Making I/O in Scala elegant and typesafe

Galilei is a library for performing safe disk I/O with Scala. It uses dependent types to provide precise static representations of paths, files, directories and other filesystem objects, enforcing filesystem-aware constraints on filenames and metadata. Galilei has a particular focus on precise error handling and, when enabled, each filesystem operation which might fail must be handled for each possible failure type; but its innovation is in minimizing that to a set of failure types which depends not just on the operation, but also the filesystem and in-scope options.

• intuitive and instructive API for most common filesystem operations
• safe and precise error handling, without unnecessary handling
• read from and write to files on disk with a variety of different types

For example, moving a file from /home/work/file to /home/work/copy should work fine if there is no pre-existing file at /home/work/copy. We can move it with, file.moveTo(destination). But if /home/work/copy already exists, then we may or may not care about what happens if we try to overwrite it.

The behavior can be specified with a contextual value in scope. Either,

import filesystemOptions.overwritePreexisting

or,

import filesystemOptions.doNotOverwritePreexisting

The moveTo operation does not assume one option or the other as a default, and Galilei's philosophy is that it would be wrong to do so. Instead, invoking moveTo without exactly one of the two contextal values in scope is a compile error, and the user is forced to decide on the correct behavior. This is both unpresumptuous and instructive, since the user may not have even considered the decision had to be made.

As a contextual value, the choice of behavior can be limited to a narrow scope, or imported globally, as needed.

If Scala 3's "safer exceptions" are turned on, then the choice of behavior also affects which exceptions must be handled when calling moveTo. The method invocation may throw an IoError under any circumstances, so that must always be handled, but with doNotOverwritePreexisting in scope, if there is a pre-existing file at the destination, then an OverwriteError will be thrown, which must be handled.

But since it cannot be thrown with overwritePreexisting in scope, the obligation to handle it is also removed.

Unlike many disk I/O libraries, Galilei provides different types for Paths, Files, Directorys and other types of node, like Symlinks. A Path represents the abstract notion of a location within a filesystem, which may or may not exist and may be a file, a directory or (on Linux, at least) one of several other filesystem node types. Types such as File and Directory should only exist to correspond to a real file or directory on disk.

Of course, the contents of a filesystem can change independently of the JVM, so the existence of an immutable File or Directory instance does not guarantee its eternal existence on disk, but we do, at least, guarantee that the filesystem node existed and had the correct type at the time of the object's creation.

Galilei is classified as fledgling. For reference, Soundness projects are categorized into one of the following five stability levels:

• embryonic: for experimental or demonstrative purposes only, without any guarantees of longevity
• fledgling: of proven utility, seeking contributions, but liable to significant redesigns
• maturescent: major design decisions broady settled, seeking probatory adoption and refinement
• dependable: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version 1.0.0 or later
• adamantine: proven, reliable and production-ready, with no further breaking changes ever anticipated

Projects at any stability level, even embryonic projects, can still be used, as long as caution is taken to avoid a mismatch between the project's stability level and the required stability and maintainability of your own project.

Galilei is designed to be small. Its entire source code currently consists of 965 lines of code.

Galilei will ultimately be built by Fury, when it is published. In the meantime, two possibilities are offered, however they are acknowledged to be fragile, inadequately tested, and unsuitable for anything more than experimentation. They are provided only for the necessity of providing some answer to the question, "how can I try Galilei?".

1. Copy the sources into your own project

   Read the fury file in the repository root to understand Galilei's build structure, dependencies and source location; the file format should be short and quite intuitive. Copy the sources into a source directory in your own project, then repeat (recursively) for each of the dependencies.

   The sources are compiled against the latest nightly release of Scala 3. There should be no problem to compile the project together with all of its dependencies in a single compilation.

2. Build with Wrath

   Wrath is a bootstrapping script for building Galilei and other projects in the absence of a fully-featured build tool. It is designed to read the fury file in the project directory, and produce a collection of JAR files which can be added to a classpath, by compiling the project and all of its dependencies, including the Scala compiler itself.

   Download the latest version of wrath, make it executable, and add it to your path, for example by copying it to /usr/local/bin/.

   Clone this repository inside an empty directory, so that the build can safely make clones of repositories it depends on as peers of galilei. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Galilei's dependencies.

   If the build was successful, the compiled JAR files can be found in the .wrath/dist directory.

Contributors to Galilei are welcome and encouraged. New contributors may like to look for issues marked beginner.

We suggest that all contributors read the Contributing Guide to make the process of contributing to Galilei easier.

Please do not contact project maintainers privately with questions unless there is a good reason to keep them private. While it can be tempting to repsond to such questions, private answers cannot be shared with a wider audience, and it can result in duplication of effort.

Galilei was designed and developed by Jon Pretty, and commercial support and training on all aspects of Scala 3 is available from Propensive OÜ.

Galilei's primary focus is handling Input and Output, or I/O, and is a pun on the name of the moon Io, one of the four moons of Jupiter discovered by Galileo Galilei.

/ˌɡælɪˈleɪ/

In general, Soundness project names are always chosen with some rationale, however it is usually frivolous. Each name is chosen for more for its uniqueness and intrigue than its concision or catchiness, and there is no bias towards names with positive or "nice" meanings—since many of the libraries perform some quite unpleasant tasks.

Names should be English words, though many are obscure or archaic, and it should be noted how willingly English adopts foreign words. Names are generally of Greek or Latin origin, and have often arrived in English via a romance language.

The logo is the Galilean planet, Io.

Galilei is copyright © 2024 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.