Starting the REPL: A Comprehensive Guide for Java Developers Transitioning to Clojure

2.4.2 Starting the REPL

The Read-Eval-Print Loop (REPL) is a powerful tool in Clojure that allows developers to interactively evaluate code, test functions, and explore libraries in real-time. For Java developers, this might be a new concept, as Java traditionally relies on compiling and running entire programs. In this section, we will explore how to start the REPL in different environments, including the command line and popular IDEs, and discuss the differences between a standalone REPL and one connected to a project.

Understanding the REPL

Before diving into the specifics of starting the REPL, let’s briefly discuss what the REPL is and why it’s beneficial. The REPL is an interactive programming environment that reads user inputs (expressions), evaluates them, prints the result, and loops back to read the next input. This cycle allows for rapid prototyping, debugging, and learning.

In Java, you might be accustomed to writing code, compiling it, and then running it to see the results. The REPL eliminates the compile step, allowing you to test snippets of code immediately. This can significantly speed up the development process and enhance your understanding of Clojure’s syntax and semantics.

Starting the REPL from the Command Line

Using clj

The clj command is part of the Clojure CLI tools and is the simplest way to start a REPL. It provides a lightweight environment for evaluating Clojure code.

1. Open your terminal.
2. Type clj and press Enter.

1clj

This command starts a REPL session. You can now enter Clojure expressions, and the REPL will evaluate them and print the results.

1;; Example: Adding two numbers
2(+ 1 2)
3;; => 3

Using lein repl

Leiningen is a popular build automation tool for Clojure projects. It provides additional features like dependency management and project scaffolding.

1. Navigate to your project directory (if you want to start a project-connected REPL).
2. Type lein repl and press Enter.

1lein repl

This command starts a REPL session with your project’s dependencies and configurations loaded. This is particularly useful for testing code within the context of a specific project.

1;; Example: Using a library function
2(require '[clojure.string :as str])
3(str/upper-case "hello")
4;; => "HELLO"

Starting the REPL from an IDE or Editor

IntelliJ IDEA with Cursive

Cursive is a popular Clojure plugin for IntelliJ IDEA that provides excellent support for Clojure development, including REPL integration.

1. Open your Clojure project in IntelliJ IDEA.
2. Navigate to the “Run” menu and select “Run REPL”.
3. Choose the appropriate configuration (e.g., standalone or project).

This will start a REPL session within the IDE, allowing you to evaluate code directly from your editor.

Visual Studio Code with Calva

Calva is a Clojure extension for Visual Studio Code that provides REPL support.

1. Open your Clojure project in Visual Studio Code.
2. Press Ctrl+Shift+P to open the command palette.
3. Type “Calva: Start REPL” and select the command.

Calva will start a REPL session, and you can evaluate code by selecting it and pressing Ctrl+Enter.

Emacs with CIDER

CIDER is a powerful Clojure development environment for Emacs.

1. Open your Clojure project in Emacs.
2. Press M-x and type cider-jack-in.

This command starts a REPL session connected to your project, allowing you to evaluate code interactively.

Standalone vs. Project-Connected REPL

A standalone REPL is started without any project context. It’s useful for experimenting with Clojure code or testing small snippets. However, it doesn’t have access to project-specific dependencies or configurations.

A project-connected REPL is started within the context of a specific project. It loads the project’s dependencies and configurations, allowing you to test and debug code within the project’s environment. This is particularly useful for larger applications where you need to ensure that your code interacts correctly with other components.

Diagrams and Visuals

To better understand the flow of data and the interaction between different components in a REPL session, let’s look at a diagram that illustrates the REPL process.

    flowchart TD
	    A[Start REPL] --> B[Read Expression]
	    B --> C[Evaluate Expression]
	    C --> D[Print Result]
	    D --> B
	    C --> E[Access Project Dependencies]
	    E --> C

Diagram Caption: This flowchart illustrates the REPL process, showing how expressions are read, evaluated, and printed. It also highlights the interaction with project dependencies in a project-connected REPL.

Try It Yourself

To get hands-on experience with the REPL, try the following exercises:

1. Start a standalone REPL using clj and evaluate a few basic expressions, such as arithmetic operations and string manipulations.
2. Create a simple Clojure project using Leiningen and start a project-connected REPL with lein repl. Experiment with requiring libraries and using their functions.
3. Explore REPL features in your preferred IDE. Try evaluating code directly from the editor and observe how the REPL responds.

Key Takeaways

• The REPL is a powerful tool for interactive programming in Clojure, allowing for rapid prototyping and testing.
• You can start the REPL from the command line using clj or lein repl, or from within an IDE like IntelliJ IDEA, Visual Studio Code, or Emacs.
• A standalone REPL is useful for quick experiments, while a project-connected REPL provides access to project-specific dependencies and configurations.

Further Reading

For more information on using the REPL and integrating it into your workflow, check out the following resources:

• Official Clojure Documentation
• ClojureDocs
• Cursive Plugin for IntelliJ IDEA
• Calva Extension for Visual Studio Code
• CIDER for Emacs

Exercises

1. Modify a Clojure expression in the REPL to explore different functions and their outputs.
2. Create a new Clojure project and add a dependency. Start a project-connected REPL and use the dependency in your code.
3. Experiment with REPL features in different IDEs to find the setup that works best for you.

Quiz Time!