Package scope rules in Go - Step-by-Step Execution

Concept Flow - Package scope rules

Start: Package declaration

↓

Declare variables, constants, functions

↓

Check if identifier starts with uppercase letter

↓

Exported: Visible outside package

↓

Package scope rules apply

↓

Use identifiers within package or from other packages

↓

End

This flow shows how Go decides if a variable, constant, or function is visible outside its package based on its name's first letter.

Execution Sample

Go

package main

var PublicVar = 10
var privateVar = 20

func main() {
    println(PublicVar, privateVar)
}

This code declares two variables with different visibility and prints both inside the same package.

Execution Table

Step	Identifier	Name	Starts with uppercase?	Scope	Accessible in main()?	Output
1	Declaration	PublicVar	Yes	Exported (package-wide and outside)	Yes
2	Declaration	privateVar	No	Unexported (package-only)	Yes
3	main() execution	PublicVar	Yes	Exported	Yes	10
4	main() execution	privateVar	No	Unexported	Yes	20
5	Outside package access	PublicVar	Yes	Exported	Yes
6	Outside package access	privateVar	No	Unexported	No	Error: undefined

💡 Execution stops after demonstrating visibility rules inside and outside the package.

Variable Tracker

Variable	Start	After Declaration	Final
PublicVar	undefined	10	10
privateVar	undefined	20	20

Key Moments - 3 Insights

Why can main() access both PublicVar and privateVar even though privateVar starts with a lowercase letter?

Because both variables are declared in the same package, package scope allows access to unexported (lowercase) identifiers within the package, as shown in execution_table rows 3 and 4.

Why can't code outside the package access privateVar?

Because privateVar starts with a lowercase letter, it is unexported and only visible inside its own package. Execution_table row 6 shows that outside access causes an error.

What determines if an identifier is exported or unexported in Go?

The first letter of the identifier: uppercase means exported (visible outside the package), lowercase means unexported (package-only), as shown in execution_table steps 1 and 2.

Visual Quiz - 3 Questions

Look at the execution_table, at step 4, what is the value of privateVar inside main()?

A 20

B undefined

C Error

D 10

💡 Hint

Check the 'Output' column at step 4 in the execution_table.

At which step does the code show that an identifier is not accessible outside the package?

A Step 3

B Step 6

C Step 1

D Step 4

💡 Hint

Look for 'Error: undefined' in the 'Output' column in the execution_table.

If we rename privateVar to PrivateVar, how would step 6 change?

A It would still cause an error

B It would be accessible outside the package

C It would be accessible only inside main()

D It would be undefined inside main()

💡 Hint

Refer to the 'Starts with uppercase?' and 'Scope' columns in the execution_table steps 1 and 6.

Concept Snapshot

Go package scope rules:
- Identifiers starting with uppercase are exported (visible outside package).
- Identifiers starting with lowercase are unexported (package-only).
- Inside the same package, all identifiers are accessible.
- Outside the package, only exported identifiers are accessible.
- This controls visibility and encapsulation in Go.

Full Transcript

In Go, the visibility of variables, constants, and functions depends on their names. If the name starts with an uppercase letter, it is exported and can be accessed from other packages. If it starts with a lowercase letter, it is unexported and only accessible within the same package. For example, PublicVar is accessible everywhere, while privateVar is only accessible inside the package. The main function can access both because it is in the same package. Outside the package, privateVar is not visible and causes an error. This rule helps organize code and control what is shared 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