NOTE: This file is auto-generated, so please don't edit manually. See the CONTRIBUTING guide to see how to update it.
Generated with Scala 3.4.0-RC1-bin-20231101-e2b38f9-NIGHTLY
NOTE: This error is no longer emitted by the compiler.
Erroneous Code Example
try {} catch {}Erroneous Code Example
try {} catch {}Example Error Output
-- [E001] Syntax Error: examples/001_EmptyCatchBlockID.scala:3:9
3 | try {} catch {}
| ^^^^^^^^
| The catch block does not contain a valid expression, try
| adding a case like - case e: Exception => to the block
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| A try expression should be followed by some mechanism to handle any exceptions
| thrown. Typically a catch expression follows the try and pattern matches
| on any expected exceptions. For example:
|
| import scala.util.control.NonFatal
|
| try {} catch {
| case NonFatal(e) => ???
| }
|
| It is also possible to follow a try immediately by a finally - letting the
| exception propagate - but still allowing for some clean up in finally:
|
| try {} finally {
| // perform your cleanup here!
| }
|
| It is recommended to use the NonFatal extractor to catch all exceptions as it
| correctly handles transfer functions like return.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
try {}Example Error Output
-- [E002] Syntax Warning: examples/002_EmptyCatchAndFinallyBlockID.scala:3:2
3 | try {}
| ^^^^^^
| A try without catch or finally is equivalent to putting
| its body in a block; no exceptions are handled.
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| A try expression should be followed by some mechanism to handle any exceptions
| thrown. Typically a catch expression follows the try and pattern matches
| on any expected exceptions. For example:
|
| import scala.util.control.NonFatal
|
| try {} catch {
| case NonFatal(e) => ???
| }
|
| It is also possible to follow a try immediately by a finally - letting the
| exception propagate - but still allowing for some clean up in finally:
|
| try {} finally {
| // perform your cleanup here!
| }
|
| It is recommended to use the NonFatal extractor to catch all exceptions as it
| correctly handles transfer functions like return.
-----------------------------------------------------------------------------
1 warning found
Erroneous Code Example
type A = Double with IntExample Error Output
-- [E003] Syntax Deprecation Warning: examples/003_DeprecatedWithOperatorID.scala:4:18
4 | type A = Double with Int
| ^
| with as a type operator has been deprecated; use & instead
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Dotty introduces intersection types - & types. These replace the
| use of the with keyword. There are a few differences in
| semantics between intersection types and using with.
-----------------------------------------------------------------------------
1 warning found
Erroneous Code Example
case class FooExample Error Output
-- [E004] Syntax Error: examples/004_CaseClassMissingParamListID.scala:3:13
3 | case class Foo
| ^^^
| A case class must have at least one parameter list
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Foo must have at least one parameter list, if you would rather
| have a singleton representation of Foo, use a "case object".
| Or, add an explicit () as a parameter list to Foo.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
List((1, 2)).map { case (num, num) => ??? }Example Error Output
-- [E005] Naming Error: examples/005_DuplicateBindID.scala:3:32
3 | List((1, 2)).map { case (num, num) => ??? }
| ^^^
| duplicate pattern variable: num
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| For each case bound variable names have to be unique. In:
|
| case (num, num) => {
| ???
| }
|
| num is not unique. Rename one of the bound variables!
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
aExample Error Output
-- [E006] Not Found Error: examples/006_MissingIdentID.scala:3:2
3 | a
| ^
| Not found: a
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Each identifier in Scala needs a matching declaration. There are two kinds of
| identifiers: type identifiers and value identifiers. Value identifiers are introduced
| by `val`, `def`, or `object` declarations. Type identifiers are introduced by `type`,
| `class`, `enum`, or `trait` declarations.
|
| Identifiers refer to matching declarations in their environment, or they can be
| imported from elsewhere.
|
| Possible reasons why no matching declaration was found:
| - The declaration or the use is mis-spelt.
