How to use the command 'clj' (with examples)

The clj command is a powerful tool for developers working with the Clojure programming language. It allows users to start a REPL (Read-Eval-Print Loop), execute functions, manage dependencies, and much more. The command is highly versatile and supports a wide range of operations through its integration with the deps.edn file, which defines project configurations, dependencies, and other settings. By using clj, developers can streamline their workflow, ensuring a quick initiation of projects, seamless function execution, and efficient management of libraries.

Use case 1: Start a REPL (interactive shell)

Code:

clj

Motivation:
Starting a REPL is often the first step in exploring a new Clojure project, testing code snippets, or debugging. The REPL provides a dynamic and interactive environment where Clojure code can be written and evaluated in real time. This immediacy and interactivity are crucial for iterative development and rapid prototyping, allowing developers to test ideas quickly and adapt as necessary.

Explanation:
The command clj without any additional arguments initializes the default REPL environment based on the configuration specified in the deps.edn file. This configuration might include specified dependencies or custom REPL options, allowing for a customizable and ready-to-use interactive shell.

Example Output:
Once invoked, the REPL console will start, typically showing a prompt such as Clojure 1.10.1 followed by user=>, indicating readiness to accept Clojure expressions.

Use case 2: Execute a function

Code:

clj -X namespace/function_name

Motivation:
Executing a function directly from the command line is beneficial for testing specific functions or scripts without needing to navigate a larger project structure. It helps in scenarios where particular functionalities need to be validated independently, thus speeding up the testing process.

Explanation:

• -X: This argument signals that the command will execute the specified function.
• namespace/function_name: This is a specification of the namespace and function to be executed. It tells clj where the defined function is located within the codebase.

Example Output:
The output will depend on the specific function executed. For a simple addition function, you might see the result of the addition printed to the terminal.

Use case 3: Run the main function of a specified namespace

Code:

clj -M -m namespace args

Motivation:
Running a main function easily through the command line simplifies the process of initiating applications or scripts. This is particularly useful for projects where certain scripts are designed to be executed directly, providing a straightforward path to start applications without needing additional setup.

Explanation:

• -M: Instructs clj to prepare for running a main function.
• -m: Designates the main function to be executed.
• namespace: Refers to the specific namespace containing the desired main function.
• args: Optional arguments that can be passed to the function being executed.

Example Output:
If the main function is a simple output message, the terminal would display that message, confirming successful execution of the script.

Use case 4: Prepare a project by resolving dependencies, downloading libraries, and making/caching classpaths

Code:

clj -P

Motivation:
Ensuring that all dependencies are properly resolved and libraries are downloaded is crucial before running any project. This step prevents runtime errors relating to missing libraries and promotes a smoother development process by establishing all needed resources in advance.

Explanation:

• -P: This option tells clj to only prepare the project dependencies without starting a REPL or executing a function. It focuses on resolving dependencies, refreshing libraries, and managing the classpath cache.

Example Output:
There may be no visible output other than logs indicating the progress of dependency resolution and caching. Completion without errors implies the project is fully prepared to run.

Use case 5: Start an nREPL server with the CIDER middleware

Code:

clj -Sdeps '{:deps {nrepl {:mvn/version "0.7.0"} cider/cider-nrepl {:mvn/version "0.25.2"}}}' -m nrepl.cmdline --middleware '["cider.nrepl/cider-middleware"]' --interactive

Motivation:
nREPL servers with CIDER middleware enhance the developer experience, especially within the context of integrated development environments like Emacs. They provide features like code completion, debugging, and enhanced interaction with running code, exponentially increasing productivity and ease of use.

Explanation:

• -Sdeps '{...}': Specifies additional dependencies, overriding or supplementing those in deps.edn. In this case, it includes nREPL and CIDER versions.
• -m nrepl.cmdline: Indicates the main entry point for the nREPL server.
• --middleware '["cider.nrepl/cider-middleware"]': Adds CIDER middleware, needed for enhanced functionalities like introspection and code navigation.
• --interactive: Keeps the REPL interactive for potential dynamic code executions.

Example Output:
A successful startup will have console logs indicating an nREPL server’s initiation, including a port number for remote connections.

Use case 6: Start a REPL for ClojureScript and open a web browser

Code:

clj -Sdeps '{:deps {org.clojure/clojurescript {:mvn/version "1.10.758"}}}' --main cljs.main --repl

Motivation:
Launching a ClojureScript REPL with browser support is crucial for developing web-based applications using ClojureScript. This setup allows developers to incrementally write and test code that directly interacts with a browser, aiding in reactive and responsive front-end development.

Explanation:

• -Sdeps '{...}': Adds ClojureScript as a dependency, specifying its version.
• --main cljs.main: Directs clj to the ClojureScript main entry point.
• --repl: Opens an interactive REPL session specifically tailored to ClojureScript.

Example Output:
Upon running, a browser session will start, interacting with the ClojureScript environment. Ideally, this setup displays logs that indicate successful connections and readiness for further code experimentation.

Conclusion:

The clj command provides a robust foundation for working with both Clojure and ClojureScript. Through its versatile use, developers can manage their development workflow effectively, ensuring quick environment setups, streamlined function execution, and efficient project preparations. By mastering these use cases, developers can significantly enhance their productivity and fully utilize the capabilities of Clojure tooling.