null

background-image: url(../img/fp-tower/website-background.svg)

class: center, middle, white

.title[Dealing with optional values]
<br><br>

---
# Scala Functional Subset
<br>

.sixty-seven-left[
.center[
# .light-green[FP functions everywhere]

# .light-green[AND]

# .light-green[Two additional requirements]
]
]

---
<br><br><br><br><br><br><br>

.sixty-seven-left[
.center[
# null is a valid reference
# .light-green[class or trait]
]
]
---
class: medium-code

# null is a valid reference

.seventy-two-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)
```
]

---
class: medium-code

# null is a valid reference

.seventy-two-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}
```
]

---
class: medium-code

# null is a valid reference

.seventy-two-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object `Reduced`  extends VAT(`1.05`)
  case object Standard extends VAT(1.20)
}
```
]

---
class: medium-code

# null is a valid reference

.seventy-two-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object `Standard` extends VAT(`1.20`)
}
```
]

---
class: medium-code

# null is a valid reference

.seventy-two-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(null              , 4, 13.45, null   )
```
]

---
class: medium-code

# null is a valid reference

.seventy-two-left[
```scala
case class Item(name: `String`, quantity: Int, unitPrice: Double, vat: `VAT`)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(`null`              , 4, 13.45, `null`   )
```
]

---
class: medium-code

# null is a valid reference

.seventy-two-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(null              , 4, 13.45, null   )
val other  : Item = `null`
```
]

---
class: medium-code

# null is a valid reference but not a primitive

.seventy-seven-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(null              , 4, 13.45, null   )
val other  : Item = null
```

```scala
val toy    : Item = Item("Puzzle 40 Pcs", `null`, `null`, VAT.Standard)
// error: an expression of type Null is ineligible for implicit conversion
// val toy: Item = Item("Puzzle 40 Pcs", null, null, VAT.Standard)
//                                       ^^^^  ^^^^
```
]

---
class: medium-code

# null causes exceptions

.seventy-seven-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(null              , 4, 13.45, null   )

def totalPrice(item: Item): Double =
  item.unitPrice * item.quantity * item.vat.rate
```
]

.thirty-seven-left[
```scala
totalPrice(book)
// res: Double = 18.879
```
]

---
class: medium-code

# null causes exceptions

.seventy-seven-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(null              , 4, 13.45, null   )

def `totalPrice`(item: Item): Double =
  item.unitPrice * item.quantity * item.vat.rate
```
]

.thirty-seven-left[
```scala
totalPrice(book)
// res: Double = 18.879
```
]

---
class: medium-code

# null causes exceptions

.seventy-seven-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(null              , 4, 13.45, null   )

def totalPrice(item: Item): Double =
  `item.unitPrice * item.quantity * item.vat.rate`
```
]

.thirty-seven-left[
```scala
totalPrice(book)
// res: Double = 18.879
```
]

---
class: medium-code

# null causes exceptions

.seventy-seven-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(null              , 4, 13.45, null   )

def totalPrice(item: Item): Double =
  item.unitPrice * item.quantity * item.vat.rate
```
]

.thirty-seven-left[
```scala
totalPrice(`book`)
// res: Double = `18.879`
```
]

---
class: medium-code

# null causes exceptions

.seventy-seven-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(null              , 4, 13.45, `null`   )

def totalPrice(item: Item): Double =
  item.unitPrice * item.quantity * item.`vat.rate`
```
]

.thirty-seven-left[
```scala
totalPrice(unknown)
// java.lang.NullPointerException
```
]

---
class: medium-code

# null causes exceptions

.seventy-seven-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(null              , 4, 13.45, null   )

def totalPrice(item: Item): Double =
  `item.unitPrice` * `item.quantity` * `item.vat`.rate
```
]

.thirty-seven-left[
```scala
totalPrice(`null`)
// java.lang.NullPointerException
```
]

---
class: medium-code

# Handling null

.seventy-seven-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(null              , 4, 13.45, null   )

def totalPrice(item: Item): Double =
  if(`item == null`)           0.0
  else if (`item.vat == null`) item.quantity * item.unitPrice
  else                       item.quantity * item.unitPrice * item.vat.rate
```
]

