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

MIT Scheme is a popular and robust language interpreter and interactive shell that caters to the Scheme programming language, which is a dialect of Lisp. The Scheme interpreter allows developers and enthusiasts to write, evaluate, and execute Scheme programs in an interactive manner through a Read-Eval-Print Loop (REPL). This article illustrates various practical use cases of the MIT Scheme command to help users effectively utilize its capabilities.

Start a REPL

Code:

scheme

Motivation: Engaging directly with the Scheme language in an interactive shell allows users to quickly test code snippets, debug, and learn the language in an environment that provides immediate feedback. This is particularly useful for both newcomers to programming and experienced developers looking to experiment with Scheme syntax and logic without pre-writing entire scripts.

Explanation:

• scheme: This command starts the MIT Scheme interpreter and opens the REPL, an interactive environment where you can enter Scheme expressions and receive immediate evaluation.

Example Output: Upon executing the command, you will be welcomed by a prompt similar to this:

MIT/GNU Scheme running under GNU/Linux
Type `,quit' to exit.
1 ]=> 

This prompt signifies that you can now enter Scheme expressions or commands.

Run a scheme program (with no REPL output)

Code:

scheme --quiet < script.scm

Motivation: Running a Scheme program without output clutter from REPL is essential when you want to execute a script in a streamlined manner, especially when you’re focused on the program’s output alone, without the need for debugging information or interactive evaluation. It’s commonly used in production environments or when the focus is on capturing the clean output of Scheme code execution.

Explanation:

• scheme: Initiates the Scheme interpreter.
• --quiet: Prevents the display of the typical startup banner and REPL output. This option is perfect for a cleaner execution environment.
• < script.scm: Directs the input to come from the specified Scheme script file, script.scm.

Example Output: Assuming script.scm contains:

(display "Hello, Scheme!")

Executing the command would simply output:

Hello, Scheme!

Load a scheme program into the REPL

Code:

scheme --load script.scm

Motivation: Loading a Scheme program into the REPL grants an interactive session with all the defined variables and functions pre-loaded. This makes it easier to test and further develop or debug the script, enhancing the interactive coding experience by allowing for real-time modifications and evaluations.

Explanation:

• scheme: Launches the Scheme interpreter.
• --load script.scm: Loads the contents of script.scm into the REPL, making all its definitions and expressions readily available for interaction.

Example Output: If script.scm contains:

(define (greet) (display "Hello from Scheme!"))

The REPL would allow you to then call:

1 ]=> (greet)

Resulting in:

Hello from Scheme!

Load scheme expressions into the REPL

Code:

scheme --eval "(define foo 'x)"

Motivation: Using scheme expressions directly within the command line is excellent for initializing the REPL with certain variables or expressions you plan to work with. This can be particularly beneficial when setting up a specific environment or performing quick tests without delving into script files.

Explanation:

• scheme: Opens the Scheme interpreter.
• --eval "(define foo 'x)": Evaluates the given Scheme expression upon startup, defining a variable foo with the value x.

Example Output: After running the command, you can verify the definition as follows:

1 ]=> foo

Which would output:

x

Open the REPL in quiet mode

Code:

scheme --quiet

Motivation: Opening the REPL in quiet mode is favorable when you prefer a distraction-free working environment. It suppresses the verbose startup information about the Scheme interpreter, allowing users to focus entirely on the task at hand without unnecessary information crowding the workspace.

Explanation:

• scheme: Initializes the Scheme interpreter.
• --quiet: Starts the session without displaying the introductory banner of the Scheme interpreter.

Example Output: Instead of the usual verbose banner, you go directly to the prompt:

1 ]=> 

Conclusion:

Each of these examples illustrates a different facet of using the MIT Scheme interpreter to enhance productivity, simplify debugging, and foster a smooth interactive programming experience. Whether you’re executing programs in a clean non-interactive environment or experimenting within the REPL, understanding how to leverage these commands can significantly improve your workflow with Scheme.