| - An import is missing.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
val foo: String = 1Example Error Output
-- [E007] Type Mismatch Error: examples/007_TypeMismatchID.scala:3:20
3 | val foo: String = 1
| ^
| Found: (1 : Int)
| Required: String
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
|
| Tree: 1
| I tried to show that
| (1 : Int)
| conforms to
| String
| but none of the attempts shown below succeeded:
|
| ==> (1 : Int) <: String
| ==> Int <: String = false
|
| The tests were made under the empty constraint
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
object Foo
Foo.iDontExistExample Error Output
-- [E008] Not Found Error: examples/008_NotAMemberID.scala:4:6
4 | Foo.iDontExist
| ^^^^^^^^^^^^^^
| value iDontExist is not a member of object Foo
1 error found
Errors encountered during compilation
Erroneous Code Example
import java.io.File
trait ListFiles:
val f: File
def getList(msg: String) = f.listFiles
end ListFiles
class A extends {
val f = new File("iDontExist")
} with ListFilesExample Error Output
-- Error: examples/009_EarlyDefinitionsNotSupportedID.scala:9:18
9 | class A extends {
| ^
| `extends` must be followed by at least one parent
-- [E009] Syntax Error: examples/009_EarlyDefinitionsNotSupportedID.scala:11:4
11 | } with ListFiles
| ^^^^
| Early definitions are not supported; use trait parameters instead
|----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Earlier versions of Scala did not support trait parameters and "early
| definitions" (also known as "early initializers") were used as an alternative.
|
| Example of old syntax:
|
| trait Logging {
| val f: File
| f.open()
| onExit(f.close())
| def log(msg: String) = f.write(msg)
| }
|
| class B extends Logging {
| val f = new File("log.data") // triggers a NullPointerException
| }
|
| // early definition gets around the NullPointerException
| class C extends {
| val f = new File("log.data")
| } with Logging
|
| The above code can now be written as:
|
| trait Logging(f: File) {
| f.open()
| onExit(f.close())
| def log(msg: String) = f.write(msg)
| }
|
| class C extends Logging(new File("log.data"))
----------------------------------------------------------------------------
-- Error: examples/009_EarlyDefinitionsNotSupportedID.scala:11:9
11 | } with ListFiles
| ^^^^^^^^^
| end of statement expected but identifier found
3 errors found
Errors encountered during compilation
NOTE: This error is no longer emitted by the compiler.
In the past top level implicit classes were not allowed. This is however no longer the case.
Erroneous Code Example
implicit class Foo(a: Int)Erroneous Code Example
implicit case class Foo(a: Int)Example Error Output
-- [E011] Syntax Error: examples/011_ImplicitCaseClassID.scala:3:22
3 | implicit case class Foo(a: Int)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| A case class may not be defined as implicit
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Implicit classes may not be case classes. Instead use a plain class:
|
| implicit class Foo...
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
implicit class FooExample Error Output
-- [E012] Syntax Error: examples/012_ImplicitClassPrimaryConstructorArityID.scala:3:17
3 | implicit class Foo
| ^^^^^^^^^^^^^^^^^^
| Implicit classes must accept exactly one primary constructor parameter
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Implicit classes may only take one non-implicit argument in their constructor. For example:
|
| implicit class RichDate(date: java.util.Date)
|
| While it’s possible to create an implicit class with more than one non-implicit argument,
| such classes aren’t used during implicit lookup.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
trait A
object Foo { self: A => }Example Error Output
-- [E013] Syntax Error: examples/013_ObjectMayNotHaveSelfTypeID.scala:4:15
4 | object Foo { self: A => }
| ^^^^^^^^^^
| objects must not have a self type
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| objects must not have a self type:
|
| Consider these alternative solutions:
| - Create a trait or a class instead of an object
| - Let the object extend a trait containing the self type:
|
| object Foo extends A
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
NOTE: This error is no longer emitted by the compiler.
Before scala/scala3#4938 tuples were limited in length to 22
Erroneous Code Example
(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23)Erroneous Code Example
final final val foo = ???Example Error Output
-- [E015] Syntax Error: examples/015_RepeatedModifierID.scala:3:8
3 | final final val foo = ???
| ^^^^^
| Repeated modifier final
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| This happens when you accidentally specify the same modifier twice.
|
| Example:
|
| private private val Origin = Point(0, 0)
|
| instead of
|
| private final val Origin = Point(0, 0)
-----------------------------------------------------------------------------
-- [E088] Syntax Error: examples/015_RepeatedModifierID.scala:3:14
3 | final final val foo = ???
| ^^^
| Expected start of definition
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| You have to provide either class, trait, object, or enum definitions after qualifiers
-----------------------------------------------------------------------------
2 errors found
Errors encountered during compilation
Erroneous Code Example
s"$new Point(1, 2)"Example Error Output
-- [E016] Syntax Error: examples/016_InterpolatedStringErrorID.scala:3:5
3 | s"$new Point(1, 2)"
| ^
| Error in interpolated string: identifier or block expected
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| This usually happens when you forget to place your expressions inside curly braces.