---
class: medium-code

# Handling null

.seventy-seven-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(null              , 4, 13.45, null   )

def totalPrice(item: Item): Double =
  if(item == null)           0.0
  else if (item.vat == null) item.quantity * item.unitPrice
  else                       item.quantity * item.unitPrice * item.vat.rate
```
]

.thirty-seven-left[
```scala
totalPrice(unknown)
// res: Double = `53.8`
```
]

.three-left[    ]

.thirty-seven-left[
```scala
totalPrice(null)
// res: Double = `0.0`
```
]

---
class: medium-code

# Handling null

.seventy-seven-left[
```scala
case class Item(name: String, quantity: Int, unitPrice: Double, vat: VAT)

sealed abstract class VAT(val rate: Double)
object VAT {
  case object Reduced  extends VAT(1.05)
  case object Standard extends VAT(1.20)
}

val book   : Item = Item("Little Red Train", 2,  8.99, VAT.Reduced)
val unknown: Item = Item(null              , 4, 13.45, null   )

def totalPrice(item: Item): Double =
  if(item == null)           0.0
  else if (item.vat == null) item.quantity * item.unitPrice
  else                       item.quantity * item.unitPrice * item.vat.rate
```
]

.thirty-seven-left[
```scala
totalPrice(unknown)
// res: Double = 53.8
```
]

.three-left[    ]

.thirty-seven-left[
```scala
totalPrice(null)
// res: Double = 0.0
```
]

---
# Ignore null

<br>

## 1. Don't write functions that returns null

## 2. Don't assign null to variables

## 3. Don't check for null

---
# Forward reference

.forty-two-left[
```scala
val defaultRate = defaultVAT.rate
// java.lang.NullPointerException

val defaultVAT  = VAT.Standard
```
]

---
# Forward reference

.forty-two-left[
```scala
val defaultRate = `defaultVAT`.rate
// java.lang.NullPointerException

val `defaultVAT`  = VAT.Standard
```
]

---
# Forward reference

.forty-two-left[
```scala
val defaultRate = defaultVAT.rate
// java.lang.NullPointerException

val defaultVAT  = VAT.Standard
```
]

<br><br><br><br><br><br><br>

.twelve-left[.center[
## Compiler

<br>
## IDE
]]

.seventy-seven-right[

```scala
// warning: Reference to uninitialized value defaultVAT
// val defaultRate = defaultVAT.rate
//                   ^^^^^^^^^^

```
<img src="../img/screenshot/forward-reference.png" style="width:800px;"/>
]

---
# Option is a better null

.fifty-two-left[
```scala
case class User(
  name : String,
  age  : Int,
  email: Option[String],
)
```
]

---
# Option is a better null

.fifty-two-left[
```scala
case class User(
  name : String,
  age  : Int,
  email: `Option`[String],
)
```
]

---
# Option is a better null

.fifty-two-left[
```scala
case class User(
  name : String,
  age  : Int,
  email: Option[String],
)
```
]

.forty-two-right[
```sql
CREATE TABLE users (
  name    TEXT `NOT NULL`,
  age     INT  `NOT NULL`,
  email   TEXT
)
```
]

---
# Option is a better null

.fifty-two-left[
```scala
case class User(
  name : String,
  age  : Int,
  email: Option[String],
)
```
]

.fifty-two-left[
```scala
def vatRate(optVat: Option[VAT]): Double =
  optVat match {
    case None      => 1.0
    case Some(vat) => vat.rate
  }
```
]

---
# Option is a better null

.fifty-two-left[
```scala
case class User(
  name : String,
  age  : Int,
  email: Option[String],
)
```
]

.forty-two-right[
```scala
List(1,2,3,4).find(_ > 2)
// res5: Option[Int] = Some(value = 3)

