How to Use the Command 'guile' (with examples)

Guile is an implementation of the Scheme programming language designed for flexibility and ease of integration with other programs. As an interpreter for Scheme, a dialect of Lisp, Guile empowers developers to work with simple and powerful language constructs, allowing for efficient program scripting and extension. With built-in support for coroutines, a web server, and bindings to popular libraries, Guile offers a rich environment suitable for rapid development and prototyping.

Start a REPL (interactive shell)

Code:

guile

Motivation:
Starting a REPL (Read-Eval-Print Loop) allows developers to interactively test snippets of Scheme code, debug, and experiment in a live environment. This can significantly speed up development times by allowing you to try out code immediately, test functionality, and make on-the-fly modifications without needing to fully integrate changes into your entire codebase.

Explanation:
When you run the command guile, it launches the Guile Scheme interpreter in interactive mode. The absence of additional arguments instructs Guile to drop into a REPL, where users can enter Scheme expressions one at a time. Each expression is evaluated, and the result is printed out immediately.

Example output:

GNU Guile 3.0.8
Copyright (C) 1995-2021 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details
type `,show w'.  This program is free software, and you
are welcome to redistribute it under certain conditions;
type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> (+ 2 3)
$1 = 5
scheme@(guile-user)> 

Execute the script in a given Scheme file

Code:

guile script.scm

Motivation:
Sometimes, you have a complete Scheme program saved in a file, and you want to execute the entire script without entering each expression manually in an interactive shell. Running a script file is essential for projects that involve larger code bases or for automating tasks using pre-defined logic in Scheme.

Explanation:
In this command, guile is invoked with a filename argument, script.scm. The Guile interpreter reads the file, processes its Scheme expressions, and executes them sequentially. This approach is useful when you have structured your code into a script that should be run from top to bottom.

Example output: Suppose script.scm contains the following:

(display "Hello, Scheme!")
(newline)

Running the command would produce:

Hello, Scheme!

Execute a Scheme expression

Code:

guile -c "(display (+ 4 5))"

Motivation:
You might find yourself in a situation where you only need to evaluate a single Scheme expression quickly without going into a REPL or writing a script file. Using the -c option allows for rapid execution of a one-liner, which can be particularly useful for scripting or automation tasks from the command line.

Explanation:
The -c flag tells Guile to evaluate the following string as a Scheme program. In this example, guile -c "(display (+ 4 5))" evaluates the expression (+ 4 5) and displays the result. The expression calculates the sum of 4 and 5, and display outputs the result to the console.

Example output:

9

Listen on a port or a Unix domain socket for remote REPL connections

Code:

guile --listen=4000

Motivation:
Sometimes, you need to manage or debug a Scheme application running on a server remotely. By setting Guile to listen on a specific port or socket, you are opening an endpoint for remote connections to the REPL. This capability is crucial for collaborative debugging, server-side execution modifications, and real-time monitoring or control of a running Scheme application.

Explanation:
The --listen option configures Guile to start a REPL server, listening for incoming connections on the specified port or Unix domain socket. In this case, 4000 is the TCP port being used. Once a connection is established, users can interact with the Scheme interpreter remotely as if they were using the local REPL.

Example output: While this doesn’t display directly in the console like other use cases, a successful initiation might result in:

;; Guile REPL server listening on port 4000

Conclusion:

The versatility of Guile as a Scheme interpreter comes from its ability to operate interactively, execute scripts, evaluate single expressions, and support remote connections. Each use case demonstrates distinct scenarios where developers and users can leverage Guile’s capabilities to streamline tasks, whether it be for rapid prototyping, batch processing, on-the-fly calculations, or remote application control. By understanding these examples, users can harness the power of Guile in their own projects effectively.