Package scope rules in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is package scope in Go?

Package scope means that a variable, function, or type is accessible anywhere within the same package but not outside it.

beginner

How do you make a function or variable accessible outside its package in Go?

By starting its name with an uppercase letter, making it exported and accessible from other packages.

beginner

Given this code snippet, where is the variable accessible?

package main

var count int = 5

func main() {
    // ...
}

The variable 'count' is accessible anywhere inside the 'main' package but not from other packages.

intermediate

What happens if two files in the same package declare variables with the same name at package scope?

It causes a compile-time error because package scope variables must have unique names within the package.

intermediate

Explain the difference between package scope and block scope in Go.

Package scope means the identifier is visible throughout the entire package, across all files. Block scope means the identifier is only visible within the enclosing block where it is declared (e.g., inside a function or if statement).

In Go, how do you make a variable accessible outside its package?

A Start its name with an uppercase letter

B Declare it inside the main function

C Use the 'public' keyword

D Prefix it with 'export_'

In Go, identifiers starting with uppercase letters are exported and accessible outside their package.

Where can a package-scoped variable be accessed?

A Only inside the function where it is declared

B Anywhere in the program

C Anywhere inside the same package

D Only inside the file where it is declared

Package scope means the variable is accessible anywhere within the same package.

What happens if two package-scoped variables have the same name in the same package?

A No problem, both exist separately

B Compile-time error

C The second variable overwrites the first

D Runtime error

Go does not allow duplicate package-scoped variable names in the same package; it causes a compile-time error.

Which of these is NOT true about package scope in Go?

A Unexported identifiers are only accessible inside the package

B Package scope variables are accessible across all files in the package

C Exported identifiers are accessible outside the package

D Package scope variables are accessible from other packages by default

Unexported package scope variables are NOT accessible from other packages.

If a variable name starts with a lowercase letter, what is its visibility in Go?

A Visible only inside its package

B Visible everywhere in the program

C Visible only inside its function

D Visible only inside its file

Lowercase names are unexported and visible only inside their package.

Describe how package scope works in Go and how it affects variable and function visibility.

Think about uppercase vs lowercase naming.

Explain why two package-scoped variables cannot have the same name in Go and what error occurs if they do.

Consider how Go enforces uniqueness in package scope.

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