List(1,2,3,4).find(_ > 10)
// res6: Option[Int] = None
```
]

.fifty-two-left[
```scala
def vatRate(optVat: Option[VAT]): Double =
  optVat match {
    case None      => 1.0
    case Some(vat) => vat.rate
  }
```
]

---
# Exception 1: Interface with Java code

.fifty-seven-left[
```scala
import org.apache.poi.ss.usermodel.{Cell, Row}

val row: Row = ...

row.getCell(0)
// res: Cell = Little Red Train

row.getCell(1)
// res: Cell = 2

row.getCell(3)
// res: Cell = null
```
]

.thirty-seven-right[
<br>
<img src="../img/screenshot/excel-poi.png" style="width:400px;"/>
]

---
# Exception 1: Interface with Java code

.fifty-seven-left[
```scala
import org.apache.poi.ss.usermodel.{Cell, Row}

val row: Row = ...

row.`getCell`(0)
// res: Cell = Little Red Train

row.getCell(1)
// res: Cell = 2

row.getCell(3)
// res: Cell = null
```
]

.thirty-seven-right[
<br>
<img src="../img/screenshot/excel-poi.png" style="width:400px;"/>
]

---
# Exception 1: Interface with Java code

.fifty-seven-left[
```scala
import org.apache.poi.ss.usermodel.{Cell, Row}

val row: Row = ...

row.getCell(`0`)
// res: Cell = `Little Red Train`

row.getCell(1)
// res: Cell = 2

row.getCell(3)
// res: Cell = null
```
]

.thirty-seven-right[
<br>
<img src="../img/screenshot/excel-poi.png" style="width:400px;"/>
]

---
# Exception 1: Interface with Java code

.fifty-seven-left[
```scala
import org.apache.poi.ss.usermodel.{Cell, Row}

val row: Row = ...

row.getCell(0)
// res: Cell = Little Red Train

row.getCell(`1`)
// res: Cell = `2`

row.getCell(3)
// res: Cell = null
```
]

.thirty-seven-right[
<br>
<img src="../img/screenshot/excel-poi.png" style="width:400px;"/>
]

---
# Exception 1: Interface with Java code

.fifty-seven-left[
```scala
import org.apache.poi.ss.usermodel.{Cell, Row}

val row: Row = ...

row.getCell(0)
// res: Cell = Little Red Train

row.getCell(1)
// res: Cell = 2

row.getCell(`3`)
// res: Cell = `null`
```
]

.thirty-seven-right[
<br>
<img src="../img/screenshot/excel-poi.png" style="width:400px;"/>
]

---
# Option constructor

.fifty-seven-left[
```scala
import org.apache.poi.ss.usermodel.{Cell, Row}

val row: Row = ...

Option(row.getCell(0))
// res: Option[Cell] = Some(Little Red Train)

Option(row.getCell(1))
// res: Option[Cell] = Some(2)

Option(row.getCell(3))
// res: Option[Cell] = None
```

```scala
object Option {
  def apply[A](value: A): Option[A] =
    if (value == null) None
    else Some(value)
}
```
]

.thirty-seven-right[
<br>
<img src="../img/screenshot/excel-poi.png" style="width:400px;"/>
]

---
# Option constructor

.fifty-seven-left[
```scala
import org.apache.poi.ss.usermodel.{Cell, Row}

val row: Row = ...

