pinguinson / scapegoat

Scala compiler plugin for static code analysis



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Scapegoat is a Scala static code analyzer, what is more colloquially known as a code lint tool or linter. Scapegoat works in a similar vein to Java's FindBugs or checkstyle, or Scala's Scalastyle.

A static code analyzer is a tool that flag suspicious language usage in code. This can include behavior likely to lead or bugs, non idiomatic usage of a language, or just code that doesn't conform to specified style guidelines.

What's the difference between this project and Scalastyle (or others)?

Scalastyle is a similar linting tool which focuses mostly on enforcing style/code standards. There's no problems running multiple analysis tools on the same codebase. In fact it could be beneficial as the total set of possible warnings is the union of the inspections of all the enabled tools. The worst case is that the same warnings might be generated by multiple tools.


Scapegoat is developed as a scala compiler plugin, which can then be used inside your build tool.


See: sbt-scapegoat for SBT integration.


Firstly you need to add scapegoat plugin as a dependency:


Then configure scala-maven-plugin by adding compilerPlugin


The only required parameter is dataDir (where report will be generated):


You can pass other configuration flags same way, e.g.


Note: You may use a separate maven profile, so that the dependency doesn't go to you runtime classpath.


Here is sample output from the console during the build for a project with warnings/errors:

[warning] [scapegoat] Unused method parameter - org.ensime.util.ClassIterator.scala:46
[warning] [scapegoat] Unused method parameter - org.ensime.util.ClassIterator.scala:137
[warning] [scapegoat] Use of var - org.ensime.util.ClassIterator.scala:22
[warning] [scapegoat] Use of var - org.ensime.util.ClassIterator.scala:157
[   info] [scapegoat]: Inspecting compilation unit [FileUtil.scala]
[warning] [scapegoat] Empty if statement - org.ensime.util.FileUtil.scala:157
[warning] [scapegoat] Expression as statement - org.ensime.util.FileUtil.scala:180

And if you prefer a prettier report, here is a screen shot of the type of HTML report scapegoat generates:



For instructions on suppressing warnings by file, by inspection or by line see the sbt-scapegoat README.

False positives

Please note that scapegoat is a new project. While it's been tested on some common open source projects, there is still a good chance you'll find false positives. Please open up issues if you run into these so we can fix them.


  • 0.94.0 - Fixed some more false positives. Added MethodNames inspection, StripMarginOnRegex inspection, and VariableShadowing inspection (the latter being a work in progress, please report feedback).

  • 0.93.2 - Fixed false positives.

  • 0.93.1 - #67 fixed var args in duplicate map check, #66 ignoring methods returning nothing when checking for unused params, #69 fixed extended classes false pos, #73 Removed incorrect inspection, #64 updated suppression to use tree.symbol.isSynthetic instead of mods.synth, Merge pull request #77 from paulp/psp, Give access to the inspection logic through the sbt console, #76 Improve the contains test.

  • 0.93.0 - Added ability to define multiple traversers that run in separate phases of the compiler, #58 Updated suppression to work on objects and classes, #60 handling case objects in suspicious match on class object, Allow all inspections to be disabled, other fixes

  • 0.92.2 - Added debug option, Made summary optional and disabled in tests, Improved var could be val #54, Split null inspections into assignment and invocation #53, Bumped count on operators to > 2, loads of fixes, loads of verboseness removed.

  • 0.92.1 - Fixed a load of false positives.

  • 0.92.0 - Added swallowed exception inspection, Added public finalizer inspection, Added use expm1(x) instead of exp(x) - 1 inspection, Added use log1p(x) instead of log(x + 1) inspection, Added use log10(x) instead of log(x)/log(10) inspection, Added use cbrt inspection

  • 0.91.0 - Updated logging format to include less [scapegoat] everywhere, Added scala.math and java.StrictMath to useSqrt, Added ignored files patterns option, Added wildcard import inspection, Added comparison to empty set inspection

  • 0.90.17 - Added looks like interpolated string inspection, Added SuspiciousMatchOnClassObject inspection, Updated varuse to not warn on vars in actors #46, Added comparison to empty list inspection, #37 Changed emptyinterpolated string to error, #37 Fixed warning on max parameters

  • 0.90.14 - Bunch of bug fixes for false positives. No new inspections.

