Type inference in Go - Mini Project: Build & Apply

Type inference in Go

📖 Scenario: You are creating a simple Go program to store and display information about a book. You will use Go's type inference feature to declare variables without explicitly stating their types.

🎯 Goal: Build a Go program that uses type inference to declare variables for a book's title, author, and number of pages, then prints these details.

📋 What You'll Learn

Declare variables using type inference with the := operator

Use string values for title and author

Use an integer value for pages

Print the book details using fmt.Println

💡 Why This Matters

🌍 Real World

Type inference helps write cleaner and shorter Go code, especially when variable types are obvious from the assigned values.

💼 Career

Understanding type inference is important for Go developers to write idiomatic and efficient code in real-world applications.

1

Create variables with type inference

Use Go's type inference to declare a variable called title with the value "The Go Programming Language".

Go

// Use type inference to declare 'title' with value "The Go Programming Language"
// Your code here

Hint

Use the := operator to let Go infer the type automatically.

2

Add more variables with type inference

Add two more variables using type inference: author with value "Alan A. A. Donovan" and pages with value 380.

Go

package main

func main() {
    title := "The Go Programming Language"
    // Declare 'author' with value "Alan A. A. Donovan"
    // Declare 'pages' with value 380
    // Your code here
}

Hint

Use := for both variables to let Go infer their types.

3

Use variables in a print statement

Import the fmt package and use fmt.Println to print the variables title, author, and pages in one line.

Go

package main

import "fmt"

func main() {
    title := "The Go Programming Language"
    author := "Alan A. A. Donovan"
    pages := 380
    // Print title, author, and pages using fmt.Println
    // Your code here
}

Hint

Remember to import fmt at the top and use fmt.Println inside main.

4

Run the program to see the output

Run the program and print the output. The output should show the book title, author, and number of pages separated by spaces.

Go

package main

import "fmt"

func main() {
    title := "The Go Programming Language"
    author := "Alan A. A. Donovan"
    pages := 380
    fmt.Println(title, author, pages)
    // Your code here
}

Hint

Run the program using go run and check the output matches exactly.

Practice

1. What does type inference mean in Go when you write x := 10 inside a function?

easy

A. You must tell Go the type of x explicitly.

B. Go automatically figures out that x is an int.

C. Go treats x as a string by default.

D. The variable x has no type.

Solution

1. Step 1: Understand the := operator in Go

   The := operator declares a variable and assigns a value, letting Go guess the type.

2. Step 2: Check the assigned value type

   Since 10 is an integer, Go infers the type of x as int automatically.

3. Final Answer:

   Go automatically figures out that x is an int. -> Option B

4. Quick Check:

   Type inference means Go guesses type from value [OK]

Hint: := means declare and infer type from value [OK]

Common Mistakes:

• Thinking you must always write the type explicitly
• Assuming variables have no type
• Believing Go defaults to string type

2. Which of the following is the correct way to declare and assign a variable with type inference inside a function in Go?

easy

A. x := 5

B. x = 5

C. var x int = 5

D. int x = 5

Solution

1. Step 1: Recall Go syntax for type inference

   Inside functions, := declares and assigns a variable with inferred type.

2. Step 2: Check each option

   var x int = 5 declares with explicit type, not inference. x = 5 misses declaration. int x = 5 is invalid Go syntax.

3. Final Answer:

   x := 5 -> Option A

4. Quick Check:

   Use := to declare and infer type [OK]

Hint: Use := inside functions for type inference [OK]

Common Mistakes:

• Using C or D which are invalid Go syntax
• Forgetting to declare variable before assignment
• Confusing var declaration with :=

3. What is the output of this Go program?

package main
import "fmt"
func main() {
    x := 3.14
    fmt.Printf("%T", x)
}

medium

A. float64

B. float32

C. int

D. string

Solution

1. Step 1: Identify the type inferred for 3.14

   In Go, untyped floating-point constants default to float64 when assigned with :=.

2. Step 2: Understand fmt.Printf with %T

   The %T verb prints the type of the variable, so it will print "float64".

3. Final Answer:

   float64 -> Option A

4. Quick Check:

   Float literals default to float64 [OK]

Hint: Float literals default to float64 type [OK]

Common Mistakes:

• Assuming float32 is default
• Thinking 3.14 is int
• Confusing %T output with value

4. What is wrong with this Go code snippet?

func main() {
    var x := 10
    fmt.Println(x)
}

medium

A. The value 10 is not a valid integer.

B. x is not declared before use.

C. You cannot use := with var keyword.

D. fmt.Println is not imported.

Solution

1. Step 1: Understand variable declaration syntax

   In Go, := is shorthand for declaration and assignment without var keyword.

2. Step 2: Identify the syntax error

   Using var and := together is invalid syntax; you must use either var x = 10 or x := 10.

3. Final Answer:

   You cannot use := with var keyword. -> Option C

4. Quick Check:

   var and := cannot be combined [OK]

Hint: Don't combine var and := in one statement [OK]

Common Mistakes:

• Mixing var and := together
• Ignoring import errors (not shown here)
• Thinking := works outside functions

5. Given this Go code, what is the type of result after execution?

func main() {
    a := 5
    b := 2.0
    result := a * int(b)
    fmt.Printf("%T", result)
}

hard

A. float64

B. string

C. float32

D. int

Solution

1. Step 1: Analyze variable types and conversion

   a is int, b is float64 (default for 2.0). The expression converts b to int before multiplication.

2. Step 2: Determine result type

   Multiplying int (a) by int (converted b) results in int type for result.

3. Final Answer:

   int -> Option D

4. Quick Check:

   int * int = int type [OK]

Hint: Type conversion affects result type [OK]

Common Mistakes:

• Assuming result is float64 without conversion
• Forgetting to convert float to int
• Confusing multiplication type rules