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Scala 3 Language Reference

Scala 3 Syntax Summary

Language

The following description of Scala tokens uses literal characters ‘c’ when referring to the ASCII fragment \u0000\u007F.

Unicode escapes are used to represent the Unicode character with the given hexadecimal code:

UnicodeEscape ::= ‘\’ ‘u’ {‘u’} hexDigit hexDigit hexDigit hexDigit
hexDigit      ::= ‘0’ | … | ‘9’ | ‘A’ | … | ‘F’ | ‘a’ | … | ‘f’

Informal descriptions are typeset as “some comment”.

Lexical Syntax

The lexical syntax of Scala is given by the following grammar in EBNF form.

whiteSpace       ::=  ‘\u0020’ | ‘\u0009’ | ‘\u000D’ | ‘\u000A’
upper            ::=  ‘A’ | … | ‘Z’ | ‘\$’ | ‘_’  “… and Unicode category Lu”
lower            ::=  ‘a’ | … | ‘z’ “… and Unicode category Ll”
letter           ::=  upper | lower “… and Unicode categories Lo, Lt, Nl”
digit            ::=  ‘0’ | … | ‘9’
paren            ::=  ‘(’ | ‘)’ | ‘[’ | ‘]’ | ‘{’ | ‘}’ | ‘'(’ | ‘'[’ | ‘'{’
delim            ::=  ‘`’ | ‘'’ | ‘"’ | ‘.’ | ‘;’ | ‘,’
opchar           ::=  “printableChar not matched by (whiteSpace | upper |
                       lower | letter | digit | paren | delim | opchar |
                       Unicode_Sm | Unicode_So)”
printableChar    ::=  “all characters in [\u0020, \u007F] inclusive”
charEscapeSeq    ::=  ‘\’ (‘b’ | ‘t’ | ‘n’ | ‘f’ | ‘r’ | ‘"’ | ‘'’ | ‘\’)

op               ::=  opchar {opchar}
varid            ::=  lower idrest
alphaid          ::=  upper idrest
                   |  varid
plainid          ::=  alphaid
                   |  op
id               ::=  plainid
                   |  ‘`’ { charNoBackQuoteOrNewline | UnicodeEscape | charEscapeSeq } ‘`’
idrest           ::=  {letter | digit} [‘_’ op]
quoteId          ::=  ‘'’ alphaid

integerLiteral   ::=  (decimalNumeral | hexNumeral) [‘L’ | ‘l’]
decimalNumeral   ::=  ‘0’ | nonZeroDigit [{digit | ‘_’} digit]
hexNumeral       ::=  ‘0’ (‘x’ | ‘X’) hexDigit [{hexDigit | ‘_’} hexDigit]
nonZeroDigit     ::=  ‘1’ | … | ‘9’

floatingPointLiteral
                 ::=  [decimalNumeral] ‘.’ digit [{digit | ‘_’} digit] [exponentPart] [floatType]
                   |  decimalNumeral exponentPart [floatType]
                   |  decimalNumeral floatType
exponentPart     ::=  (‘E’ | ‘e’) [‘+’ | ‘-’] digit [{digit | ‘_’} digit]
floatType        ::=  ‘F’ | ‘f’ | ‘D’ | ‘d’

booleanLiteral   ::=  ‘true’ | ‘false’

characterLiteral ::=  ‘'’ (printableChar | charEscapeSeq) ‘'’

stringLiteral    ::=  ‘"’ {stringElement} ‘"’
                   |  ‘"""’ multiLineChars ‘"""’
stringElement    ::=  printableChar \ (‘"’ | ‘\’)
                   |  UnicodeEscape
                   |  charEscapeSeq
multiLineChars   ::=  {[‘"’] [‘"’] char \ ‘"’} {‘"’}
processedStringLiteral
                 ::=  alphaid ‘"’ {[‘\’] processedStringPart | ‘\\’ | ‘\"’} ‘"’
                   |  alphaid ‘"""’ {[‘"’] [‘"’] char \ (‘"’ | ‘$’) | escape} {‘"’} ‘"""’
processedStringPart
                 ::= printableChar \ (‘"’ | ‘$’ | ‘\’) | escape
escape           ::=  ‘$$’
                   |  ‘$’ letter { letter | digit }
                   |  ‘{’ Block  [‘;’ whiteSpace stringFormat whiteSpace] ‘}’
stringFormat     ::=  {printableChar \ (‘"’ | ‘}’ | ‘ ’ | ‘\t’ | ‘\n’)}

symbolLiteral    ::=  ‘'’ plainid // until 2.13

comment          ::=  ‘/*’ “any sequence of characters; nested comments are allowed” ‘*/’
                   |  ‘//’ “any sequence of characters up to end of line”

nl               ::=  “new line character”
semi             ::=  ‘;’ |  nl {nl}

Optional Braces

The lexical analyzer also inserts indent and outdent tokens that represent regions of indented code at certain points.

In the context-free productions below we use the notation <<< ts >>> to indicate a token sequence ts that is either enclosed in a pair of braces { ts } or that constitutes an indented region indent ts outdent. Analogously, the notation :<<< ts >>> indicates a token sequence ts that is either enclosed in a pair of braces { ts } or that constitutes an indented region indent ts outdent that follows a : at the end of a line.