  • 0.90.13 - Fixed NPE in VarClosure inspection, Added Object Names inspection, Added classnames inspection, Added avoid to minus one inspection.

  • 0.90.12 - New inspections: unnecessary override, duplicate import, pointless type bounds, max parameters, var closure, method returning any. Updated repeated case body to ignore bodies with two or less statements #28. Removed false positives on getter/setter #27.

  • 0.90.11 - Added empty for inspection, AnyUse inspection, Added ArrayEquals inspection, Added double negation inspection, Disabled expression as statement inspection by default, Added avoid operator overload inspection, #25 improving repeated case bodies, Added lonely sealed trait. Added postInspection call to inspections

  • 0.90.10 - Added type shadowing inspection, var could be val inspection, unreachable catch inspection and unnecessary toString inspection

  • 0.90.09 - Added new inspections: bounded by final type, empty while block, prefer vector empty, finalizer without super, impossible option size condition, filter dot head, repeated case body. Added infos to HTML output header

  • 0.90.8 - Fixed erroneous partial functions inspection. Added inspection for empty case class. Changed levels in output to lowercase. Added console output option. Fixed seq empty on non empty seq. Changed return usage to info. Fixed odd issue with empty tree. Changed unused parameter in override to be info. Ignoring all synthetic method added to case classes. Fixed while(true) being detected by ConstantIf


There are currently 116 inspections. An overview list is given, followed by a more detailed description of each inspection after the list (todo: finish rest of detailed descriptions)

