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A linting compiler plugin
Branch: master

Merge pull request #7 from non/topic/explicit-returns-for-implicits

Added a new warning for implicit methods lacking explicit return type.
latest commit f5e38a4da5
@jorgeortiz85 authored

Linter Compiler Plugin

This is a compiler plugin that adds additional lint checks to protect against sharp corners in the Scala compiler and standard libraries.

It's a work in progress. For an overview of writing compiler plugins, see


Add it as a compiler plugin in your project by editing your build.sbt file. For example, once published:

addCompilerPlugin("com.foursquare.lint" %% "linter" % "x.y.z")

Or, until published:

scalacOptions += "-Xplugin:..path-to-jar../linter.jar"

Optionally, run sbt console in this project to see it in action.

Currently suported warnings

Unsafe ==

scala> Nil == None
<console>:29: warning: Comparing with == on instances of different types (object Nil, object None) will probably return false.
              Nil == None

Unsafe contains

scala> List(1, 2, 3).contains("4")
<console>:29: warning: SeqLike[Int].contains(java.lang.String) will probably return false.
              List(1, 2, 3).contains("4")

Wildcard import from scala.collection.JavaConversions

scala> import scala.collection.JavaConversions._
<console>:29: warning: Conversions in scala.collection.JavaConversions._ are dangerous.
       import scala.collection.JavaConversions._

Any and all wildcard imports

scala> import scala.collection.JavaConversions._
<console>:10: warning: Wildcard imports should be avoided.
       import scala.reflect.generic._

Calling Option#get

scala> Option(1).get
<console>:29: warning: Calling .get on Option will throw an exception if the Option is None.

Future Work

  • Add more warnings
  • Pick and choose which warnings you want
  • Choose whether they should be warnings or errors

Ideas for new warnings

Feel free to implement these, or add your own ideas. Pull requests welcome!

  • Modularize the wildcard import warnings like the "Avoid Star Import" configuration of checkstyle (
  • Require explicit override whenever a method is being overwritten
  • Implicit methods should always have explicit return types
  • Expressions spanning multiple lines should be enclosed in parentheses
  • Unused method argument warnings
  • Warn on unrestricted catch clauses (case e => ...)
  • Traversable#head, Traversable#last, Traversable#maxBy
  • Warn on shadowing variables, especially those of the same type
  • Warn on inexhaustive pattern matching
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