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<!DOCTYPE html>
<html>
<head>
<title>Haskell - The Legend of DSLs</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);
</style>
</head>
<body>
<textarea id="source">
class: center, middle, title-slide
count: false
.less-line-height[
Alejandro Serrano @ INFOFP 2023-24
.grey[🐦 @trupill - 👨💻 JetBrains <br /> `serranofp.com`]
]
---
# In previous years...
```
Your knowledge of C#
+ Your knowledge of Haskell
---------------------------------
Ready for mainstream programming!
```
---
# In previous years...
```
Your knowledge of C#
+ Your knowledge of Haskell
---------------------------------
Ready for mainstream programming!
```
Last decade has been a good one for programing languages
- Java or C# were stuck in 2000s
- Now we have Scala, Kotlin, Swift, Elixir!
- And Java and C# are catching up
---
# Higher order functions are just there
### .grey[You understand this!]
From the Java documentation
.code70[
```java
int sum =
widgets.stream()
.filter(w -> w.getColor() == RED)
.mapToInt(w -> w.getWeight())
.sum();
```
]
.font60[
Reference: <br /> `https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html`
]
---
# In previous years...
```
Your knowledge of C#
+ Your knowledge of Haskell
---------------------------------
Ready for mainstream programming!
```
> Most of these ideas come from FP
INFOFP has become really relevant
--
## **Let's do something else instead!**
---
# <img src="img/pikachu.png" width="32px" /> Pokémon Trading Card Game
Players take turns drawing and playing cards
<table>
<tr>
<td style="vertical-align: top"><img src="img/pikachucard.png" width="100%" /></td>
<td width="68%" style="padding-left: 20px; line-height: 1.3;">
<div style="margin-bottom: -20px">Goal: knock out 6 of your opponent's Pokémon</div>
<ul>
<li>For this your use <b>attacks</b></li>
<li>Those attacks cost <b>energy</b></li>
<li>Each attack does <b>damage</b></li>
<li><b>HP</b> define the maximum damage before knock-out</li>
</ul>
</td>
</tr>
</table>
---
# 🏗️ Our approach
### .grey[Explanations interleaved with tasks]
1. Representing cards
2. Representing actions
3. Testing actions
--
## .grey[Domain-specific Language (DSL)]
Implementation of the _Ubiquitous Language_ idea from DDD, the code speaks the domain
---
# <img src="img/pikachu.png" width="32px" /> Representing cards
<table>
<tr>
<td style="vertical-align: top"><img src="img/pikachucard.png" width="100%" /></td>
<td width="68%" style="padding-left: 20px; line-height: 1.3;">
<div style="margin-bottom: -20px">Each card comes with...</div>
<ul style="margin-bottom: 20px">
<li>Name: <span class="remark-code-zoom">Pikachu</span></li>
<li>Type: <img src="img/energy/lightning.webp" height="32px" /></li>
<li>HP: 70</li>
<li>Attack(s)</li>
</ul>
(forget about the rest for now)
</td>
</tr>
</table>
---
# ⚙️ Algebraic Data Types (ADTs)
<table style="font-size: 70%">
<tr>
<td style="vertical-align: top; text-align: right;"><img src="img/pikachucard.png" width="60%" /></td>
<td width="68%" style="padding-left: 20px; line-height: 1.3;">
<div style="margin-bottom: -20px">Each card comes with...</div>
<ul style="margin-bottom: 20px">
<li>Name: <span class="remark-code-zoom">Pikachu</span></li>
<li>Type: <img src="img/energy/lightning.webp" height="20px" /></li>
<li>HP: 70</li>
<li>Attack(s)</li>
</ul>
</td>
</tr>
</table>
Straightforward translation of the description
.code70[
```haskell
data Card = Card { name :: Text
, typ :: Energy
, hp :: Natural
, attacks :: [Attack] }
```
]
---
