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A Journey into Scheme: Building a Simple Symlink Manager with Guile Scheme

• By Glenn Thompson

Introduction

I've spent my career as an electrical engineer, not a software developer. However, my recent journey in to GNU/Liniux required a tool for managing symlinks, and that's how I began learning Scheme—specifically Guile Scheme. I'm writing this post to share how I built stash, a utility that mimics GNU Stow's functionality, and how my learning journey was shaped by David Wilson's "Hands-On Guile Scheme for Beginners" course from System Crafters, more about this below.

How I Started with Scheme

My programming background was VERY limited, I produce documents in (La)Tex but I decided to take the plunge into learning Scheme, thanks to a course led by David Wilson from System Crafters. The course, "Hands-On Guile Scheme for Beginners", was incredibly helpful in making Scheme accessible even for someone like me, without a traditional programming background. I know (La)Tex isn't a programming language, it's typesetting. But how hard can it be? Right?

The course took me through the basics of Scheme, from simple expressions to more complex concepts like functions, recursion, and working with files. This structured learning environment gave me the confidence to start building stash.

The course was "instructor-led" with live meet-up sessions weekly. David has since made this course on-demand, and will be, if not already, available at the above link. Highly recommended if you are interested in taking your first steps with scheme.

Why Build Stash?

After completing David Wilson's course, I wanted to put my newly found Guile Scheme skills into practice with a real project. It wasn't enough just to understand the language conceptually—I needed to build something tangible that solved a problem I encountered regularly in my workflow. Writing stash gave me that opportunity. It allowed me to apply what I'd learned while also deepening my understanding of file manipulation, command-line tools, and conflict resolution—all within the Guile Scheme environment.

After migrating to GNU/Linux and speaking with other System Crafters Community members, I found I needed a way to manage symbolic links and organize directories. Existing tools like GNU Stow helped, but I wanted to learn how such tools are built. I decided to write my own version using Guile Scheme to enhance my understanding of the language and to have more control over the functionality.

The goal of stash is simple: allow users to move directories and create symlinks with conflict resolution, offering options to overwrite, back up, skip, or cancel the operation.

Breaking Down Stash

The core of stash revolves around:

  1. Moving Directories: Using Scheme's file manipulation functions, I learned how to move directories and files around.
  2. Creating Symlinks: I implemented functions to create symlinks to the moved directories, ensuring that the original structure remains accessible.
  3. Conflict Resolution: One of the key features I wanted was handling conflicts when a file or symlink already exists at the target location. This required prompting the user for input and responding accordingly (backup, overwrite, skip, or cancel).

Here's an excerpt of the core functionality that handles moving a source directory and creating a symlink:

;;; Helper function to move source to target
(define (move-source-to-target source-dir target-dir)
  "Move the entire source directory to the target directory."
  (let* ((source-dir (expand-home source-dir))
         (target-dir (expand-home target-dir))
         (source-name (basename source-dir))
         (target-source-dir (string-append target-dir "/" source-name)))
    (if (file-exists? target-source-dir)
        ;; Conflict handling here...
        ...)
    (rename-file source-dir target-source-dir)
    (display (format #f "Moved ~a to ~a\n" source-dir target-source-dir))))

What I Learned

This project taught me a lot about not just Scheme, but programming in general:

  • File and Directory Manipulation: Scheme's file handling functions were different from what I had experienced before, but they allowed for powerful manipulation of file systems.
  • Command-Line Utilities: Scheme isn't just a language for academic exercises; you can write real, useful command-line tools with it.
  • Problem Solving: From parsing command-line arguments to resolving conflicts with existing files, every part of the program required careful thought and consideration of edge cases.

Guile Scheme Support Resources

  1. Guile Scheme Documentation
    The official documentation for Guile Scheme, which includes tutorials, references, and the Guile Manual.
  2. Guile Reference Manual
    A comprehensive manual covering core language concepts, libraries, and functions available in Guile Scheme.
  3. Scheme Wiki
    A community-maintained wiki that covers various Scheme dialects, including Guile Scheme, with tutorials, guides, and general information on Scheme programming.
  4. Guile at Schemers.org
    A site dedicated to Scheme with resources, libraries, tools, and documentation for Scheme and its implementations, including Guile.
  5. System Crafters
    Led by David Wilson, System Crafters provides tutorials and blog posts on Guile Scheme and other GNU tools.
  6. Guile Users Mailing List
    Join the Guile mailing list to ask questions and engage with the Guile Scheme community.
  7. Guile Cookbook
    An unofficial GitHub repository with practical code snippets and tips for Guile Scheme, covering various common use cases and tasks.
  8. #guile and #scheme on Libera Chat IRC
    A helpful IRC channel where you can connect with other Guile users for real-time support and advice.
  9. #systemcrafters on Libera Chat IRC
    A SUPER helpful IRC channel not only for guile and scheme, there are a huge variety of different people here. Tell them glenneth sent you.

Next Steps

I am still refining stash, especially around its conflict resolution system and the way it handles symbolic links. But it's in a usable state, and I'm excited to continue iterating on it. You can check out the code on Codeberg.

If you're curious about Scheme and how it can be used practically, I highly recommend checking out David Wilson's course. It's been instrumental in helping me grasp the concepts I needed to build this tool. Here's the link, again :) "Hands-On Guile Scheme for Beginners"