Package scope rules in Go

Introduction

Package scope rules help decide where variables and functions can be used in Go programs. This keeps code organized and safe.

When you want to share a variable or function with all files in the same package.

When you want to hide details inside a package so other packages cannot access them.

When you want to organize code into reusable parts without exposing everything.

When you want to avoid name conflicts by limiting where names are visible.

Syntax

Go

package mypackage

var packageVar = 10 // visible inside the package

func packageFunc() {
    // function visible inside the package
}

func ExportedFunc() {
    // function visible outside the package because it starts with uppercase
}

Names starting with a lowercase letter are visible only inside the package.

Names starting with an uppercase letter are visible outside the package (exported).

Examples

Variable 'count' is visible to all functions in the package 'main'.

Go

package main

var count int = 5 // package scope variable

func increment() {
    count++ // accessible here
}

func main() {
    increment()
    println(count) // prints 6
}

Variable 'helperVar' is only visible inside 'utils' package because it starts with lowercase.

Go

package utils

var helperVar = 100 // package scope, not exported

func HelperFunc() int {
    return helperVar
}

Function 'ExportedFunc' starts with uppercase, so it can be used outside the 'utils' package.

Go

package utils

func ExportedFunc() string {
    return "Hello"
}

Sample Program

This program shows a variable with package scope used inside two functions in the same package.

Go

package main

import "fmt"

var packageVar = 42 // package scope variable

func printVar() {
    fmt.Println(packageVar) // accessible here
}

func main() {
    printVar() // prints 42
    // fmt.Println(packageVar) also works here because main is the same package
}

Important Notes

Package scope means the name is visible to all files in the same package.

Use uppercase first letter to export names outside the package.

Use lowercase first letter to keep names private to the package.

Summary

Package scope controls where variables and functions can be used inside a package.

Lowercase names are private to the package; uppercase names are public (exported).

This helps organize code and control access between packages.

Practice

1. In Go, what does it mean if a variable name starts with a lowercase letter inside a package?

easy

A. It is private to the package and cannot be accessed from other packages.

B. It is accessible from any package.

C. It is a constant value.

D. It is a global variable accessible everywhere.

Solution

1. Step 1: Understand Go naming conventions

   In Go, identifiers starting with a lowercase letter are unexported, meaning they are private to the package.

2. Step 2: Apply package scope rule

   Such variables cannot be accessed from outside their package, ensuring encapsulation.

3. Final Answer:

   It is private to the package and cannot be accessed from other packages. -> Option A

4. Quick Check:

   Lowercase start = private [OK]

Hint: Lowercase start means private inside package [OK]

Common Mistakes:

• Thinking lowercase variables are accessible outside package
• Confusing lowercase with constants
• Assuming lowercase means global

2. Which of the following is the correct way to declare an exported function in a Go package?

easy

A. func MyFunction() {}

B. func myFunction() {}

C. func _MyFunction() {}

D. func my_function() {}

Solution

1. Step 1: Recall Go export rules

   Functions starting with an uppercase letter are exported and accessible outside the package.

2. Step 2: Check each option

   func MyFunction() {} starts with uppercase 'M', so it is exported. Others start with lowercase or invalid characters.

3. Final Answer:

   func MyFunction() {} -> Option A

4. Quick Check:

   Uppercase start = exported function [OK]

Hint: Function name uppercase means exported [OK]

Common Mistakes:

• Using lowercase to export functions
• Using underscores which are allowed but don't affect export
• Confusing export with naming style

3. Given the following Go code in package 'mathops':

package mathops

var result = 10

func Result() int {
    return result
}

What will happen if another package tries to access mathops.result?

medium

A. It will access the value 10 successfully.

B. It will cause a compile-time error due to unexported variable.

C. It will cause a runtime panic.

D. It will access a zero value.

Solution

1. Step 1: Identify variable scope

   The variable 'result' starts with lowercase, so it is unexported and private to the package.

2. Step 2: Understand access from other packages

   Accessing 'mathops.result' from outside the package is not allowed and causes a compile-time error.

3. Final Answer:

   It will cause a compile-time error due to unexported variable. -> Option B

4. Quick Check:

   Lowercase variable = inaccessible outside package [OK]

Hint: Lowercase vars cause compile error if accessed outside [OK]

Common Mistakes:

• Assuming private variables can be accessed
• Thinking runtime panic occurs instead of compile error
• Confusing function export with variable export

4. Consider this Go code snippet inside a package:

package example

func calculate() int {
    return 42
}

func Calculate() int {
    return calculate()
}

What is the main reason for defining both calculate and Calculate functions?

medium

A. To confuse the compiler with similar names.

B. To create two independent exported functions.

C. To cause a compile-time error due to duplicate names.

D. To have a private helper function and a public wrapper function.

Solution

1. Step 1: Analyze function names and scope

   'calculate' starts lowercase, so it is private to the package; 'Calculate' starts uppercase, so it is exported.

2. Step 2: Understand design pattern

   This pattern allows internal logic in 'calculate' while exposing a public function 'Calculate' that calls it.

3. Final Answer:

   To have a private helper function and a public wrapper function. -> Option D

4. Quick Check:

   Lowercase private + uppercase public wrapper [OK]

Hint: Use lowercase for private helpers, uppercase for public API [OK]

Common Mistakes:

• Thinking both functions are exported
• Expecting compile error from similar names
• Assuming no reason for two functions

5. You want to create a Go package that exposes only a function GetData but keeps the data source variable private. Which code snippet correctly implements this using package scope rules?

hard

A. package data

var datasource = "secret"

func getData() string {
return datasource
}

B. package data

var DataSource = "secret"

func GetData() string {
return DataSource
}

C. package data

var datasource = "secret"

func GetData() string {
return datasource
}

D. package data

var DataSource = "secret"

func getData() string {
return DataSource
}

Solution

1. Step 1: Identify private data variable

   The data source variable should start lowercase to be private to the package, so 'datasource' is correct.

2. Step 2: Identify exported function

   The function 'GetData' starts uppercase, so it is exported and accessible outside the package.

3. Step 3: Verify function returns private variable

   The function returns the private variable, exposing data safely.

4. Final Answer:

   package data var datasource = "secret" func GetData() string { return datasource } -> Option C

5. Quick Check:

   Lowercase var + uppercase func = private data, public access [OK]

Hint: Private vars lowercase, public funcs uppercase [OK]

Common Mistakes:

• Making data variable exported by uppercase
• Making function unexported by lowercase
• Returning unexported function instead of exported