# ⚙️ Algebraic Data Types (ADTs)
<table style="font-size: 70%">
<tr>
<td style="vertical-align: top; text-align: right;"><img src="img/pikachucard.png" width="60%" /></td>
<td width="68%" style="padding-left: 20px; line-height: 1.3;">
<div style="margin-bottom: -20px">Each card comes with...</div>
<ul style="margin-bottom: 20px">
<li>Name: <span class="remark-code-zoom">Pikachu</span></li>
<li>Type: <img src="img/energy/lightning.webp" height="20px" /></li>
<li>HP: 70</li>
<li>Attack(s)</li>
</ul>
</td>
</tr>
</table>
.code70[
```haskell
data Card = Card { name :: Text
, typ :: Energy
, hp :: HP
, attacks :: [Attack] }
newtype HP = HP Natural
deriving (Eq, Show, Num)
```
]
---
# ☯️ Energies
There are 10 types of energy in the game,
- 9 regular energies <img src="img/energy/grass.webp" width="32px" /> <img src="img/energy/fire.png" width="32px" /> <img src="img/energy/water.webp" width="32px" /> <img src="img/energy/lightning.webp" width="32px" /> <img src="img/energy/fighting.webp" width="32px" /> <img src="img/energy/psychic.webp" width="32px" /> <img src="img/energy/darkness.webp" width="32px" /> <img src="img/energy/metal.webp" width="32px" /> <img src="img/energy/dragon.webp" width="32px" />
- Colorless energy <img src="img/energy/colorless.webp" width="32px" />
- Any card providing a regular energy may also provide colorless energy
<img src="img/attack.png" height="48px" style="margin-top: 10px; margin-bottom: -10px;" /> = 2 <img src="img/energy/lightning.webp" width="32px" /> + 1 of any other
---
# ☯️ Energies
There are 10 types of energy in the game,
- 9 regular energies <img src="img/energy/grass.webp" width="32px" /> <img src="img/energy/fire.png" width="32px" /> <img src="img/energy/water.webp" width="32px" /> <img src="img/energy/lightning.webp" width="32px" /> <img src="img/energy/fighting.webp" width="32px" /> <img src="img/energy/psychic.webp" width="32px" /> <img src="img/energy/darkness.webp" width="32px" /> <img src="img/energy/metal.webp" width="32px" /> <img src="img/energy/dragon.webp" width="32px" />
- Colorless energy <img src="img/energy/colorless.webp" width="32px" />
.code70[
```haskell
data Energy = Colorless
| Grass | Fire | Water
| Lightning | Fighting | Psychic
| Darkness | Metal | Dragon
data Card = PokemonCard { ... }
| EnergyCard { typ :: Energy }
```
]
---
# ⚔️ Attacks
We consider only "simple" attacks for now
<div style="text-align: center"><img src="img/attack.png" height="60px" /></div>
.code70[
```haskell
data Attack = Attack { attackName :: Text
, cost :: [Energy]
, damage :: Natural }
```
]
---
# 🧑💻 Time for practice!
.very-little-margin-top[
### `serranofp.com/infofp.zip`
]
Define values for the following cards
<div style="margin-top: -20px">
<img src="img/grookeycard.png" width="40%" />
<img src="img/goomycard.png" width="40%" />
</div>
---
# ⚔️ Attacks, redux
.code70[
```haskell
data Attack = Attack { ..., damage :: Natural }
```
]
## .grey[This is a ~~lie~~ simplification]
---
# ⚔️ Attacks, redux
<img src="img/attack1.png" width="80%" />
<img src="img/attack2.png" width="80%" />
<img src="img/attack3.png" width="80%" />
<img src="img/attack4.png" width="80%" />
---
# ⚔️ Attacks, redux
.code70[
```haskell
data Attack = Attack { ..., damage :: Natural }
```
]
## .grey[This is a ~~lie~~ simplification]
.top-margin[
- More actions than mere damage
- Draw and discard cards
- Actions may depend on the state
- Attached cards
- Coin flips
- Actions may involve conditionals and loops
]
---
# ⚔️ Attacks, redux
.code70[
```haskell
data Attack = Attack { ..., action :: ??? }
```
]
