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<!DOCTYPE html>
<html>
<head>
<title>Haskell - Adventures in IO</title>
<meta charset="utf-8">
<style>
/* @import url(https://fonts.googleapis.com/css?family=Yanone+Kaffeesatz);
@import url(https://fonts.googleapis.com/css?family=Droid+Serif:400,700,400italic);
@import url(https://fonts.googleapis.com/css?family=Ubuntu+Mono:400,700,400italic); */
@import url(css/metropolis.css);
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</head>
<body>
<textarea id="source">
class: center, middle, title-slide
count: false
.less-line-height[
Alejandro Serrano @ ZuriHac 2022
.grey[🐦 @trupill - 🐈⬛ serras - 👨💻 Tweag]
]
---
# 🥅 Overall goal
### .grey[How do we build software with Haskell?]
1. ~~Domain-specific languages <br /> Representing actions and scripts <br /> Property-based testing~~
2. Communicating over the network <br /> Serialization (without boilerplate) <br /> Error handling <br /> Concurrency across threads
---
# 🃏 Overall goal
### .grey[Build an interactive card game]
1. Represent the cards and the actions
2. Communicate different clients
<table>
<tr>
<td style="vertical-align: top"><img src="img/pikachucard.png" width="100%" /></td>
<td width="82%" style="padding-left: 20px; line-height: 1.3;">
<h3 style="margin-bottom: -20px"><img src="img/pikachu.png" width="32px" /> Pokémon Trading Card Game</h3>
<p>Goal: knock out 6 of your opponent's Pokémon using attacks</p>
</td>
</tr>
</table>
---
# 🃏 Overall goal
### .grey[Build an interactive card game]
1. Represent the cards and the actions
2. Communicate different clients
<table>
<tr>
<td style="vertical-align: top"><img src="img/pikachucard.png" width="100%" /></td>
<td width="82%" style="padding-left: 20px; line-height: 1.3;">
<h3><img src="img/pikachu.png" width="32px" /> Pokémon Trading Card Game</h3>
<h2>Waaaay too complex for 1.5h!</h2>
</td>
</tr>
</table>
---
# 🎲 Dice roll
<img src="img/diceroll.jpg" width="90%" />
---
# 🎲 Dice roll in the cloud ☁️
<img src="img/diceroll.jpg" width="90%" />
---
# 📚 Our stack
<img src="img/haskell-rainbow.gif" height="40px" /> Haskell (of course)
`network-simple` for networking
`binary` and `aeson` for serialization
`stm` for concurrency
---
# 🎲 One-person dice roll
🔢 Client requests a roll of _n_ faces
☁️ Server returns a random value
<img src="img/random_number.png" width="80%" />
.smaller[.little-margin-top[
_(mandatory XKCD strip)_
]]
---
# 🖧 Server-side `network-simple`
Let's practice signature-reading skills
.code70[
```haskell
serve
:: MonadIO m
=> HostPreference
-- ^ Host to bind.
-> ServiceName
-- ^ Server service port to bind.
-> ((Socket, SockAddr) -> IO ())
-- ^ Computation to run in a different thread
-- once an incoming connection is accepted.
-> m a -- ^ This function never returns.
```
]
---
# 🖧 Server-side `network-simple`
`serve` takes care of:
- Listening all the time at the given port
- Whenever a new connection request comes <br/> 1️⃣ accept it, <br /> 2️⃣ **spawn a new thread**, <br /> 3️⃣ run the callback.
---
# ➿ `go` pattern
Once we accept, we go into an (infinite) _loop_
- Usually with a recursive `go` function
.code70[
```haskell
diceServer :: IO ()
diceServer = serve "*" "8080" $ \(skt, _) -> do
-- initialization
go skt
where go :: Socket -> IO ()
go skt = do
-- read and parse request
-- send a response
go skt -- and over again!
```
]
---
# 🖧 Server-side `network-simple`
Requests and responses are 64-bit numbers
.little-margin-top[.code70[
```haskell
go :: Socket -> IO ()
go skt = do
-- read and parse request
mayBytes <- recv skt 8
case mayBytes of
Nothing -> pure () -- done
Just bytes -> do
let Right max = decode @Word64 bytes
response <- encode <$> randomRIO (0, max)
-- send a response
send skt response
go skt -- and over again!
```
]]
---
# 🏁 Recognizing end-of-connection
If `recv` returns nothing, we stop the loop
```haskell
go skt = do
-- read and parse request
mayBytes <- recv skt 8
case mayBytes of
Nothing -> pure () -- done
Just bytes -> do
...
go skt -- and over again!
```
---
# 🔄 Serialization
Conversion to and from `ByteString`
- `cereal` for binary encoding
- `aeson` for JSON
- `avro` for Avro (used in Kafka)
- ...
---
# 🔄 Serialization
Users implement instances of some classes
- Just one, like `Serialize` in `cereal`
- Two, like `ToJSON` and `FromJSON`
And then conversion is provided by generic
```haskell
encode :: Serialize a {- or -} ToFormat a
=> a -> ByteString
decode :: Serialize a {- or -} FromFormat a
=> ByteString -> Either Error a
```
---
# 🪢 Type applications
Help the compiler to disambiguate types
```haskell
decode @Word64 bytes
```
Other possibility is `ScopedTypeVariables`
---
# 🎲 One-person dice roll client
## .grey[🧑💻 Time for practice!] .font50[`serras.github.io/zurihac-workshop`]
.code70[
```haskell
connect
:: (MonadIO m, MonadMask m)
=> HostName
-- ^ Server hostname or IP address.
-> ServiceName
-- ^ Server service port.
-> ((Socket, SockAddr) -> m r)
-- ^ Computation taking the communication
-- socket and the server address.
-> m r
```
]
---
# 🎲 One-person dice roll client
## .grey[🤔 How do I test this?] .font50[`serras.github.io/zurihac-workshop`]
.code60[
```haskell
$ cabal repl session2
> import DiceRoll
DiceRoll> diceServer
^C -- use Ctrl + C to stop
```
```haskell
$ cabal repl session2
> import DiceRoll
DiceRoll> diceClient 6
3
DiceRoll> diceClient 10
9
```
]
---
# ‼️ Exceptions
You forget to start the server and...
```
> DiceRoll.diceClient 3
*** Exception: Network.Socket.connect:
<socket: 12>: does not exist
(Connection refused)
```
## 😱 WAAAAAAT????
---
# ‼️ Exceptions
Haskell's `IO` runtime uses **exceptions**
- Network or connection problems
- File (or resource) not found
- and many more input/output problems
Even some built-in features use exceptions
- Pattern match failure
- `undefined` and `error`
---
# ⁉️ Dealing with exceptions
~~Use `Control.Exception` from `base`~~
🅰️ Use `Control.Exception.Safe` <br /> from `safe-exceptions`
🅱️ Use `UnliftIO.Exception` from `unliftio`
### .grey[All three provide the same API]
---
# ⁉️ Dealing with exceptions
If you want to perform some handling
```haskell
catch :: (MonadCatch m, Exception e)
=> m a -> (e -> m a) -> m a
```
If you want to "purify" the problem
```haskell
try :: (MonadCatch m, Exception e)
=> m a -> m (Either e a)
```
---
# ⁉️ Idiomatic usage of `catch`
.code70[
```haskell
diceClient :: Word64 -> IO (Maybe Word64)
diceClient n =
connect "127.0.0.1" "8080" (\(skt, _) -> do
...)
`catch`
(\(e :: IOException) -> pure Nothing)
```
]
- Use `catch` infix
- Use `ScopedTypeVariables` to indicate which exceptions are to be caught
---
# 🫅 Exception hierarchy
Exception types for a hierarchy
- The top is called `SomeException`
However, if you want to "swallow" all exceptions, better use
.code70[
```haskell
catchAny :: MonadCatch m
=> m a -> (SomeException -> m a) -> m a
tryAny :: MonadCatch m
=> m a -> m (Either SomeException a)
```
]
---
# 🧰 Resource management
### .grey[Exception + laziness = 😵💫]
This makes resource management challenging <br /> (even more than usual)
🅰️ Use `resourcet` <br />
🅱️ Use `managed` <br />
➕ `resource-pool` to handle pools of resources
---
# 🎲 Dice roll and increment
### .grey[We want to extend the functionality]
How do we (de)serialize command and data?
1. Express them as data types
2. Implement `Serialize` instances
---
# 🎲 Dice roll and increment
### .grey[We want to extend the functionality]
How do we (de)serialize command and data?
1. Express them as data types
```haskell
data Request
= DiceRoll { max :: Word64 }
| Increment { number :: Word64 }
type Response = Word64
```
---
# 🎲 Dice roll and increment
### .grey[We want to extend the functionality]
How do we (de)serialize command and data?
1. Express them as data types
2. ~~Implement `Serialize` instances~~
## Use automatic deriving!
---
# 🤖 Automatic deriving
Compiler writes instances for us
- `Eq`, `Show`, and others are built-in
- Extensible with the `Generic` mechanism
.code70[
```haskell
{-# language DeriveGeneric, DeriveAnyClass #-}
data Request
= DiceRoll { max :: Word64 }
| Increment { number :: Word64 }
deriving (Generic, Serialize)
```
]
---
# 🖧 Updated server-side
```haskell
let Right max = decode @Word64 bytes
response <- encode <$> randomRIO (1, max)
```
Deserialize with the new type
```haskell
let Right req = decode @Request bytes
response <- case req of
DiceRoll max -> randomRIO (1, max)
Increment n -> pure (n + 1)
```
---
# 🖧 Updated server-side
```haskell
let Right max = decode @Word64 bytes
response <- encode <$> randomRIO (1, max)
```
Deserialize with the new type
```haskell
let Right req = decode bytes
response <- case req of
DiceRoll max -> randomRIO (1, max)
Increment n -> pure (n + 1)
```
The type `Request` can be inferred
---
# 💬 Greeting
## .grey[🧑💻 Time for practice!] .font50[`serras.github.io/zurihac-workshop`]
Add a new command to say "hi!" to people
Make it as flexible as you want:
- Name of the person to be greeted
- Language
- Time of the day
---
# ☁️🎲 Cloudy Rolly
☁️ Central server which processes requests
1️⃣ Player #1 joins and gets a code
2️⃣ Player #2 joins the session using that code
.grey[
🎲 Players send a new request for rolling
- The server sends the winner to both players
]
---
# ☁️🎲 Cloudy Rolly
.grey[
☁️ Central server which processes requests
1️⃣ Player #1 joins and gets a code
2️⃣ Player #2 joins the session using that code
]
## 🧵 Each player runs in a different thread
🪡 How to communicate across boundaries?
---
# 🪡 Communication across threads
~~Use `Control.Concurrent` from `base`~~
Use `stm` (Software Transactional Memory)
- Similar concept as in databases
- Transactions are isolated and atomic
- One cannot read the dirty state of another
---
# 🪡 Software Transactional Memory
One cannot read the dirty state of another
```haskell
do v <- newTVarIO 0
replicateM 3 $ async $ -- 3 threads
atomically $ do -- tx. boundary
n <- readTVar v -- ║
writeTVar (n + 1) -- ═╝
```
The end value of `v` is **guaranteed** to be 3
---
# ♻️ Server as state machine
When a user connects...
- If they send `NewGame`, send a `GameCode`
- When the other player connects, </br> send `GameStarts`
- If they send `JoinGame` and the code
- If the code exists, send `GameStarts`
- Otherwise, send `GameNotFound` and close
- Otherwise, close the connection
---
# 🪡 Shared state
Dictionary (key-value map)
- Keys are `GameCode`s
- Values are the sockets
--
```haskell
crServer = do
-- initialize shared state
state <- newTVarIO @State Map.empty
serve "127.0.0.1" "8080" $ \(skt, _) ->
worker state skt
worker :: TVar State -> Socket -> IO ()
```
---
# 👷 Cloudy Rolly worker
Trick to avoid repeating `state` and `skt`
.code70[
```haskell
worker :: TVar State -> Socket -> IO ()
worker state skt = start
where
start = do
...
newGame = ...
joinGame code = ...
play = putStrLn "play!"
