Larceny

Scala 3 compiler plugin for testing compiletime errors

Unlike runtime errors, compilation errors prevent successful compilation, which makes them harder to test, since we can't even compile the units tests we want to write and run to test them!

Larceny makes it possible to write those tests. Code which would normally fail compilation, for any reason (provided it parses as well-formed Scala) is permitted inside certain blocks of code, but instead of being compiled and run, instead returns a list of compilation errors, as runtime values, which are perfect for testing.

Features

• suppresses compilation errors on ordinary code blocks
• code must at least parse, but all errors will be lifted to runtime values
• allows compilation errors to be tested in unit testing frameworks
• unit tests on compilation errors can be written in the most natural way

Availability

Getting Started

Larceny is a compiler plugin, and can be included in a compilation with the -Xplugin:larceny.jar parameter to scalac:

scalac -d bin -Xplugin:larceny.jar -classpath larceny.jar *.scala

The compiler plugin identifies code blocks whose compilation errors should be suppressed, which are inside a demilitarize block (using any valid Scala syntax), for example:

package com.example

import larceny.*

@main def run(): Unit =
  demilitarize("Hello world".substring("5"))

  demilitarize:
    val x = 8
    println(x.missingMethod)

Here, the code inside each demilitarize block will never compile: the first, because substring takes an Int as a parameter, and the second because missingMethod is not a member of Int.

But despite this, if the Larceny plugin is enabled, then the code will compile.

Any invalid code that is not within a demilitarize block will still result in the expected compilation errors.

The compilation error from each demilitarize block will be returned (in a List) from each block. We could adjust the code to see them, like so:

@main def run(): Unit =
  val errors = demilitarize:
    "Hello world".substring("5")

  errors.foreach:
    case CompileError(ordinal, message, code, position, offset) =>
      println(s"[$id] Found error '$message' in the code '$code' with offset $offset")

The parameters of CompileError need some explanation:

• ordinal is the ordinal identifier representing the type of error; the Scala compiler defines about 200 such error types (though some occur more frequently than others)
• message is the human-readable error message text that would be output by the compiler
• code is the fragment of code which would be marked as problematic (in an IDE, this would usually be done with a wavy red underline)
• position is the location of the code from the start of the source file
• offset is the number of characters from the start of code that is marked as the exact point of the error

Taking the second example above,

demilitarize:
  val x = 8
  println(x.missingMethod)

the message would be:

value missingMethod is not a member of Int

while the code value would be x.missingMethod (note that the surrounding println is not considered erroneous), and the offset would be 2. The value 2 is because the erroneous code begins x., but the point of the error is considered to be the m of missingMethod, which is character 2.

The error IDs are defined in the Scala compiler and correspond to an enumeration of values. For convenience, these values have been copied into the CompileErrorId enumeration, and can be accessed by the id method of CompileError.

CompileErrorId is also an extractor on CompileError, so it's possible to write:

demilitarize(summon[Ordering[Exception]]) match
  case ErrorId(ErrorId.MissingImplicitArgumentID) => "expected"
  case _                                          => "unexpected"

Implementation

Here are the details of how Larceny works. It should not be necessary to understand its implementation for normal usage, but as experimental software, it may behave unexpectedly, and this explanation may help to diagnose misbehavior.

Larceny runs on each source file before typechecking, but after parsing. Any blocks named demilitarize found in the the untyped AST will trigger a new and independent compilation of the same source file (with the same classpath, but without the Larceny plugin) from within the main compilation.

Since the demilitarize blocks should contain compile errors, this child compilation is expected to fail, but its compilation errors will be captured. Each compilation error which is positioned within a demilitarize block will be converted to static code which constructs a new CompileError instance, and inserts it into the demilitarize block, in place of entire erroneous contents.

If there are multiple demilitarize blocks in the same source file, some errors which occur in earlier phases of compilation may suppress later phases from running, and the errors from those later phases will not be captured during the first compilation. Larceny will rerun the compiler as many times as necessary to capture errors from later phases, each time removing more code which would have precluded these later phases.

The main compilation is then allowed to continue to typechecking, which will only see the CompileError constructions, not the original code. As long as there are no compilation errors outside of a demilitarize block, compilation should succeed. When the code is run, each demilitarize block will simply return a list of CompileErrors.

Testing Frameworks

Larceny should work with any Scala unit testing framework or library. For example, with Probably, we could write a compile error test with:

test(t"cannot sort data without an Ordering"):
  demilitarize(data.sorted).head.message
.assert(_.startsWith("No implicit Ordering"))

Status

Larceny is classified as maturescent. 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.

Larceny is designed to be small. Its entire source code currently consists of 346 lines of code.

Building

Larceny 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 Larceny?".

1. Copy the sources into your own project

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

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

Contributing

Contributors to Larceny 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 Larceny 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

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

Name

Larceny is the act of unlawfully taking something from someone. Larceny unlawfully takes errors from compiletime and gives them to runtime.

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.

Logo

The logo shows a shape of a medieval fortification, alluding to a "demilitarized zone" akin to the demilitarized scopes Larceny provides.

License

Larceny is copyright © 2025 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.