April 25, 2017
Recommended Scalac Flags for 2.12
[Updated 10-May-2017]
Greetings fellow traveler.
Here’s a first stab at a set of scalacOptions
for Scala 2.12. Let me know what you think … some of these options are new and I don’t quite know what they do but they sound good so I included them. I will update this post to reflect further suggestions. Confidence high!
This is current as of Lightbend Scala 2.12.2 and Typelevel Scala 4 (see below).
scalacOptions ++= Seq(
"-deprecation", // Emit warning and location for usages of deprecated APIs.
"-encoding", "utf-8", // Specify character encoding used by source files.
"-explaintypes", // Explain type errors in more detail.
"-feature", // Emit warning and location for usages of features that should be imported explicitly.
"-language:existentials", // Existential types (besides wildcard types) can be written and inferred
"-language:experimental.macros", // Allow macro definition (besides implementation and application)
"-language:higherKinds", // Allow higher-kinded types
"-language:implicitConversions", // Allow definition of implicit functions called views
"-unchecked", // Enable additional warnings where generated code depends on assumptions.
"-Xcheckinit", // Wrap field accessors to throw an exception on uninitialized access.
"-Xfatal-warnings", // Fail the compilation if there are any warnings.
"-Xfuture", // Turn on future language features.
"-Xlint:adapted-args", // Warn if an argument list is modified to match the receiver.
"-Xlint:by-name-right-associative", // By-name parameter of right associative operator.
"-Xlint:constant", // Evaluation of a constant arithmetic expression results in an error.
"-Xlint:delayedinit-select", // Selecting member of DelayedInit.
"-Xlint:doc-detached", // A Scaladoc comment appears to be detached from its element.
"-Xlint:inaccessible", // Warn about inaccessible types in method signatures.
"-Xlint:infer-any", // Warn when a type argument is inferred to be `Any`.
"-Xlint:missing-interpolator", // A string literal appears to be missing an interpolator id.
"-Xlint:nullary-override", // Warn when non-nullary `def f()' overrides nullary `def f'.
"-Xlint:nullary-unit", // Warn when nullary methods return Unit.
"-Xlint:option-implicit", // Option.apply used implicit view.
"-Xlint:package-object-classes", // Class or object defined in package object.
"-Xlint:poly-implicit-overload", // Parameterized overloaded implicit methods are not visible as view bounds.
"-Xlint:private-shadow", // A private field (or class parameter) shadows a superclass field.
"-Xlint:stars-align", // Pattern sequence wildcard must align with sequence component.
"-Xlint:type-parameter-shadow", // A local type parameter shadows a type already in scope.
"-Xlint:unsound-match", // Pattern match may not be typesafe.
"-Yno-adapted-args", // Do not adapt an argument list (either by inserting () or creating a tuple) to match the receiver.
"-Ypartial-unification", // Enable partial unification in type constructor inference
"-Ywarn-dead-code", // Warn when dead code is identified.
"-Ywarn-extra-implicit", // Warn when more than one implicit parameter section is defined.
"-Ywarn-inaccessible", // Warn about inaccessible types in method signatures.
"-Ywarn-infer-any", // Warn when a type argument is inferred to be `Any`.
"-Ywarn-nullary-override", // Warn when non-nullary `def f()' overrides nullary `def f'.
"-Ywarn-nullary-unit", // Warn when nullary methods return Unit.
"-Ywarn-numeric-widen", // Warn when numerics are widened.
"-Ywarn-unused:implicits", // Warn if an implicit parameter is unused.
"-Ywarn-unused:imports", // Warn if an import selector is not referenced.
"-Ywarn-unused:locals", // Warn if a local definition is unused.
"-Ywarn-unused:params", // Warn if a value parameter is unused.
"-Ywarn-unused:patvars", // Warn if a variable bound in a pattern is unused.
"-Ywarn-unused:privates", // Warn if a private member is unused.
"-Ywarn-value-discard" // Warn when non-Unit expression results are unused.
)
Note that the REPL can’t really cope with -Ywarn-unused:imports
or -Xfatal-warnings
so you should turn them off for the console.
scalacOptions in (Compile, console) --= Seq("-Ywarn-unused:imports", "-Xfatal-warnings")
If you’re like me and you find that the standard library gets in the way (especially Predef
implicits) you might consider one of these (you don’t need both):
"-Yno-predef" // no automatic import of Predef (removes irritating implicits)
"-Yno-imports" // no automatic imports at all; all symbols must be imported explicitly
Modifications for Typelevel Scala 4
Typelevel Scala 4 is highly recommended for the following additional options.
scalacOptions ++ Seq(
"-Yinduction-heuristics", // speeds up the compilation of inductive implicit resolution
"-Ykind-polymorphism", // type and method definitions with type parameters of arbitrary kinds
"-Yliteral-types", // literals can appear in type position
"-Xstrict-patmat-analysis", // more accurate reporting of failures of match exhaustivity
"-Xlint:strict-unsealed-patmat" // warn on inexhaustive matches against unsealed traits
)
Typelevel Scala 4 offers a more general solution to the -Yno-predef
business above. See the -Ysysdef
and -Ypredef
options at the link above.