extremely elegant. Your store is a hook:

import create from 'zustand'
import State from "./States.ts"
import Action from "./Actions.ts"
import {fsmReducer} from "./Reducer.ts"

// Let's type our store to help avoid unexpected behavior
type FSMStore = State & {
// syntax from Zustand's documentation
dispatch: (a: Action) => void;
};

// We define the store as a hook
const useStore = create<FSMStore>((set) => ({
type: "Search",
pkmId: null,
mapId: null,
sectionId: null,
dispatch: (action: Action) => set((state) => fsmReducer(action, state)),
}));


And then you just throw them into your components. No need to wrap your app in a provider:

const MyComponent = () => {
const fsmType = useStore((state) => state.type);
const dispatch = useStore((state) => state.dispatch);
// then use those values in the component's JSX
return (...)
}


You could also create an utility action creator :

// possible action creator,
const createAction = (
type: ActionTypes,
pkmId: number = 1,
mapId: number = 1,
sectionId: number = 1,
):Action => {
return { type: type, pkmId, mapId, sectionId };
};


Thats it!

But you can wrap it up in a provider if you really want. And it may be the case that you need to do so. But I don't for this use case.

## Trying out the FSM

Let's code a separate page to play around with the behaviour of the FSM. As I mentioned, doing things this way allows me to swap the business logic of the transitions, from the machine handling the transitions.

Here's how it looks.

You can play with it by cloning the GitHub repo and running npm run dev

and visiting localhost:3000/fsm-playground

# Check the GitHub repo!

Here it is, pokedex-next

# So what?

One of the goals is learning. And not necessarily achieving the most simple solution. There are libraries, like coproduct that would have made using union types much simpler.

I could have also implemented the state machine just with Zustand, but here I am using it as a quick way to wire my state machine to the app.

TypeScript not behaving as "proper" Haskell may seem like a disadvantage. But it doesn't take much effort to make it do (to some reasonable extent).

That makes me like TypeScript even more. In my opinion, JavaScript's idiosyncratic object-orientedness is much more elegant than pretty much any other language.

It's a tradeoff, in exchange for the flexibility of structural typing. Which is one of the really nice characteristics of both JavaScript and TypeScript. And I think it is well worth it.

The only thing in my wishlist is to make TypeScript much less verbose and a little bit more consistent.

# Epilogue.

Hopefully this was interesting!

I did learn a TON. Like, an obscene amount. I hope you get to share my learniness :D

If you made it all the way here, consider leaving a comment or feedback. I appreciate it a lot! I am learning this stuff. So any feedback will be greatly appreciated!

Next post will be about using this state machine to build the interface. With TailwindCSS! So, hopefully will not be as heavy as this one :D

I will also be updating on Twitter with the hashtag #BuildInPublic