Name Brief Description
ArrayEquals Checks for comparison of arrays using == which will always return false
ArraysInFormat Checks for arrays passed to String.format
ArraysToString Checks for explicit toString calls on arrays
AsInstanceOf Checks for use of asInstanceOf
AvoidOperatorOverload Checks for mental symbolic method names
AvoidSizeEqualsZero Traversable.size can be slow for some data structure, prefer .isEmpty
AvoidSizeNotEqualsZero Traversable.size can be slow for some data structure, prefer .nonEmpty
AvoidToMinusOne Checks for loops that use x to n-1 instead of x until n
BigDecimalDoubleConstructor Checks for use of BigDecimal(double) which can be unsafe
BigDecimalScaleWithoutRoundingMode setScale() on a BigDecimal without setting the rounding mode can throw an exception
BoundedByFinalType Looks for types with upper bounds of a final type
BrokenOddness checks for a % 2 == 1 for oddness because this fails on negative numbers
CatchException Checks for try blocks that catch Exception
CatchFatal Checks for try blocks that catch fatal exceptions: VirtualMachineError, ThreadDeath, InterruptedException, LinkageError, ControlThrowable
CatchNpe Checks for try blocks that catch null pointer exceptions
CatchThrowable Checks for try blocks that catch Throwable
ClassNames Ensures class names adhere to the style guidelines
CollectionIndexOnNonIndexedSeq Checks for indexing on a Seq which is not an IndexedSeq
CollectionNamingConfusion Checks for variables that are confusingly named
CollectionNegativeIndex Checks for negative access on a sequence eg list.get(-1)
CollectionPromotionToAny Checks for collection operations that promote the collection to Any
ComparingFloatingPointTypes Checks for equality checks on floating point types
ComparingUnrelatedTypes Checks for equality comparisons that cannot succeed
ComparisonToEmptyList Checks for code like a == List() or a == Nil
ComparisonToEmptySet Checks for code like a == Set() or a == Set.empty
ComparisonWithSelf Checks for equality checks with itself
ConstantIf Checks for code where the if condition compiles to a constant
DivideByOne Checks for divide by one, which always returns the original value
DoubleNegation Checks for code like !(!b)
DuplicateImport Checks for import statements that import the same selector
DuplicateMapKey Checks for duplicate key names in Map literals
DuplicateSetValue Checks for duplicate values in set literals
EitherGet Checks for use of .get on Left or Right
EmptyCaseClass Checks for case classes like case class Faceman()
EmptyFor Checks for empty for loops
EmptyIfBlock Checks for empty if blocks
EmptyInterpolatedString Looks for interpolated strings that have no arguments
EmptyMethod Looks for empty methods
EmptySynchronizedBlock Looks for empty synchronized blocks
EmptyTryBlock Looks for empty try blocks
EmptyWhileBlock Looks for empty while loops
ExistsSimplifiableToContains exists(x => x == b) replaceable with contains(b)
FilterDotHead .filter(x => ).head can be replaced with find(x => ) match { .. }
FilterDotHeadOption .filter(x =>).headOption can be replaced with find(x => )
FilterDotIsEmpty .filter(x => ).isEmpty can be replaced with !exists(x => )
FilterDotSize .filter(x => ).size can be replaced more concisely with with count(x => )
FilterOptionAndGet .filter(_.isDefined).map(_.get) can be replaced with flatten
FinalModifierOnCaseClass Using Case classes without final modifier can lead to surprising breakage
FinalizerWithoutSuper Checks for overridden finalizers that do not call super
FindAndNotEqualsNoneReplaceWithExists .find(x => ) != None can be replaced with exist(x => )
FindDotIsDefined find(x => ).isDefined can be replaced with exist(x => )
IllegalFormatString Looks for invalid format strings
ImpossibleOptionSizeCondition Checks for code like option.size > 2 which can never be true
IncorrectNumberOfArgsToFormat Checks for wrong number of arguments to String.format
IncorrectlyNamedExceptions Checks for exceptions that are not called *Exception and vice versa
InvalidRegex Checks for invalid regex literals
IsInstanceOf Checks for use of isInstanceOf
JavaConversionsUse Checks for use of implicit Java conversions
ListAppend Checks for List :+ which is O(n)
ListSize Checks for List.size which is O(n).
LonelySealedTrait Checks for sealed traits which have no implementation
LooksLikeInterpolatedString Finds strings that look like they should be interpolated but are not
MapGetAndGetOrElse Map.get(key).getOrElse(value) can be replaced with Map.getOrElse(key, value)
MaxParameters Checks for methods that have over 10 parameters
MethodNames Warns on method names that don't adhere to the Scala style guidelines
MethodReturningAny Checks for defs that are defined or inferred to return Any
ModOne Checks for x % 1 which will always return 0
NanComparison Checks for x == Double.NaN which will always fail
NegationIsEmpty !Traversable.isEmpty can be replaced with Traversable.nonEmpty
NegationNonEmpty !Traversable.nonEmpty can be replaced with Traversable.isEmpty
NoOpOverride Checks for code that overrides parent method but simply calls super
NullAssignment Checks for use of null in assignments
NullParameter Checks for use of null in method invocation
ObjectNames Ensures object names adhere to the Scala style guidelines
OptionGet Checks for Option.get
OptionSize Checks for Option.size
ParameterlessMethodReturnsUnit Checks for def foo : Unit
PartialFunctionInsteadOfMatch Warns when you could use a partial function directly instead of a match block
PointlessTypeBounds Finds type bounds of the form [A <: Any] or [A >: Nothing]
PreferSeqEmpty Checks for Seq() when could use Seq.empty
PreferSetEmpty Checks for Set() when could use Set.empty
ProductWithSerializableInferred Checks for vals that have Product with Serializable as their inferred type
PublicFinalizer Checks for overridden finalizes that are public
RedundantFinalModifierOnMethod Redundant final modifier on method that cannot be overridden
RedundantFinalModifierOnVar Redundant final modifier on var that cannot be overridden
RedundantFinalizer Checks for empty finalizers.
RepeatedCaseBody Checks for case statements which have the same body
ReverseFunc reverse followed by head, headOption, iterator, ormap can be replaced, respectively, with last, lastOption, reverseIterator, or reverseMap
ReverseTailReverse .reverse.tail.reverse can be replaced with init
ReverseTakeReverse .reverse.take(...).reverse can be replaced with takeRight
SimplifyBooleanExpression b == false can be simplified to !b
StripMarginOnRegex Checks for .stripMargin on regex strings that contain a pipe
SubstringZero Checks for String.substring(0)
SuspiciousMatchOnClassObject Finds code where matching is taking place on class literals
SwallowedException Finds catch blocks that don't handle caught exceptions
SwapSortFilter sort.filter can be replaced with filter.sort for performance
TraversableHead Looks for unsafe usage of Traversable.head
TraversableLast Looks for unsafe usage of Traversable.last
TryGet Checks for use of Try.get
TypeShadowing Checks for shadowed type parameters in methods
UnnecessaryIf Checks for code like if (expr) true else false
UnnecessaryReturnUse Checks for use of return keyword in blocks
UnnecessaryToInt Checks for unnecessary toInt on instances of Int
UnnecessaryToString Checks for unnecessary toString on instances of String
UnreachableCatch Checks for catch clauses that cannot be reached
UnsafeContains Checks for List.contains(value) for invalid types
UnusedMethodParameter Checks for unused method parameters
UseCbrt Checks for use of math.pow for calculating math.cbrt
UseExpM1 Checks for use of math.exp(x) - 1 instead of math.expm1(x)
UseLog10 Checks for use of math.log(x)/math.log(10) instead of math.log10(x)
UseLog1P Checks for use of math.log(x + 1) instead of math.log1p(x)
UseSqrt Checks for use of math.pow for calculating math.sqrt
VarClosure Finds closures that reference var
VarCouldBeVal Checks for vars that could be declared as vals
WhileTrue Checks for code that uses a while(true) or do { } while(true) block.
ZeroNumerator Checks for dividing by 0 by a number, eg 0 / x which will always return 0
Arrays to string