|
| s"$new Point(0, 0)"
|
| should be written as
|
| s"${new Point(0, 0)}"
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
{ _ }Example Error Output
-- [E017] Syntax Error: examples/017_UnboundPlaceholderParameterID.scala:3:4
3 | { _ }
| ^
| Unbound placeholder parameter; incorrect use of _
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| The _ placeholder syntax was used where it could not be bound.
| Consider explicitly writing the variable binding.
|
| This can be done by replacing _ with a variable (eg. x)
| and adding x => where applicable.
|
| Example before:
|
| { _ }
|
| Example after:
|
| x => { x }
|
| Another common occurrence for this error is defining a val with _:
|
| val a = _
|
| But this val definition isn't very useful, it can never be assigned
| another value. And thus will always remain uninitialized.
| Consider replacing the val with var:
|
| var a = _
|
| Note that this use of _ is not placeholder syntax,
| but an uninitialized var definition.
| Only fields can be left uninitialized in this manner; local variables
| must be initialized.
|
| Another occurrence for this error is self type definition.
| The _ can be replaced with this.
|
| Example before:
|
| trait A { _: B => ...
|
| Example after:
|
| trait A { this: B => ...
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
val x = : Example Error Output
-- [E018] Syntax Error: examples/018_IllegalStartSimpleExprID.scala:3:10
3 | val x = :
| ^
| expression expected but �[31m:�[0m found
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| An expression cannot start with �[31m:�[0m.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
trait Foo:
def fooExample Error Output
-- [E019] Syntax Error: examples/019_MissingReturnTypeID.scala:4:11
4 | def foo
| ^
| Missing return type
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| An abstract declaration must have a return type. For example:
|
| trait Shape:
| def area: Double // abstract declaration returning a Double
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
for i <- 1 to 3 println(i)Example Error Output
-- [E020] Syntax Error: examples/020_YieldOrDoExpectedInForComprehensionID.scala:5:0
5 |
|^
|yield or do expected
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| When the enumerators in a for comprehension are not placed in parentheses or
| braces, a do or yield statement is required after the enumerators
| section of the comprehension.
|
| You can save some keystrokes by omitting the parentheses and writing
|
| val numbers = for i <- 1 to 3 yield i
|
| instead of
|
| val numbers = for (i <- 1 to 3) yield i
|
| but the yield keyword is still required.
|
| For comprehensions that simply perform a side effect without yielding anything
| can also be written without parentheses but a do keyword has to be
| included. For example,
|
| for (i <- 1 to 3) println(i)
|
| can be written as
|
| for i <- 1 to 3 do println(i) // notice the 'do' keyword
-----------------------------------------------------------------------------
-- [E008] Not Found Error: examples/020_YieldOrDoExpectedInForComprehensionID.scala:3:18
3 | for i <- 1 to 3 println(i)
| ^^^^^^^^^^^^^^
|value println is not a member of scala.collection.immutable.Range.Inclusive
-- [E006] Not Found Error: examples/020_YieldOrDoExpectedInForComprehensionID.scala:3:26
3 | for i <- 1 to 3 println(i)
| ^
| Not found: i
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Each identifier in Scala needs a matching declaration. There are two kinds of
| identifiers: type identifiers and value identifiers. Value identifiers are introduced
| by `val`, `def`, or `object` declarations. Type identifiers are introduced by `type`,
| `class`, `enum`, or `trait` declarations.
|
| Identifiers refer to matching declarations in their environment, or they can be
| imported from elsewhere.
|
| Possible reasons why no matching declaration was found:
| - The declaration or the use is mis-spelt.
| - An import is missing.
-----------------------------------------------------------------------------
3 errors found
Errors encountered during compilation
Erroneous Code Example
trait Foo:
/** Some docs
* @usecase val foo = ()
*/
def foo: UnitExample Error Output
-- [E021] Doc Comment Error: examples/021_ProperDefinitionNotFoundID.scala:6:14
6 | * @usecase val foo = ()
| ^
| Proper definition was not found in @usecase
7 | */
8 | def foo: Unit
9 |// END
|----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Usecases are only supported for defs. They exist because with Scala's
| advanced type-system, we sometimes end up with seemingly scary signatures.
| The usage of these methods, however, needs not be - for instance the map
| function
|
| List(1, 2, 3).map(2 * _) // res: List(2, 4, 6)
|
| is easy to understand and use - but has a rather bulky signature:
|
| def map[B, That](f: A => B)(implicit bf: CanBuildFrom[List[A], B, That]): That
|
| to mitigate this and ease the usage of such functions we have the @usecase
| annotation for docstrings. Which can be used like this:
|
| /** Map from List[A] => List[B]
| *
| * @usecase def map[B](f: A => B): List[B]
| */
| def map[B, That](f: A => B)(implicit bf: CanBuildFrom[List[A], B, That]): That
|
|
| When creating the docs, the signature of the method is substituted by the
| usecase and the compiler makes sure that it is valid. Because of this, you're
| only allowed to use defs when defining usecases.
