The Way to Simplicity

# The Way to [Simplicity]()

### how Haskell simplifies code maintenance

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## — Simplicity is the result of [maturity]().

???

That's why novices tend to create too complex solutions. But when we become more experienced
we're looking for a simplicity.

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---

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How simple the code can be repaired, can be refactored and can be improved.

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---

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### []() Don't forget to encrypt

### []() Are you launching missiles?

### []() Build a page with types

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### []()  Types

### []()  Purity

### []()  Pattern-matching

---

### Did we encrypt it?!

---

```cpp
class Message {
  std::string message;
public:
  Message (const std::string& msg);
  std::string getMessage() const;
};
```

---

```haskell
data Message = Message Text
```

---

```cpp
class Message {
  std::string message;
* bool encrypted = false;
public:
  Message (const std::string& msg);
  std::string getMessage() const;
  
  bool isEncrypted() const;
  void encrypt();
  void decrypt();
};
```

---

```haskell
data Message = Plain Text
             | Encrypted Text

encrypt :: Message -> Message

decrypt :: Message -> Message
```

---

```haskell
main :: IO ()
main = send secret "dev@dev.by"
  where
    secret = encrypt plain
*   plain = Plain "Hi Minsk!"
```

---

```haskell
main :: IO ()
main = send secret "dev@dev.by"
  where
*   secret = Encrypted $
      readFile "/tmp/encrypted.msg"
```

---

```cpp
void
send (const Message& msg,
      const Recipient& recipient) {
* if (msg.isEncrypted())
    realSending(msg.getMessage(),
                recipient);
  else
    throw SendException
      ("Unencrypted message!");
}
```

---

```haskell
send :: Message
     -> Recipient
     -> IO ()
send (Encrypted text) recipient =
  realSending text recipient
send (Plain _) _ =
  throw SendUnencryptedMessage
```

---

```haskell
data Message state = Message Text

-- States of the message
data Plain
data Encrypted
```

---

```haskell
encrypt :: Message Plain
        -> Message Encrypted

decrypt :: Message Encrypted
        -> Message Plain
```

---

```haskell
send :: Message Encrypted
     -> Recipient
     -> IO ()
send (Message text) recipient =
  realSending text recipient
```

---

```haskell
data Message state = Message Text

-- States of the message
data Plain
data Encrypted
```

---

```haskell
data Message state = Message Text

-- States of the message
data Plain
data Encrypted
*data InBase64
```

---

```haskell
messageLength :: Message state
              -> Int
messageLength (Message text) =
  length text
```

---

```haskell
messageLength :: Message state
              -> Int
messageLength (Message text) =
  length text

toBase64 :: Message state
         -> Message InBase64
toBase64 (Message text) =
  Message $ encode text
```

---

### What about missiles?

---

```cpp
std::vector<Result>
calculate (const FoodInfo& info,
 const Food& commonFood);
```

---

```cpp
std::vector<Result>
calculate (const FoodInfo& info,
 const Food& commonFood);
```

---

```haskell
calculate :: FoodInfo
          -> Food
          -> [Result]
```

---

```haskell
calculate :: FoodInfo
          -> Food
          -> [Result]
```

---

```haskell
calculate :: FoodInfo
          -> Food
*         -> IO [Result]
```

---

```cpp
double calc_sin (double degree) {
  return sin (degree * PI / 180);
}
```

---

```cpp
double calc_sin (double degree) {
* launch_missiles();
  return sin (degree * PI / 180);
}
```

---

```cpp
double calc_sin (double degree) {
* should_I_launch_missiles = true;
  return sin (degree * PI / 180);
}
```

---

```haskell
calculate :: FoodInfo
          -> Food
          -> [Result]
```

---

### Building the page…

---

```haskell
".buy-form" ? do
  margin     (em 0) auto
  maxWidth   (em 30.0)
  fontFamily ["Roboto"] [sansSerif]
```

---

```haskell
body $
  div ! class_ "container" $
    div ! class_ ".buy-form"
      ...
```

---

```haskell
body $
  div ! class_ "container" $
*   div ! class_ ".buy-form"
      ...
```

---

```haskell
data ClassName
  = LanguageSwitcher
  | CurrentLanguage
  | BuyForm
  ...
  | Copyright
  | SocialButtons
  deriving (Show)
```

---

```haskell
data ClassName
  = LanguageSwitcher
  | CurrentLanguage
* | BuyForm
  ...
  | Copyright
  | SocialButtons
  deriving (Show)
```

---

```haskell
data ClassName
  = LanguageSwitcher
  | CurrentLanguage
  | BuyForm
  ...
  | Copyright
  | SocialButtons
* deriving (Show)
```

---

```haskell
*cl_ BuyForm ? do
  margin     (em 0) auto
  maxWidth   (em 30.0)
  fontFamily ["Roboto"] [sansSerif]
```

---

```haskell
*cl_ BuyForm ? do
  margin     (em 0) auto
  maxWidth   (em 30.0)
  fontFamily ["Roboto"] [sansSerif]
```

```haskell
*"." <> "BuyForm" ? do
 margin (em 0) auto
 maxWidth (em 30.0)
 fontFamily ["Roboto"] [sansSerif]
```

---

```haskell
body $
  div ! class_ "container" $
*   div ! class_ (toValue BuyForm)
      ...
```

---

```haskell
body $
  div ! class_ "container" $
*   div ! class_ (toValue BuyForm)
      ...
```

```html
<body>
 <div class="container">
* <div class="BuyForm">
 ...
```

---

### []()  Types

### []()  Purity

### []()  Pattern-matching

---

### Thanks!

## [Your]() questions…