Checks for explicit toString calls on arrays. Since toString on an array does not perform a deep toString, like say scala's List, this is usually a mistake.


Checks for calls of .apply(idx) on a Seq where the index is not a literal and the Seq is not an IndexedSeq.

Rationale If code which expects O(1) positional access to a Seq is given a non-IndexedSeq (such as a List, where indexing is O(n)) then this may cause poor performance.


Checks for equality comparisons that cannot succeed because the types are unrelated. Eg "string" == BigDecimal(1.0). The scala compiler has a less strict version of this inspection.


Checks for if statements where the condition is always true or false. Not only checks for the boolean literals, but also any expression that the compiler is able to turn into a constant value. Eg, if (0 < 1) then else that


Checks for a format string that is not invalid, such as invalid conversions, invalid flags, etc. Eg, "% s", "%qs", %.-4f"


Checks for an incorrect number of arguments to String.format. Eg, "%s %s %f".format("need", "three") flags an error because the format string specifies 3 parameters but the call only provides 2.


Checks for invalid regex literals that would fail at compile time. Either dangling metacharacters, or unclosed escape characters, etc that kind of thing.

List size

Checks for .size on an instance of List. Eg, val a = List(1,2,3); a.size

Rationale List.size is O(n) so for performance reasons if .size is needed on a list that could be large, consider using an alternative with O(1), eg Array, Vector or ListBuffer.

Redundant finalizer

Checks for empty finalizers. This is redundant code and should be removed. Eg, override def finalize : Unit = { }


Indicates where code using Set() could be replaced with Set.empty. Set() instantiates a new instance each time it is invoked, whereas Set.empty returns a pre-instantiated instance.


Checks for use of return in a function or method. Since the final expression of a block is always the return value, using return is unnecessary. Eg, def foo = { println("hello"); return 12; }


Checks for catch clauses that cannot be reached. This means the exception is dead and if you want that exception to take precedence you should move up further up the case list.


Checks for List.contains(value) for invalid types. The method for contains accepts any types. This inspection finds situations when you have a list of type A and you are checking for contains on type B which cannot hold.

While true

Checks for code that uses a while(true) or do { } while(true) block.

Rationale: This type of code is usually not meant for production as it will not return normally. If you need to loop until interrupted then consider using a flag.

Other static analysis tools: