Functional reactive Twitter bots

class: center, middle

# Functional reactive<br>Twitter bots
#### Omar Rizwan
July 22, 2014 at LambdaJam Chicago

---

# What does *this* have to do with functional programming?

.center[![Counter bot](images/counter-bot.png)]

---

# Signals

- What is a Signal?

- A Signal is a time-varying value.

---

# Signals

- Suppose we have `Mouse.position` of type<br>`Signal (Int, Int)`.

- We can see `Mouse.position` as a *function* from<br>`Time -> (Int, Int)`.

- At a certain time, `Mouse.position` gives us a certain value:
<div id="mouse-position"></div>

---

# Operating on Signals

- One other way to see a Signal, in Elm, is as a *stream of events*.

---

# Operating on Signals

- `Mouse.position` is like a list that extends into the future:
<div id="mouse-position-stream"></div>

---

# Operating on Signals: lifting

- We use `map` to make a function `a -> b` into a function `[a] -> [b]`

- Similarly, we use `lift` to make a function<br>
.center[`a -> b`]
.center[into]
.center[`Signal a -> Signal b`]

---

# Operating on Signals: lifting

---

# What does this have to do with Twitter?

.center[![Counter bot](images/counter-bot.png)]

---

# Twitter feeds are like Signals!

- Represent an individual tweet as a value of type<br>
`StatusUpdate a`

- Then we'll represent a Twitter feed as<br>
`Signal (StatusUpdate a)`

---

# The Counter bot

- We need to generate a Signal of status updates

```scala
module Bot where

import Birdhouse as BH

port updates : Signal (BH.StatusUpdate {})
*port updates = ???
```

---

# The Counter bot

- We need to generate a Signal of status updates

- Each status update has the previous update's number + 1

---

.center[# What are our tools?]

---

# Starting with a primitive Signal:<br>a tick

- We want a Signal that ticks every time a fixed time interval passes

- The `every` function
.center[![Every](images/every.png)]

- Pass it a `Time` to get the Signal we'll start with
.times.center[![Times](images/times.png)]

---

# Starting with a primitive Signal:<br>a tick

- `every second` looks like this:
<div id="primitive"></div>
<button id="start-primitive">Start `every second`</button>

---

# Transforming `every second`:<br>ticks to counting

- We need to replace those timestamps with counting!

- The `count` function
.count.center[![Count](images/count.png)]

---

# Transforming `every second`:<br>ticks to counting

- Now we have `count (every second)`:
<div id="count"></div>
<button id="start-count">Start `count (every second)`</button>

---

<h1 class="smaller">Transforming <code>count (every second)</code>:<br>counting to tweets</h1>

- Now we want to turn those counts from `count (every second)` into tweets.

- First, lift the `show` function to turn the counts into Strings:

```scala
show : a -> String

lift show : Signal a -> Signal String
```

---

<h1 class="smaller">Transforming <code>count (every second)</code>:<br>counting to tweets</h1>

- Now we want to turn those counts from `count (every second)` into tweets.

- Then, lift the `update` function from the Birdhouse library:

```scala
update : String -> StatusUpdate {}

lift update : Signal String -> Signal (StatusUpdate {})
```

---

# Our final Signal

```scala
module Bot where

import Birdhouse as BH

port updates : Signal (BH.StatusUpdate {})
*port updates = lift BH.update (lift show ((count (every second))))
```

---

# Our final Signal

```scala
module Bot where

import Birdhouse as BH

port updates : Signal (BH.StatusUpdate {})
*port updates = BH.update . show <~ count (every second)
```

---

```scala
*port updates = BH.update . show <~ count (every second)
```

---