## How do we model .grey[actions]?
.top-margin[
- More actions than mere damage
- Draw and discard cards
- Actions may depend on the state
- Attached cards
- Coin flips
- Actions may involve conditionals and loops
]
---
# 🪙 Coin flips
<img src="img/attack5.png" width="80%" />
.code70[
```haskell
data FlipOutcome = Heads | Tails
data Action
= FlipCoin (FlipOutcome -> Action)
| Damage Natural
surpriseAttackAction
= FlipCoin $ \case Heads -> Damage 30
Tails -> Damage 0
```
]
---
# 🪙 Coin flips
<img src="img/attack1.png" width="80%" />
## .grey[🧑💻 Time for practice!] .font70[`serranofp.com/infofp.zip`]
---
# 🪙 Coin flips
<img src="img/attack1.png" width="80%" />
```haskell
ironTailAction = go 0
where
go acc = FlipCoin $ \case
Tails -> Damage acc
Heads -> go (acc + 30)
```
---
# 🧞 Syntax/algebra and interpretation
`Action` defines the **syntax** of our DSL <br /> (also known as **algebra** in some circles)
> "The language itself", "what we can say"
---
# 🧞 Syntax/algebra and interpretation
`Action` defines the **syntax** of our DSL <br /> (also known as **algebra** in some circles)
An **interpretation** defines how each value behaves in a certain context
> "What a sentence means"
1 syntax / algebra ⟷ ∞ interpretations
---
# 🎰 Randomness interpretation
During the actual game, we expect to generate random coin flips to obtain the actual damage
```haskell
interpretRandom :: Action -> IO Natural
```
## .grey[🧑💻 Time for practice!] .font70[`serranofp.com/infofp.zip`]
---
# 🎰 Randomness interpretation
During the actual game, we expect to generate random coin flips to obtain the actual damage
```haskell
interpretRandom :: Action -> IO Natural
interpretRandom (Damage d) = pure d
interpretRandom (FlipCoin f) = do
outcome <- flipCoin
interpretRandom (f outcome)
-- one-liner
-- flipCoin >>= interpretRandom . f
```
---
# 🎴 Actions about cards
<img src="img/attack2.png" width="80%" />
<img src="img/attack3.png" width="80%" />
<img src="img/attack4.png" width="80%" />
---
# 🎴 Actions about cards
.code70[
```haskell
data Action
= FlipCoin (FlipOutcome -> Action)
| DrawCard (Maybe Card -> Action)
-- ^ there may not be more cards
| QueryAttached ([Card] -> Action)
-- ^ get info. about the current Pokémon
| Damage Natural
```
]
### .grey[Can you spot the pattern? 🔍]
---
# 🎴 Actions about cards
.code70[
```haskell
data Action
= FlipCoin (FlipOutcome -> Action)
| DrawCard (Maybe Card -> Action)
-- ^ there may not be more cards
| QueryAttached ([Card] -> Action)
-- ^ get info. about the current Pokémon
| Damage Natural
```
]
- `Damage` is a **final** action
- The rest "generate" a value, <br /> which is consumed to keep going
---
# 🎴 Actions about cards
<img src="img/attack4.png" width="80%" />
## .grey[🧑💻 Time for practice!] .font70[`serranofp.com/infofp.zip`]
.margin-top[
1. Write a function to **draw *n* ** cards
2. Add an additional operation to **discard** cards
- Must include a predicate to select cards
- Outcome: whether a card was discarded
]
---
# 🎴 Actions about cards
## .grey[🧑💻 Time for practice!] .font70[`serranofp.com/infofp.zip`]
Write a function to **draw *n* ** cards
_What should be the function signature?_
--
```haskell
drawN :: Natural -- amount
-> ([Card] -> Action) -- "next"
-> Action
drawN n next = _
```
---
# 🎴 Actions about cards
Write a function to **draw *n* ** cards
.code70[
```haskell
drawN n next = go n []
where
go n acc
| n <= 0
= next (reverse acc)
| otherwise
= DrawCard $ \case
Nothing -> next (reverse acc)
Just c -> go (n - 1) (c : acc)
```
]
---
# 🩻 Property-based testing
Generate many _random_ tests for the same function (or set of them)
Focus on **properties** rather than examples
- PBT frameworks are good at generating corner cases (extreme values, empty lists, ...)