----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
This ErrorMessageID has no valid example yet. See the CONTRIBUTING guide to see how you can help.
Erroneous Code Example
def foo[A] = ()
foo[String, String]Example Error Output
-- [E023] Syntax Error: examples/023_WrongNumberOfTypeArgsID.scala:4:5
4 | foo[String, String]
| ^^^^^^^^^^^^^^^^^^^
| Too many type arguments for 023_WrongNumberOfTypeArgsID$package.foo[A]
| expected: [A]
| actual: [String, String]
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| You have supplied too many type parameters
|
| For example List takes a single type parameter (List[A])
| If you need to hold more types in a list then you need to combine them
| into another data type that can contain the number of types you need,
| In this example one solution would be to use a Tuple:
|
| val tuple2: (Int, String) = (1, "one")
| val list: List[(Int, String)] = List(tuple2)
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
Option((1, 2)) match
case (1, n) | (n, 1) => "got a one!"
case _ => "didn't get a 1"Example Error Output
-- [E024] Syntax Error: examples/024_IllegalVariableInPatternAlternativeID.scala:4:13
4 | case (1, n) | (n, 1) => "got a one!"
| ^
| Illegal variable n in pattern alternative
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Variables are not allowed within alternate pattern matches. You can workaround
| this issue by adding additional cases for each alternative. For example, the
| illegal function:
|
| def g(pair: (Int,Int)): Int = pair match {
| case (1, n) | (n, 1) => n
| case _ => 0
| }
| could be implemented by moving each alternative into a separate case:
|
| def g(pair: (Int,Int)): Int = pair match {
| case (1, n) => n
| case (n, 1) => n
| case _ => 0
| }
-----------------------------------------------------------------------------
-- [E024] Syntax Error: examples/024_IllegalVariableInPatternAlternativeID.scala:4:19
4 | case (1, n) | (n, 1) => "got a one!"
| ^
| Illegal variable n in pattern alternative
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Variables are not allowed within alternate pattern matches. You can workaround
| this issue by adding additional cases for each alternative. For example, the
| illegal function:
|
| def g(pair: (Int,Int)): Int = pair match {
| case (1, n) | (n, 1) => n
| case _ => 0
| }
| could be implemented by moving each alternative into a separate case:
|
| def g(pair: (Int,Int)): Int = pair match {
| case (1, n) => n
| case (n, 1) => n
| case _ => 0
| }
-----------------------------------------------------------------------------
2 errors found
Errors encountered during compilation
This ErrorMessageID has no valid example yet. See the CONTRIBUTING guide to see how you can help.
Erroneous Code Example
class Foo(a: Int):
def this(using a: Int) = this(a)Example Error Output
-- [E026] Syntax Error: examples/026_AuxConstructorNeedsNonImplicitParameterID.scala:4:27
4 | def this(using a: Int) = this(a)
| ^
| Auxiliary constructor needs non-implicit parameter list
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Only the primary constructor is allowed an implicit parameter list;
| auxiliary constructors need non-implicit parameter lists. When a primary
| constructor has an implicit argslist, auxiliary constructors that call the
| primary constructor must specify the implicit value.
|
| To resolve this issue check for:
| - Forgotten parenthesis on this (def this() = { ... })
| - Auxiliary constructors specify the implicit value
-----------------------------------------------------------------------------
-- [E120] Naming Error: examples/026_AuxConstructorNeedsNonImplicitParameterID.scala:4:8
4 | def this(using a: Int) = this(a)
| ^
|Double definition:
|def <init>(a: Int): Foo in class Foo at line 3 and
|def <init>(using a: Int)(): Foo in class Foo at line 4
|have the same type after erasure.
|
|Consider adding a @targetName annotation to one of the conflicting definitions
|for disambiguation.
2 errors found
Errors encountered during compilation
This ErrorMessageID has no valid example yet. See the CONTRIBUTING guide to see how you can help.
This ErrorMessageID has no valid example yet. See the CONTRIBUTING guide to see how you can help.
Erroneous Code Example
enum Foo:
case a, b, c
val foo: Foo = Foo.a
foo match
case Foo.a => "What about the rest?"Example Error Output
-- [E029] Pattern Match Exhaustivity Warning: examples/029_PatternMatchExhaustivityID.scala:7:2
7 | foo match
| ^^^
| match may not be exhaustive.
|
| It would fail on pattern case: b, c
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| There are several ways to make the match exhaustive:
| - Add missing cases as shown in the warning
| - If an extractor always return Some(...), write Some[X] for its return type
| - Add a case _ => ... at the end to match all remaining cases
-----------------------------------------------------------------------------
1 warning found
Erroneous Code Example
val myList = List(1, 2, 3)
myList match
case foo => "this catches everything"
case bar => "So I'll never get here"
case null => "extra null check to not see E121"Example Error Output
-- [E030] Match case Unreachable Warning: examples/030_MatchCaseUnreachableID.scala:6:9
6 | case bar => "So I'll never get here"
| ^^^
| Unreachable case
1 warning found
This ErrorMessageID has no valid example yet. See the CONTRIBUTING guide to see how you can help.
Erroneous Code Example
package foo { object bar }
package foo.bar {}Example Error Output
-- [E033] Naming Error: examples/033_PkgDuplicateSymbolID.scala:5:12
5 |package foo.bar {}
| ^^^^^^^
| Trying to define package with same name as class bar
1 error found
Errors encountered during compilation
Erroneous Code Example
trait Foo[A, B]:
type F = A forSome { type B}Example Error Output
-- [E034] Syntax Error: examples/034_ExistentialTypesNoLongerSupportedID.scala:4:15
4 | type F = A forSome { type B}
| ^^^^^^^
| Existential types are no longer supported -
| use a wildcard or dependent type instead
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| The use of existential types is no longer supported.
|
| You should use a wildcard or dependent type instead.
|
| For example:
|
| Instead of using forSome to specify a type variable
|
| List[T forSome { type T }]
|
| Try using a wildcard type variable
|
| List[?]
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
val foo: _ = ???Example Error Output
-- [E035] Syntax Error: examples/035_UnboundWildcardTypeID.scala:3:11
3 | val foo: _ = ???
| ^
| Unbound wildcard type
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| The wildcard type syntax (_) was used where it could not be bound.
| Replace _ with a non-wildcard type. If the type doesn't matter,
| try replacing _ with Any.
|
| Examples:
|
| - Parameter lists
|
| Instead of:
| def foo(x: _) = ...
|
| Use Any if the type doesn't matter:
| def foo(x: Any) = ...
|
| - Type arguments
|
| Instead of:
| val foo = List[?](1, 2)
|
| Use:
| val foo = List[Int](1, 2)
|
| - Type bounds
|
| Instead of:
| def foo[T <: _](x: T) = ...
|
| Remove the bounds if the type doesn't matter:
| def foo[T](x: T) = ...
|
| - val and def types
|
| Instead of:
| val foo: _ = 3
|
| Use:
| val foo: Int = 3
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
NOTE: This error is no longer emitted by the compiler.
Erroneous Code Example
class Foo:
override val foo = ???Example Error Output
-- [E037] Declaration Error: examples/037_OverridesNothingID.scala:4:17
4 | override val foo = ???
| ^
| value foo overrides nothing
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| There must be a field or method with the name foo in a super
| class of class Foo to override it. Did you misspell it?
| Are you extending the right classes?
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
trait Foo1:
def bar(i: Int) = ???
trait Foo2 extends Foo1:
override def bar(s: String) = ???Example Error Output
-- [E038] Declaration Error: examples/038_OverridesNothingButNameExistsID.scala:7:15
7 | override def bar(s: String) = ???
| ^
| method bar has a different signature than the overridden declaration
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| There must be a non-final field or method with the name bar and the
| same parameter list in a super class of trait Foo2 to override it.
|
| override def bar(s: String): Nothing
|
| The super classes of trait Foo2 contain the following members
| named bar:
| def bar(i: Int): Nothing
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
foo.toUpperCase
val foo: String = ???Example Error Output
-- [E039] Reference Error: examples/039_ForwardReferenceExtendsOverDefinitionID.scala:3:2
3 | foo.toUpperCase
| ^^^
| foo is a forward reference extending over the definition of foo
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| foo is used before you define it, and the definition of foo
| appears between that use and the definition of foo.
|
| Forward references are allowed only, if there are no value definitions between
| the reference and the referred method definition.
|
| Define foo before it is used,
| or move the definition of foo so it does not appear between
| the declaration of foo and its use,
| or define foo as lazy.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
val foo: = ???Example Error Output
-- [E040] Syntax Error: examples/040_ExpectedTokenButFoundID.scala:3:11
3 | val foo: = ???
| ^
| an identifier expected, but '=' found
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
|
| If you want to use '=' as identifier, you may put it in backticks: `=`.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
case class I(j: Int):
def +-(i: Int) = i
def +:(i: Int) = i
I(1) +- I(4) +: I(4)Example Error Output
-- [E041] Syntax Error: examples/041_MixedLeftAndRightAssociativeOpsID.scala:7:10
7 | I(1) +- I(4) +: I(4)
| ^
|+- (which is left-associative) and +: (which is right-associative) have same precedence and may not be mixed
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| The operators +- and +: are used as infix operators in the same expression,
| but they bind to different sides:
| +- is applied to the operand to its left
| +: is applied to the operand to its right
| As both have the same precedence the compiler can't decide which to apply first.
|
| You may use parenthesis to make the application order explicit,
| or use method application syntax operand1.+-(operand2).
|
| Operators ending in a colon : are right-associative. All other operators are left-associative.
|
| Infix operator precedence is determined by the operator's first character. Characters are listed
| below in increasing order of precedence, with characters on the same line having the same precedence.
| (all letters)
| |
| ^
| &
| = !
| < >
| :
| + -
| * / %
| (all other special characters)
| Operators starting with a letter have lowest precedence, followed by operators starting with `|`, etc.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
trait Foo
new FooExample Error Output
-- [E042] Type Error: examples/042_CantInstantiateAbstractClassOrTraitID.scala:4:6
4 | new Foo
| ^^^
| Foo is a trait; it cannot be instantiated
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Abstract classes and traits need to be extended by a concrete class or object
| to make their functionality accessible.
|
| You may want to create an anonymous class extending Foo with
| class Foo { }
|
| or add a companion object with
| object Foo extends Foo
|
| You need to implement any abstract members in both cases.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
case class CC[T](key: T)
type Alias[T] = Seq[CC[T]]
def g(xs: Alias[?]) = xs map { case CC(x) => CC(x) }Example Error Output
-- [E043] Type Error: examples/043_UnreducibleApplicationID.scala:6:12
6 | def g(xs: Alias[?]) = xs map { case CC(x) => CC(x) }
| ^^^^^^^^
|unreducible application of higher-kinded type [T] =>> Seq[CC[T]] to wildcard arguments
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| An abstract type constructor cannot be applied to wildcard arguments.
| Such applications are equivalent to existential types, which are not
| supported in Scala 3.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
class Foo:
def foo = ???
def foo(a: Int) = foo()Example Error Output
-- [E044] Cyclic Error: examples/044_OverloadedOrRecursiveMethodNeedsResultTypeID.scala:5:22
5 | def foo(a: Int) = foo()
| ^
| Overloaded or recursive method foo needs return type
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Case 1: method foo is overloaded
| If there are multiple methods named method foo and at least one definition of
| it calls another, you need to specify the calling method's return type.
|
| Case 2: method foo is recursive
| If method foo calls itself on any path (even through mutual recursion), you need to specify the return type
| of method foo or of a definition it's mutually recursive with.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
val factorial = (x: Int) => if x == 0 then 1 else x * factorial(x - 1)Example Error Output
-- [E045] Cyclic Error: examples/045_RecursiveValueNeedsResultTypeID.scala:3:56
3 | val factorial = (x: Int) => if x == 0 then 1 else x * factorial(x - 1)
| ^
| Recursive value factorial needs type
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| The definition of value factorial is recursive and you need to specify its type.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
This ErrorMessageID has no valid example yet. See the CONTRIBUTING guide to see how you can help.
This ErrorMessageID has no valid example yet. See the CONTRIBUTING guide to see how you can help.
Erroneous Code Example
class Foo:
val foo = Foo.super[Bar].fooExample Error Output
-- [E048] Reference Error: examples/048_SuperQualMustBeParentID.scala:4:14
4 | val foo = Foo.super[Bar].foo
| ^^^^^^^^^^^^^^
| Bar does not name a parent of class Foo
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| When a qualifier T is used in a super prefix of the form C.super[T],
| T must be a parent type of C.
|
| In this case, the parents of class Foo are:
| - Object
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
import foo.*
foo
object foo:
def foo = ()Example Error Output
-- [E049] Reference Error: examples/049_AmbiguousReferenceID.scala:4:2
4 | foo
| ^^^
| Reference to foo is ambiguous.
| It is both defined in package <empty>
| and imported subsequently by import foo._
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| The identifier foo is ambiguous because a name binding of lower precedence
| in an inner scope cannot shadow a binding with higher precedence in
| an outer scope.