```
]
---
# 👷 Cloudy Rolly worker
Wait for the first message to come
.little-margin-top[.code70[
```haskell
worker state skt = start
where
start = do
Just req <- recvJson skt
case req of
NewGame -> newGame
JoinGame code -> joinGame code
```
```haskell
-- read a line and deserialize
recvJson :: (MonadIO m, FromJSON a)
=> Socket -> m (Maybe a)
```
]
]
---
# 👷 Cloudy Rolly worker
.code60[
```haskell
worker state skt = start
where
newGame = do
-- record ourselves in new code
code <- randomCode
atomically $
modifyTVar state (Map.insert code [skt])
sendJson skt (GameCode code)
-- wait for the other message
atomically $ do
Just skts <- Map.lookup code <$> readTVar state
check (length skts > 1)
sendJson skt GameStarts
-- go!
play
```
]
---
# 🌀 Retry transactions
.code70[
```haskell
-- wait for the other message
atomically $ do
Just skts <- Map.lookup code <$> readTVar state
check (length skts > 1)
```
]
- `check` aborts the tx. if the condition is `False`
- The tx. is only retried when any `TVar` mentioned changes its value
### `TVar` works as a synchronization mechanism
---
# 👷 Cloudy Rolly worker
.code60[
```haskell
worker state skt = start
where
joinGame code = do
found <- atomically $ do
-- tx. boundary ════════════════════════════════╗
result <- Map.lookup code <$> readTVar state -- ║
case result of -- ║
Nothing -> pure False -- ║
Just skts -> do -- ║
modifyTVar state $ -- ║
Map.insertWith (<>) code [skt] -- ║
pure True -- ║
-- ═════════════════════════════════════════════╝
if found
then sendJson skt GameStarts >> play
else sendJson skt GameNotFound
```
]
---
# 👷 Cloudy Rolly worker
.code60[
```haskell
worker state skt = start
where
joinGame code = do
found <- atomically $ do
-- tx. boundary ════════════════════════════════╗
result <- Map.lookup code <$> readTVar state -- ║
case result of -- ║
Nothing -> pure False -- ║
Just skts -> do -- ║
modifyTVar state $ -- ║
Map.insertWith (<>) code [skt] -- ║
pure True -- ║
-- ═════════════════════════════════════════════╝
```
]
Transaction may include pure computations
---
# ☁️🎲 Cloudy Rolly
## .grey[🏠🧑💻 (No more) time for practice!]
### Finish the implementation of the server
1. Change `State` to represent on-going games
2. Use `TVar` as synchronization mechanism
3. Write a single `play` implementation
### Finish the implementation of the clients
---
# 🪡 STM-enabled data structures
Many users `==>` lots of contention
- Every `JoinGame` "wakes up" every `NewGame`
- But we only care about one code!
--
### More fine-grained data structures
.margin-top[
- `stm-containers` brings maps and sets
- `stm` includes unbounded queues
- `stm-chans` adds bounded queues
]
---
# 📋 Summary
### .grey[Lots of practice in the `IO` monad]
.margin-top[
- GHC uses exceptions for I/O errors
- High-level networking with `network-simple`
- Automatic deriving = <br /> serialization without the boilerplate
- `stm` for communication across threads
]
--
- Spawning and controlling threads with `async`
- `MonadIO` and `MonadUnliftIO`
---
# 😸 A word from our sponsor
## `leanpub.com/haskell-stdlibs`
<table style="margin-top: 40px;">
<tr>
<td style="vertical-align: top"><img src="img/libs.png" width="100%" /></td>
<td width="70%">
<ul style="margin-top: -60px; margin-bottom: -30px">
<li>More on exceptions and resource management</li>
<li>I/O using streaming</li>
<li>Servers and clients with HTTP</li>
</ul>
</td>
</tr>
</table>
<div class="grey"><b>Many other useful libraries in the ecosystem</b></div>
---
class: center, middle, title-slide
# 🤩 It's been a pleasure
## Enjoy the rest of ZuriHac!
</textarea>
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