Kotlin REPL and script mode

Introduction

Kotlin REPL and script mode let you quickly try out Kotlin code without making a full project. It helps you learn and test ideas fast.

You want to test a small piece of Kotlin code quickly.

You are learning Kotlin and want to experiment with commands one by one.

You want to write a simple Kotlin script to automate a task.

You want to check how Kotlin functions or expressions behave without creating a full app.

Syntax

Kotlin

REPL: Run `kotlinc-jvm` in terminal and type Kotlin code line by line.

Script mode: Save code in a file with `.kts` extension and run with `kotlinc-jvm -script filename.kts`.

REPL stands for Read-Eval-Print Loop. It reads your code, runs it, shows the result, and waits for more.

Script mode runs Kotlin code from a file without needing a full project setup.

Examples

Start REPL by typing kotlinc-jvm in your terminal, then type Kotlin commands like println.

Kotlin

kotlinc-jvm
println("Hello from REPL!")

Save this code in a file named hello.kts and run it with kotlinc-jvm -script hello.kts.

Kotlin

// hello.kts
println("Hello from script!")

Sample Program

This script sets a variable and prints a greeting. Run it with kotlinc-jvm -script greet.kts.

Kotlin

// Save as greet.kts
val name = "Friend"
println("Hello, $name!")

Important Notes

In REPL, you can define variables and functions and use them in following lines.

Script files use the .kts extension to tell Kotlin to run them as scripts.

REPL is great for quick tests, scripts are better for reusable small programs.

Summary

Kotlin REPL lets you run code interactively line by line.

Script mode runs Kotlin code saved in .kts files.

Both help you try Kotlin quickly without full project setup.

Practice

1. What is the main purpose of Kotlin REPL?

easy

A. To run Kotlin code interactively line by line

B. To compile Kotlin code into Java bytecode

C. To create Android applications

D. To manage Kotlin project dependencies

Solution

1. Step 1: Understand REPL meaning

   Kotlin REPL stands for Read-Eval-Print Loop, which runs code interactively.

2. Step 2: Identify REPL use

   It lets you type and run Kotlin code line by line without compiling a full program.

3. Final Answer:

   To run Kotlin code interactively line by line -> Option A

4. Quick Check:

   Kotlin REPL = Interactive code running [OK]

Hint: REPL runs code line by line interactively [OK]

Common Mistakes:

• Confusing REPL with full project compilation
• Thinking REPL is for app deployment
• Assuming REPL manages dependencies

2. Which file extension is used for Kotlin script files?

easy

A. .kts

B. .skr

C. .kotlin

D. .kt

Solution

1. Step 1: Recall Kotlin file types

   Kotlin source files usually use .kt extension for normal programs.

2. Step 2: Identify script file extension

   Kotlin scripts use .kts extension to run code without full compilation.

3. Final Answer:

   .kts -> Option A

4. Quick Check:

   Kotlin script files = .kts [OK]

Hint: Scripts use .kts extension, not .kt [OK]

Common Mistakes:

• Choosing .kt which is for normal Kotlin files
• Assuming .kotlin or .skr are valid extensions
• Mixing script files with compiled files

3. What is the output of this Kotlin REPL session?

val x = 5
val y = 10
x + y

medium

A. 510

B. Error: undefined variables

C. x + y

D. 15

Solution

1. Step 1: Evaluate variable assignments

   x is assigned 5, y is assigned 10 in REPL environment.

2. Step 2: Calculate expression result

   x + y equals 5 + 10 which is 15.

3. Final Answer:

   15 -> Option D

4. Quick Check:

   5 + 10 = 15 [OK]

Hint: Add values assigned to variables for output [OK]

Common Mistakes:

• Concatenating numbers as strings (510)
• Printing expression text instead of result
• Assuming variables are undefined in REPL

4. Identify the error in this Kotlin script file example.kts:

val name = "Alice"
println(Name)

medium

A. Script files cannot use println

B. Variable 'Name' is not defined (case sensitive)

C. Missing semicolon after println statement

D. String literals must use single quotes

Solution

1. Step 1: Check variable usage

   Variable declared as 'name' but used as 'Name' which differs in case.

2. Step 2: Understand Kotlin case sensitivity

   Kotlin variables are case sensitive, so 'Name' is undefined causing error.

3. Final Answer:

   Variable 'Name' is not defined (case sensitive) -> Option B

4. Quick Check:

   Variable names are case sensitive [OK]

Hint: Check variable name case carefully [OK]

Common Mistakes:

• Thinking semicolons are mandatory
• Believing println is disallowed in scripts
• Using single quotes for strings in Kotlin

5. You want to quickly test a Kotlin function without creating a full project. Which approach is best?

hard

A. Write the function in Java and call it from Kotlin

B. Create a full Kotlin project and add the function

C. Use Kotlin REPL to define and test the function interactively

D. Write the function in a .kts script file and run it

Solution

1. Step 1: Compare quick testing methods

   Creating a full project is slow; writing Java adds complexity.

2. Step 2: Choose between script and REPL

   REPL lets you define and test functions interactively without saving files.

3. Step 3: Understand REPL advantage

   REPL is fastest for quick tests; scripts require saving and running files.

4. Final Answer:

   Use Kotlin REPL to define and test the function interactively -> Option C

5. Quick Check:

   REPL = fastest interactive testing [OK]

Hint: Use REPL for fastest interactive function testing [OK]

Common Mistakes:

• Assuming scripts are faster than REPL
• Thinking full projects are needed for tests
• Confusing Java code with Kotlin quick tests