Function as inputs

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

class: center, middle, white

.title[Function as inputs]

---
# Function as inputs

```scala
def filter(
  text     : String,
  predicate: Char => Boolean
): String = ...
```

---
# Function as inputs

.thirty-seven-left[
 
 
```scala
filter(
 "Hello World!",
 (c: Char) => c.isUpper
)
// res0: String = "HW"
```
]

.fifty-seven-right[
<img src="../img/foundation/filter-isUpper.svg" alt="isUpper" style="width:600px;"/>
]

---
# Function as inputs

.thirty-seven-left[
 
 
```scala
filter(
 "Hello World!",
 (c: Char) => c.isLetter
)
// res1: String = "HelloWorld"
```
]

.fifty-seven-right[
<img src="../img/foundation/filter-isLetter.svg" alt="isUpper" style="width:600px;"/>
]

---
.h1[# Reduce code duplication]

.forty-seven-left[
```scala
def upperCase(text: String): String = {
 val characters = text.toArray
 for (i <- 0 until text.length) {
 characters(i) = characters(i).toUpper
 }
 new String(characters)
}
```

```scala
upperCase("Hello")
// res2: String = "HELLO"
```
]

.forty-seven-right[
```scala
def lowerCase(text: String): String = {
 val characters = text.toArray
 for (i <- 0 until text.length) {
 characters(i) = characters(i).toLower
 }
 new String(characters)
}
```

```scala
lowerCase("Hello")
// res3: String = "hello"
```
]

---
# Capture pattern

.fifty-seven-left[

```scala
def map(text: String, update: Char => Char): String = {
 val characters = text.toArray
 for (i <- 0 until text.length) {
 characters(i) = update(characters(i))
 }
 new String(characters)
}
```

```scala
def upperCase(text: String): String =
  map(text, c => c.toUpper)

def lowerCase(text: String): String =
  map(text, c => c.toLower)
```
]

.thirty-seven-right[
<img src="../img/foundation/map-string.svg" alt="isUpper" style="width:400px;"/>
]

---
# Capture pattern

.fifty-seven-left[
```scala
def map(text: String, update: Char => Char): String = {
 val characters = text.toArray
 for (i <- 0 until text.length) {
 characters(i) = update(characters(i))
 }
 new String(characters)
}
```

```scala
def upperCase(text: String): String =
  map(text, c => c.toUpper)

def lowerCase(text: String): String =
  map(text, c => c.toLower)
```
]

.thirty-seven-right[
<img src="../img/foundation/higher-order-function-1.svg" alt="higher order function hierarchy" style="width:400px;"/>
]

---
# Property based testing

.fifty-seven-left[

```scala
test("map does not modify the size of a text") {
  forAll((
    text  : String,
    update: Char => Char
  ) =>
    val outputText = map(text, update)
    outputText.length == text.length
  )
}
```
]

.thirty-seven-right[
<img src="../img/foundation/higher-order-function-1.svg" alt="higher order function hierarchy" style="width:400px;"/>
]

---
background-image: url(../img/foundation/higher-order-function-2.svg)

# Hierarchy of functions

---
background-image: url(../img/foundation/file-processing-1.svg)

# File processing

---
background-image: url(../img/foundation/file-processing-2.svg)

# File processing

---
background-image: url(../img/foundation/file-processing-fail.svg)

# File processing

---
background-image: url(../img/foundation/file-processing-fail-safe.svg)

# File processing

---
# Write tricky code once

```scala
import scala.io.Source

def usingFile(fileName: String, processing: Iterator[String] => Int): Int = {
  val source = Source.fromResource(fileName)
  try {
    processing(source.getLines())
  } finally {
    source.close()
  }
}
```

.forty-seven-left[
```scala
val countLines: Iterator[String] => Int =
  lines => lines.size
```
]

.forty-seven-right[
```scala
val countWords: Iterator[String] => Int =
  lines => ...
```
]

---
# Write tricky code once

```scala
import scala.io.Source

.forty-seven-left[
```scala
usingFile("50-word-count.txt", countLines)
// res7: Int = 2
```
]

.forty-seven-right[
```scala
usingFile("50-word-count.txt", countWords)
// res8: Int = 50
```
]

---
# Summary

.large[
* Higher order function

* Reduce code duplication

* Improve code quality
]