Scala compiler plugin to detect unused expressions (non-Unit).

In purely functional programming, expressions are always pure - their evaluation has no side effects. Side effects are therefore expressed with an IO-like type (e.g. Cats Effect IO) and they only happen when the IO is run - preferably with a single, "impure" invocation hidden somewhere in library code.

Because of that, in pure FP it never makes sense to discard the result of an expression, e.g.

val number = {
  discardedExpression // pointless!
  42
}

This is an easy mistake to make and it may cause very tricky bugs, e.g. when the discarded expression was an IO that did something important. The Scala compiler does not detect this kind of mistake because Scala is not a purely functional language so the compiler cannot assume that all expressions are pure.

This plugin reports a warning for any discarded expression whose type is not Unit.

Enable the plugin in build.sbt:

libraryDependencies += compilerPlugin("com.github.ghik" % "zerowaste" % "<version>" cross CrossVersion.full)

The plugin issues warnings, but it is often a good idea to turn them into compilation errors:

scalacOptions += "-Werror"

Note that these warnings, despite being converted to errors, can be suppressed with the @nowarn annotation:

import scala.annotation.nowarn

val number = {
  discardedExpression: @nowarn("msg=discarded expression")
  42
}

Zerowaste is currently available for Scala 2.12.17+, 2.13.10+ and 3.2.1+

Compiler plugins must be cross-built for every minor and patch version of Scala. If zerowaste is not available for your Scala version, please file an issue or submit a PR.

1. Add your desired Scala version to crossScalaVersions in build.sbt
2. Run sbt githubWorkflowGenerate
3. Commit the changes in build.sbt and github workflows
4. Submit a PR