Why functions are first-class in Swift - See It in Action

Why functions are first-class in Swift

📖 Scenario: Imagine you are organizing a small event and want to assign different tasks to helpers. You want to keep track of these tasks and be able to pass them around easily to different people.

🎯 Goal: You will create simple functions representing tasks, store them in a list, and then call them one by one. This will show how functions in Swift can be treated like any other value.

📋 What You'll Learn

Create functions that print simple task messages

Store these functions in an array

Call each function from the array

Print the output of each function call

💡 Why This Matters

🌍 Real World

In real apps, you often pass functions as callbacks or handlers to respond to user actions or events.

💼 Career

Understanding first-class functions is key for Swift developers to write clean, modular, and reusable code.

1

Create simple task functions

Create two functions called taskOne and taskTwo. Each function should print a message: "Task One is done" and "Task Two is done" respectively.

Swift

// Create two functions taskOne and taskTwo that print messages
// Your code here

Hint

Define each function with func keyword and use print inside the function body.

2

Store functions in an array

Create a variable called tasks and assign it an array containing the two functions taskOne and taskTwo.

Swift

func taskOne() {
    print("Task One is done")
}

func taskTwo() {
    print("Task Two is done")
}

// Create an array called tasks containing taskOne and taskTwo
// Your code here

Hint

Use square brackets [] to create an array and list the function names without parentheses.

3

Call each function from the array

Use a for loop with variable task to iterate over tasks and call each function inside the loop.

Swift

func taskOne() {
    print("Task One is done")
}

func taskTwo() {
    print("Task Two is done")
}

let tasks = [taskOne, taskTwo]

// Use a for loop to call each function in tasks
// Your code here

Hint

Inside the loop, call the function by adding parentheses () after the variable task.

4

Print the final output

Run the program to print the messages from both tasks. The output should show Task One is done and Task Two is done on separate lines.

Swift

func taskOne() {
    print("Task One is done")
}

func taskTwo() {
    print("Task Two is done")
}

let tasks = [taskOne, taskTwo]

for task in tasks {
    task()
}

// Run the program to see the output
// Your code here

Hint

Just run the program. The print statements inside the functions will show the messages.

Practice

1. Why are functions considered first-class citizens in Swift?

easy

A. Because functions can be assigned to variables and passed as arguments

B. Because functions cannot return values

C. Because functions must always be declared globally

D. Because functions cannot be nested inside other functions

Solution

1. Step 1: Understand what first-class means in Swift

   First-class means functions can be treated like any other value, such as assigned to variables or passed around.

2. Step 2: Identify the correct property of functions in Swift

   Swift allows functions to be stored in variables and passed as arguments, which matches the definition of first-class citizens.

3. Final Answer:

   Because functions can be assigned to variables and passed as arguments -> Option A

4. Quick Check:

   Functions as values = Because functions can be assigned to variables and passed as arguments [OK]

Hint: Remember: first-class means functions act like normal values [OK]

Common Mistakes:

• Thinking functions cannot be passed as arguments
• Believing functions must be global only
• Assuming functions cannot return values

2. Which of the following is the correct way to assign a function to a variable in Swift?

easy

A. let greet = func sayHello() { print("Hello") }

B. let greet = sayHello

C. func greet = sayHello()

D. var greet: sayHello = func()

Solution

1. Step 1: Recall Swift syntax for assigning functions

   In Swift, you assign a function by its name without parentheses to a variable or constant.

2. Step 2: Check each option for correct syntax

   let greet = sayHello correctly assigns the function name without calling it. Other options have syntax errors or misuse keywords.

3. Final Answer:

   let greet = sayHello -> Option B

4. Quick Check:

   Assign function by name without () = let greet = sayHello [OK]

Hint: Assign function by name only, no parentheses [OK]

Common Mistakes:

• Adding parentheses when assigning function
• Using 'func' keyword in assignment
• Incorrect variable declaration syntax

3. What will be the output of this Swift code?

func multiplyByTwo(_ number: Int) -> Int {
    return number * 2
}

let operation = multiplyByTwo
print(operation(5))

medium

A. 5

B. multiplyByTwo

C. 10

D. Error: Cannot assign function to variable

Solution

1. Step 1: Understand function assignment and call

   The function multiplyByTwo is assigned to 'operation' without calling it, so 'operation' is a function reference.

2. Step 2: Call the function through the variable

   Calling operation(5) runs multiplyByTwo(5), which returns 5 * 2 = 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   Function call via variable returns 10 [OK]

Hint: Calling assigned function runs original logic [OK]

Common Mistakes:

• Expecting variable to hold result, not function
• Confusing function name with string output
• Thinking assignment causes error

4. Identify the error in this Swift code snippet:

func add(_ a: Int, _ b: Int) -> Int {
    return a + b
}

let sum = add(3, 4)
let operation = add(3, 4)
print(operation(5, 6))

medium

A. Function add is not defined properly

B. Missing return statement in add function

C. Cannot assign function call to a constant

D. Cannot call operation as a function because it holds an Int, not a function

Solution

1. Step 1: Analyze variable assignments

   sum is assigned the result of add(3, 4), which is an Int (7). operation is also assigned add(3, 4), so operation is an Int, not a function.

2. Step 2: Check the call to operation(5, 6)

   Since operation is an Int, calling operation(5, 6) causes an error because Int is not callable.

3. Final Answer:

   Cannot call operation as a function because it holds an Int, not a function -> Option D

4. Quick Check:

   Calling Int as function causes error = Cannot call operation as a function because it holds an Int, not a function [OK]

Hint: Assign function name, not function call, to variable [OK]

Common Mistakes:

• Assigning function call instead of function reference
• Expecting function call to be callable again
• Ignoring type mismatch errors

5. How can you write a Swift function that returns another function which adds a fixed number to its input? For example, calling makeAdder(3)(5) should return 8.

hard

A. func makeAdder(_ x: Int) -> (Int) -> Int {
return { y in x + y }
}

B. func makeAdder(_ x: Int) -> Int {
return x + y
}

C. func makeAdder(x: Int, y: Int) -> Int {
return x + y
}

D. func makeAdder(_ x: Int) -> (Int) {
return x
}

Solution

1. Step 1: Understand the requirement for returning a function

   The function makeAdder must return another function that takes an Int and adds it to x.

2. Step 2: Check which option returns a function type

   func makeAdder(_ x: Int) -> (Int) -> Int { return { y in x + y } } returns a closure (function) of type (Int) -> Int that adds x and y, matching the requirement.

3. Final Answer:

   func makeAdder(_ x: Int) -> (Int) -> Int { return { y in x + y } } -> Option A

4. Quick Check:

   Return function adding x to input = func makeAdder(_ x: Int) -> (Int) -> Int { return { y in x + y } } [OK]

Hint: Return a closure capturing x to add later [OK]

Common Mistakes:

• Returning Int instead of function
• Not using closure syntax for returned function
• Incorrect function signature