|
| The precedence of the different kinds of name bindings, from highest to lowest, is:
| - Definitions in an enclosing scope
| - Inherited definitions and top-level definitions in packages
| - Names introduced by import of a specific name
| - Names introduced by wildcard import
| - Definitions from packages in other files
| Note:
| - As a rule, definitions take precedence over imports.
| - Definitions in an enclosing scope take precedence over inherited definitions,
| which can result in ambiguities in nested classes.
| - When importing, you can avoid naming conflicts by renaming:
| import scala.{foo => fooTick}
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
def foo = 1
foo(1)Example Error Output
-- [E050] Type Error: examples/050_MethodDoesNotTakeParametersId.scala:4:2
4 | foo(1)
| ^^^
| method foo does not take parameters
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| You have specified more parameter lists than defined in the method definition(s).
| Nullary methods may not be called with parenthesis
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
object Test:
extension [A](a: A) def render: String = "Hi"
extension [B](b: B) def render(using DummyImplicit): Char = 'x'
Test.render(1)Example Error Output
-- [E051] Reference Error: examples/051_AmbiguousOverloadID.scala:7:7
7 | Test.render(1)
| ^^^^^^^^^^^
|Ambiguous overload. The overloaded alternatives of method render in object Test with types
| [B](b: B)(using x$2: DummyImplicit): Char
| [A](a: A): String
|both match arguments ((1 : Int))
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| There are 2 methods that could be referenced as the compiler knows too little
| about the expected type.
| You may specify the expected type e.g. by
| - assigning it to a value with a specified type, or
| - adding a type ascription as in instance.myMethod: String => Int
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
object Foo:
val a = 1
Foo.a = 2Example Error Output
-- [E052] Type Error: examples/052_ReassignmentToValID.scala:13:8
13 | Foo.a = 2
| ^^^^^^^^^
| Reassignment to val a
|----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| You can not assign a new value to a as values can't be changed.
| Keep in mind that every statement has a value, so you may e.g. use
| val a = if (condition) 2 else 5
| In case you need a reassignable name, you can declare it as
| variable
| var a = ...
----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
val hello: String[String] = ???Example Error Output
-- [E053] Type Error: examples/053_TypeDoesNotTakeParametersID.scala:3:13
3 | val hello: String[String] = ???
| ^^^^^^^^^^^^^^
| String does not take type parameters
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| You specified �[0ma type parameter Ident(String)�[0m for String, which is not
| declared to take any.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
NOTE: This error is no longer emitted by the compiler.
Erroneous Code Example
trait Foo(x: Int)
val foo = new Foo {}Erroneous Code Example
class Foo(val a: => Boolean)Example Error Output
-- [E055] Syntax Error: examples/055_VarValParametersMayNotBeCallByNameID.scala:3:19
3 | class Foo(val a: => Boolean)
| ^^
| val parameters may not be call-by-name
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| var and val parameters of classes and traits may no be call-by-name. In case you
| want the parameter to be evaluated on demand, consider making it just a parameter
| and a def in the class such as
| class MyClass(aTick: => String) {
| def a() = aTick
| }
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
trait X[F[_] <: AnyKind] { type L = F[Int]; def a: L = ??? } // error: cannot be used as a value typeExample Error Output
-- [E056] Syntax Error: examples/056_MissingTypeParameterForID.scala:3:53
3 | trait X[F[_] <: AnyKind] { type L = F[Int]; def a: L = ??? } // error: cannot be used as a value type
| ^
| X.this.L cannot be used as a value type
1 error found
Errors encountered during compilation
Erroneous Code Example
trait Animal
class Dog extends Animal
class Cup
class Foo[A <: Animal]
Foo[Cup]Example Error Output
-- [E057] Type Mismatch Error: examples/057_DoesNotConformToBoundID.scala:8:6
8 | Foo[Cup]
| ^
| Type argument Cup does not conform to upper bound Animal
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| I tried to show that
| Cup
| conforms to
| Animal
| but none of the attempts shown below succeeded:
|
| ==> Cup <: Animal = false
|
| The tests were made under the empty constraint
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
trait User
trait Phone:
this: User =>
class Brand extends PhoneExample Error Output
-- [E058] Type Mismatch Error: examples/058_DoesNotConformToSelfTypeID.scala:6:8
6 | class Brand extends Phone
| ^
|illegal inheritance: self type Brand of class Brand does not conform to self type User
|of parent trait Phone
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| I tried to show that
| Brand
| conforms to
| User
| but none of the attempts shown below succeeded:
|
| ==> Brand <: User = false
|
| The tests were made under the empty constraint
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
trait A
class B {
self: A =>
}
val b = new BExample Error Output
-- [E059] Type Mismatch Error: examples/059_DoesNotConformToSelfTypeCantBeInstantiatedID.scala:9:14
9 | val b = new B
| ^
| B does not conform to its self type A; cannot be instantiated
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| I tried to show that
| B
| conforms to
| A
| but none of the attempts shown below succeeded:
|
| ==> B <: A = false
|
| The tests were made under the empty constraint
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
trait Foo:
final def bar: StringExample Error Output
-- [E060] Syntax Error: examples/060_AbstractMemberMayNotHaveModifierID.scala:4:16
4 | final def bar: String
| ^
| abstract method bar may not have `final` modifier
1 error found
Errors encountered during compilation
NOTE: This error is no longer emitted by the compiler.
