propensive/digression

Utilities for working with exceptions in Scala

Digression

Make Scala stack traces more understandable

Stack traces correspond to the classes, interfaces and methods as they exist on the running JVM. The Scala definitions from which they originate may have undergone various name changes during compilation, and features of Scala which do not exist in Java may have been encoded in method and class names, such that the original Scala names and features are harder to discern. Digression believes that a Scala developer (particularly a beginner, who will still be exposed to stack traces) should not have to understand how Scala is encoded in order to work with the language.

Features

• decodes Scala encodings in stack traces
• disambiguates between objects and types in stack frames
• syntax highlights stack traces
• tabulates stack traces to make them easier to read
• introduces a Codepoint contextual value containing the sourcefile and line

Availability Plan

Digression has not yet been published. The medium-term plan is to build Digression with Fury and to publish it as a source build on Vent. This will enable ordinary users to write and build software which depends on Digression.

Subsequently, Digression will also be made available as a binary in the Maven Central repository. This will enable users of other build tools to use it.

For the overeager, curious and impatient, see building.

Getting Started

safely and unsafely

When language.experimental.saferExceptions is enabled, any expression which may throw an exception must be handled with a try/catch, or the exception declared in its signature. Sometimes, we know enough about the execution state to circumvent this.

If we know that the exception will not be thrown (or we are confident that it doesn't matter!), then we can wrap the expression in unsafely, for example, invoking a send() method which could throw a SendError,

def send(): Response throws SendError = ...

can be performed without any error handling with:

import digression.*
import language.experimental.saferExceptions

val response: Response = unsafely(send())

The unsafely method works by providing a contextual CanThrow[Exception] instance in its parameter.

The safely method can be used in the same way, but will catch thrown exceptions and convert them to the Unset value, effectively changing the return type from ReturnType to Maybe[ReturnType].

This is useful in cases where an exception may be expected, but enough is known about it to handle it without examining it. An additional use-case is fire-and-forgat method calls where we care neither about the normal nor an exceptional result.

Codepoint

A method may request a Codepoint value in using clause, like so:

def callMe()(using codepoint: Codepoint): Unit

The codepoint parameter will contain, at runtime, two values which will be computed during compilation at the callsite: the source file of the callsite, and the line number of that file.

So, compiling a file called src/tests.scala which contains calls to callMe() on lines 138 and 211 will provide the instances, Codepoint("src/tests.scala", 138) and Codepoint("src/tests.scala", 211) as using parameters to these invocations.

Subsequent recompilations may, of course, potentially change the line numbers if the invocations move within the file.

The Error type and err"" messages

Error messages for exceptions are usually constructed as a single string. For most purposes, this is sufficient, but by eagerly converting the exception's parameters to strings, it makes a compromise on the flexibility of the message at a later time when it may be presented to a user.

A typical exception will include one or more parameters, which capture, somehow, the conditions in which the exception was thrown. An error message will present those parameters in context, in a human-readable form, for (usually) a programmer to decipher. However, converting those parameters to strings at the point of costruction makes it impossible to inspect them later, and capturing the parameters in the exception independently of the message would duplicate them unnecessarily. One particular compromise made is the inability to distinguish between the parts of the message that are static for all instances of the exception, and those which vary depending on its state.

The essence of an uncompromising, structured error message would be a Tuple of n parameters, with n + 1 fragments of text interleaving it. For example, an error message about a missing file could be structured,

case class MissingFile(dir: Directory, child: Text) extends Exception

with a message that reads:

t"The directory $dir did not contain the file $child."

By representing this as the tuple, (dir: Directory, child: Text) and the text fragments, List(t"The directory ", t" did not contain the file ", t"."), we would store precisely the state we want, and is provided in Rudiments' ErrorMessage type:

import rudiments.*
val dir: Directory = ...
val child: Text = ...
val message = ErrorMessage[(Directory, Text)](List(t"The directory ",
    t" did not contain the file ", t"."), (dir, child))

Unfortunately, this is a very inconvenient way to write a message.

The err"" interpolator constructs an instance of ErrorMessage without such convoluted code:

val message = err"The directory $directory did not contain the file $child."

Combining this with Digression's Error class, distinct from java.lang.Error, provides a convenient way of defining a new exception type:

import digression.*
case class MissingFile(dir: Directory, child: Text)
extends Error(err"The directory $directory did not contain the file $child.")

Rewritten stack traces

Digression provides an immutable representation of an exception's stack trace, called StackTrace, constructed from a Throwable, but with stack frames rewritten to make it easier to relate Java's raw method names with the Scala code which created them.

This includes the following rewrites:

• symbolic names are decoded
• class vs object calls are distinguished with # or .
• primitives are written with full names
• package file objects and extenion methods are indicated as such
• lambdas and anonymous classes are identified
• Function types are written inline

Escapade provides a contextual AnsiShow instance for StackTrace, and by extension, Exception.

Status

Digression is classified as fledgling. For reference, Scala One 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.

Digression is designed to be small. Its entire source code currently consists of 268 lines of code.

Building

Digression 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 Digression?".

1. Copy the sources into your own project

   Read the fury file in the repository root to understand Digression'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 Digression 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 digression. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Digression's dependencies.

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

Contributing

Contributors to Digression 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 Digression 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.

Author

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

Name

A digression is a deviation from the main subject, much like an exception departs from the main path.

In general, Scala One 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.

Logo

The logo is an explosion, indicative of the most disastrous of exceptions.

License

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