---
# ✅ Testing actions
### .grey[How can we test `ironTailAction`?]
<img src="img/attack1.png" width="80%" />
---
# ✅ Testing actions
### .grey[How can we test `ironTailAction`?]
<img src="img/attack1.png" width="80%" />
- If we get a tail as first result, we get 0
- If our outcomes start with `n` heads, <br /> then the result is `30 * n`
---
# ✅ Testing actions
### .grey[How can we test `ironTailAction`?]
<img src="img/attack1.png" width="80%" />
❌ Using `interpretRandom` would not work
- The outcome is random
- Testing `IO` is cumbersome
---
# 🧮 Pure interpretation of flipping
We pass the future outcomes as a parameter
```haskell
interpretPure :: [FlipOutcome]
-> Action -> Natural
```
---
# 🧮 Pure interpretation of flipping
We pass the future outcomes as a parameter
```haskell
interpretPure :: [FlipOutcome]
-> Action -> Natural
```
Now we control the future 🔮
```haskell
> interpretPure [Heads, Heads, Tails]
ironTailAction
60
```
---
# 🧮 Pure interpretation of flipping
.code70[
```haskell
interpretPure :: [FlipOutcome]
-> Action -> Natural
interpretPure (result : future) (FlipCoin next) =
interpretPure future (next result)
interpretPure _future (Damage n) = n
```
]
---
# 🏃 QuickCheck + Tasty 🥧
**QuickCheck** is a well-known library for property-based testing
- Define properties of functions
- Support for custom generators
**Tasty** is a test runner
- Runs and reports over a set of tests
---
# 🏃 QuickCheck + Tasty 🥧
🥧 `testGroup` + 🏃 `testProperty`
- `outcomes` is randomly selected
.code70[
```haskell
tests :: TestTree
tests = testGroup "Iron Tail"
[ testProperty "non-negative" $ \outcomes ->
interpretPure (outcomes ++ [Tails])
ironTailAction >= 0
, ... ]
```
]
---
# ✋ A wrong property
```haskell
interpretPure ... ironTailAction > 0
```
A counter-example is found by QuickCheck
.code70[
```
Iron Tail
non-negative: FAIL
*** Failed! Falsified (after 1 test):
[]
Use --quickcheck-replay=139730 to reproduce.
Use -p '/non-negative/' to rerun this test only.
```
]
---
# <img src="img/pikachu.png" width="32px" /> The "times 30" property
To create good properties you must...
- Be creative with the inputs
- Ensure that inputs are correct
.code70[
```haskell
testProperty "30 * # heads" $ \(hs :: Int) ->
hs > 0 ==>
let outcomes = replicate hs Heads ++ [Tails]
in interpretPure outcomes ironTailAction
== fromIntegral (hs * 30)
```]
---
class: center, middle, title-slide
count: false
# 👻 Spooky slides ahead
---
# 👎 This is terrible
<img src="img/attack1.png" width="80%" />
```haskell
ironTailAction = go 0
where
go acc = FlipCoin $ \case
Tails -> Damage acc
Heads -> go (acc + 30)
```
---
# 👍 This reads better
<img src="img/attack1.png" width="80%" />
```haskell
ironTailAction = do
heads <- while (/= Tails) flipCoin
return (30 * length heads)
```
--
## We are building our own .grey[language]
---
# 🧙 Monads!
```haskell
data Action
= FlipCoin (FlipOutcome -> Action)
| Damage Natural
```
Why is it **not** possible to write `Monad` for this?
--
`Monad` applies to .grey[**type constructors**]
- We need to turn this into `Action a`
---
# 🧙 Monads!
```haskell
data Action a
= FlipCoin (FlipOutcome -> Action a)
| Return a -- "ends" the computation
```
--
## .grey[🧑💻 Time for practice!] .font70[`serranofp.com/infofp.zip`]
Write the `Monad Action` instance
---
# 🧙 Monads!
```haskell
data Action a
= FlipCoin (FlipOutcome -> Action a)
| Return a -- "ends" the computation
instance Monad Action where
return = _
x >>= f = _
```
---
# 🧙 Monads!
```haskell
data Action a
= FlipCoin (FlipOutcome -> Action a)