In the past top level implicit were not allowed. This is however no longer the case as of scala/scala3#5754
Erroneous Code Example
implicit val foo: String = "foo"Erroneous Code Example
implicit trait FooExample Error Output
-- [E062] Syntax Error: examples/062_TypesAndTraitsCantBeImplicitID.scala:3:15
3 |implicit trait Foo
|^^^^^^^^^^^^^^^^^^
|implicit modifier cannot be used for types or traits
1 error found
Errors encountered during compilation
Erroneous Code Example
class Foo:
abstract def foo: UnitExample Error Output
-- [E063] Syntax Error: examples/063_OnlyClassesCanBeAbstractID.scala:4:15
4 | abstract def foo: Unit
| ^
|abstract modifier can be used only for classes; it should be omitted for abstract members
1 error found
Errors encountered during compilation
Erroneous Code Example
trait Foo:
def foo: Unit
abstract class Bar extends Foo:
abstract override def foo: UnitExample Error Output
-- [E064] Syntax Error: examples/064_AbstractOverrideOnlyInTraitsID.scala:7:26
7 | abstract override def foo: Unit
| ^
| abstract override modifier only allowed for members of traits
1 error found
Errors encountered during compilation
Erroneous Code Example
final trait FooExample Error Output
-- [E065] Syntax Error: examples/065_TraitsMayNotBeFinalID.scala:3:14
3 | final trait Foo
| ^
| trait Foo may not be final
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| A trait can never be final since it is abstract and must be extended to be useful.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
class Foo:
@native def foo: Unit = ()Example Error Output
-- [E066] Syntax Error: examples/066_NativeMembersMayNotHaveImplementationID.scala:4:16
4 | @native def foo: Unit = ()
| ^
| @native members may not have an implementation
1 error found
Errors encountered during compilation
Erroneous Code Example
object Foo:
def foo: UnitExample Error Output
-- [E067] Syntax Error: examples/067_OnlyClassesCanHaveDeclaredButUndefinedMembersID.scala:4:8
4 | def foo: Unit
| ^
|Declaration of method foo not allowed here: only classes can have declared but undefined members
1 error found
Errors encountered during compilation
Erroneous Code Example
trait Foo extends AnyValExample Error Output
-- [E068] Syntax Error: examples/068_CannotExtendAnyValID.scala:3:8
3 | trait Foo extends AnyVal
| ^
| trait Foo cannot extend AnyVal
|-----------------------------------------------------------------------------
| Explanation (enabled by `-explain`)
|- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Only classes (not traits) are allowed to extend AnyVal, but traits may extend
| Any to become �[32m"universal traits"�[0m which may only have def members.
| Universal traits can be mixed into classes that extend AnyVal.
-----------------------------------------------------------------------------
1 error found
Errors encountered during compilation
Erroneous Code Example
class Base:
class Inner
class Other extends Base:
class InnerExample Error Output
-- [E069] Naming Error: examples/069_CannotHaveSameNameAsID.scala:7:10
7 | class Inner
| ^
|class Inner cannot have the same name as class Inner in class Base -- class definitions cannot be overridden
1 error found
Errors encountered during compilation
Erroneous Code Example
class Foo(a: Int) extends AnyVal:
class InnerExample Error Output
-- [E070] Syntax Error: examples/070_ValueClassesMayNotDefineInnerID.scala:5:8
5 | class Inner
| ^^^^^^^^^^^
| Value classes may not define an inner class
1 error found
Errors encountered during compilation
Erroneous Code Example
trait Foo(x: Int)
val foo = new Foo {}Example Error Output
-- [E171] Type Error: examples/171_MissingArgumentID.scala:4:16
4 | val foo = new Foo {}
| ^^^
|missing argument for parameter x of constructor Foo in trait Foo: (x: Int): Foo
1 error found
Errors encountered during compilation