Option(row.getCell(0))
// res: Option[Cell] = `Some(Little Red Train)`

Option(row.getCell(1))
// res: Option[Cell] = Some(2)

Option(row.getCell(3))
// res: Option[Cell] = `None`
```

```scala
object Option {
  def apply[A](value: A): Option[A] =
    if (`value == null`) None
    else Some(value)
}
```
]

.thirty-seven-right[
<br>
<img src="../img/screenshot/excel-poi.png" style="width:400px;"/>
]

---
class: medium-code

# Scala wrapper

.sixty-two-left[
```scala
import org.apache.poi.ss.usermodel.{Cell, Row}

case class SafeRow(row: Row) {
  def getDouble(column: Int): Option[Double] =
    if (row == null) None
    else {
      val cell = row.getCell(column)
      if (cell == null || cell.getCellType() != Cell.NUMERIC)
        None
      else
        Some(cell.getNumericCellValue())
    }
}
```
]

.thirty-four-right[
```scala
val row: SafeRow = ...

row.getDouble(0)
// res: Option[Double] = None

row.getDouble(1)
// res: Option[Double] = Some(2)

row.getDouble(3)
// res: Option[Double] = None
```
]

---
class: medium-code

# Scala wrapper

.sixty-two-left[
```scala
import org.apache.poi.ss.usermodel.{Cell, Row}

case class `SafeRow`(row: `Row`) {
  def getDouble(column: Int): Option[Double] =
    if (row == null) None
    else {
      val cell = row.getCell(column)
      if (cell == null || cell.getCellType() != Cell.NUMERIC)
        None
      else
        Some(cell.getNumericCellValue())
    }
}
```
]

.thirty-four-right[
```scala
val row: SafeRow = ...

row.getDouble(0)
// res: Option[Double] = None

row.getDouble(1)
// res: Option[Double] = Some(2)

row.getDouble(3)
// res: Option[Double] = None
```
]

---
class: medium-code

# Scala wrapper

.sixty-two-left[
```scala
import org.apache.poi.ss.usermodel.{Cell, Row}

case class SafeRow(row: Row) {
  def getDouble(column: Int): Option[Double] =
    if (`row == null`) None
    else {
      val cell = row.getCell(column)
      if (`cell == null` || `cell.getCellType() != Cell.NUMERIC`)
        None
      else
        Some(cell.getNumericCellValue())
    }
}
```
]

.thirty-four-right[
```scala
val row: SafeRow = ...

row.getDouble(0)
// res: Option[Double] = None

row.getDouble(1)
// res: Option[Double] = Some(2)

row.getDouble(3)
// res: Option[Double] = None
```
]

---
# Exception 2: Low level code

.sixty-two-left[
```scala
if (hasUnboxed) {
  popNextBind(bFirst, bRest) match {
    case null =>
      cb(Right(unboxed))
      return
    case bind =>
      val fa =
        try bind(unboxed)
        catch { case NonFatal(ex) => RaiseError(ex) }
      hasUnboxed = false
      unboxed = null
      bFirst = null
      currentIO = fa
  }
}
```
.medium[.center[from cats-effect library]]
]

---
class: white

background-image: url(../img/fp-tower/website-background.svg)

# .white[Summary]

<br>

## .white[Ignore null: don't use it, don't handle it]

## .white[Use Option instead]

## .white[ Exception 1: interfacing with Java code]

## .white[ Exception 2: high-performance]

---
# Explicit Nulls in Scala 3

.forty-seven-left[
```scala
scalacOptions += "-Yexplicit-nulls"
```

<br>
```scala
val item: Item = null
// error
//    Found:    Null
//    Required: Item
```
]

---
# Explicit Nulls in Scala 3

.forty-seven-left[
```scala
scalacOptions += `"-Yexplicit-nulls"`
```

<br>
```scala
val item: Item = null
// error
//    Found:    Null
//    Required: Item
```
]

---
# Explicit Nulls in Scala 3

.forty-seven-left[
```scala
scalacOptions += "-Yexplicit-nulls"
```

<br>
```scala
val item: Item = null
// error
//    Found:    Null
//    Required: Item
```
]
<br><br><br><br><br><br><br><br><br><br><br>

.seventy-seven-left[
```scala
val item: `Item | Null` = null

val book: `Item | Null` = Item("Little Red Train", 2,  8.99, Reduced)
```
]