| Return a -- "ends" the computation
instance Monad Action where
-- a -> Action a
return = _
x >>= f = _
```
---
# 🧙 Monads!
```haskell
data Action a
= FlipCoin (FlipOutcome -> Action a)
| Return a -- "ends" the computation
instance Monad Action where
-- a ↰ ↱ Action a
return x = _
x >>= f = _
```
---
# 🧙 Monads!
```haskell
data Action a
= FlipCoin (FlipOutcome -> Action a)
| Return a -- "ends" the computation
instance Monad Action where
-- a ↰ ↱ Action a
return x = Return x
x >>= f = _
```
---
# 🧙 Monads!
```haskell
data Action a
= FlipCoin (FlipOutcome -> Action a)
| Return a -- "ends" the computation
instance Monad Action where
return x = Return x
-- Action a -> (a -> Action b) -> Action b
x >>= f = _
```
--
Pattern matching on `Action` keeps us going
---
# 🧙 Monads!
```haskell
data Action a
= FlipCoin (FlipOutcome -> Action a)
| Return a -- "ends" the computation
instance Monad Action where
return x = Return x
-- a ↰ ↱ (a -> Action b)
Return x >>= f = _ -- Action b
FlipCoin next >>= f = _
```
---
# 🧙 Monads!
```haskell
data Action a
= FlipCoin (FlipOutcome -> Action a)
| Return a -- "ends" the computation
instance Monad Action where
return x = Return x
-- a ↰ ↱ (a -> Action b)
Return x >>= f = f x -- Action b
FlipCoin next >>= f = _
```
---
# 🧙 Monads!
```haskell
data Action a
= FlipCoin (FlipOutcome -> Action a)
| Return a -- "ends" the computation
instance Monad Action where
return x = Return x
Return x >>= f = f x -- Action b
-- FlipCoin -> Action a
-- ↑ ↱ (a -> Action b)
FlipCoin next >>= f = _
```
--
We are mixing two blocks of actions
---
# 🧙 Monads!
```haskell
data Action a
= FlipCoin (FlipOutcome -> Action a)
| Return a -- "ends" the computation
instance Monad Action where
return x = Return x
Return x >>= f = f x -- Action b
-- FlipCoin -> Action a
-- ↑ ↱ (a -> Action b)
FlipCoin next >>= f = FlipCoin _
-- FlipCoin -> Action b ↵
```
---
# 🧙 Monads!
```haskell
data Action a
= FlipCoin (FlipOutcome -> Action a)
| Return a -- "ends" the computation
instance Monad Action where
return x = Return x
Return x >>= f = f x -- Action b
-- FlipCoin -> Action a
-- ↑ ↱ (a -> Action b)
FlipCoin next >>= f =
FlipCoin (\oc -> _ :: Action b)
```
---
# 🧙 Monads!
```haskell
data Action a
= FlipCoin (FlipOutcome -> Action a)
| Return a -- "ends" the computation
instance Monad Action where
return x = Return x
Return x >>= f = f x -- Action b
-- FlipCoin -> Action a
-- ↑ ↱ (a -> Action b)
FlipCoin next >>= f =
FlipCoin (\oc -> next oc >>= f)
```
---
# 😮💨 What have we gained?
Being a `Monad` gives you `do` notation
```haskell
flipCoin :: Action FlipOutcome
flipCoin = FlipCoin Return
flipTwo = do
x <- flipCoin
y <- flipCoin
if (x == Heads && y == Heads)
then return 50 else return 0
```
---
# 😮💨 What have we gained?
Being a `Monad` gives you `do` notation
```haskell
flipTwo =
FlipCoin $ \oc1 ->
FlipCoin $ \oc2 ->
if (oc1 == Heads && oc2 == Heads)
then Return 50 else Return 0
```
No need to handle constructors manually!
---
# 😮💨 What have we gained?
Being a `Monad` gives you access to many others
- Everything in `Control.Monad`
- `Control.Monad.Extra` in `extra`
- `monad-loops`
--
```haskell
ironTailAction = do
heads <- while (/= Tails) flipCoin
return (30 * length heads)
```
---
# 📋 Summary
### .grey[Haskell is a great language for DSLs]