 <<< ts >>>   ::=  ‘{’ ts ‘}’
                |  indent ts outdent
:<<< ts >>>   ::=  [nl] ‘{’ ts ‘}’
                |  `:` indent ts outdent

Keywords

Regular keywords

abstract  case      catch     class     def       do        else
enum      export    extends   false     final     finally   for
given     if        implicit  import    lazy      match     new
null      object    override  package   private   protected return
sealed    super     then      throw     trait     true      try
type      val       var       while     with      yield
:         =         <-        =>        <:        >:        #
@         =>>       ?=>

Soft keywords

as  derives  end  extension  infix  inline  opaque  open  transparent  using  |  *  +  -

See the separate section on soft keywords for additional details on where a soft keyword is recognized.

Context-free Syntax

The context-free syntax of Scala is given by the following EBNF grammar:

Literals and Paths

SimpleLiteral     ::=  [‘-’] integerLiteral
                    |  [‘-’] floatingPointLiteral
                    |  booleanLiteral
                    |  characterLiteral
                    |  stringLiteral
Literal           ::=  SimpleLiteral
                    |  processedStringLiteral
                    |  symbolLiteral
                    |  ‘null’

QualId            ::=  id {‘.’ id}
ids               ::=  id {‘,’ id}

SimpleRef         ::=  id
                    |  [id ‘.’] ‘this’
                    |  [id ‘.’] ‘super’ [ClassQualifier] ‘.’ id

ClassQualifier    ::=  ‘[’ id ‘]’

Types

Type              ::=  FunType
                    |  HkTypeParamClause ‘=>>’ Type
                    |  FunParamClause ‘=>>’ Type
                    |  MatchType
                    |  InfixType
FunType           ::=  FunTypeArgs (‘=>’ | ‘?=>’) Type
                    |  HKTypeParamClause '=>' Type
FunTypeArgs       ::=  InfixType
                    |  ‘(’ [ FunArgTypes ] ‘)’
                    |  FunParamClause
FunParamClause    ::=  ‘(’ TypedFunParam {‘,’ TypedFunParam } ‘)’
TypedFunParam     ::=  id ‘:’ Type
MatchType         ::=  InfixType `match` <<< TypeCaseClauses >>>
InfixType         ::=  RefinedType {id [nl] RefinedType}
RefinedType       ::=  AnnotType {[nl] Refinement}
AnnotType         ::=  SimpleType {Annotation}

SimpleType        ::=  SimpleLiteral
                    |  ‘?’ TypeBounds
                    |  id
                    |  Singleton ‘.’ id
                    |  Singleton ‘.’ ‘type’
                    |  ‘(’ Types ‘)’
                    |  Refinement
                    |  ‘$’ ‘{’ Block ‘}’
                    |  SimpleType1 TypeArgs
                    |  SimpleType1 ‘#’ id
Singleton         ::=  SimpleRef
                    |  SimpleLiteral
                    |  Singleton ‘.’ id

FunArgType        ::=  Type
                    |  ‘=>’ Type
FunArgTypes       ::=  FunArgType { ‘,’ FunArgType }
ParamType         ::=  [‘=>’] ParamValueType
ParamValueType    ::=  Type [‘*’]
TypeArgs          ::=  ‘[’ Types ‘]’
Refinement        ::=  ‘{’ [RefineDcl] {semi [RefineDcl]} ‘}’
TypeBounds        ::=  [‘>:’ Type] [‘<:’ Type]
TypeParamBounds   ::=  TypeBounds {‘:’ Type}
Types             ::=  Type {‘,’ Type}

Expressions

Expr              ::=  FunParams (‘=>’ | ‘?=>’) Expr
                    |  HkTypeParamClause ‘=>’ Expr
                    |  Expr1
BlockResult       ::=  FunParams (‘=>’ | ‘?=>’) Block
                    |  HkTypeParamClause ‘=>’ Block
                    |  Expr1
FunParams         ::=  Bindings
                    |  id
                    |  ‘_’
Expr1             ::=  [‘inline’] ‘if’ ‘(’ Expr ‘)’ {nl} Expr [[semi] ‘else’ Expr]
                    |  [‘inline’] ‘if’  Expr ‘then’ Expr [[semi] ‘else’ Expr]
                    |  ‘while’ ‘(’ Expr ‘)’ {nl} Expr
                    |  ‘while’ Expr ‘do’ Expr
                    |  ‘try’ Expr Catches [‘finally’ Expr]
                    |  ‘try’ Expr [‘finally’ Expr]
                    |  ‘throw’ Expr
                    |  ‘return’ [Expr]
                    |  ForExpr
                    |  [SimpleExpr ‘.’] id ‘=’ Expr
                    |  SimpleExpr1 ArgumentExprs ‘=’ Expr
                    |  PostfixExpr [Ascription]
                    |  ‘inline’ InfixExpr MatchClause
Ascription        ::=  ‘:’ InfixType
                    |  ‘:’ Annotation {Annotation}
Catches           ::=  ‘catch’ (Expr | ExprCaseClause)
PostfixExpr       ::=  InfixExpr [id]                                          -- only if language.postfixOperators is enabled
InfixExpr         ::=  PrefixExpr
                    |  InfixExpr id [nl] InfixExpr
                    |  InfixExpr MatchClause
MatchClause       ::=  ‘match’ <<< CaseClauses >>>
PrefixExpr        ::=  [‘-’ | ‘+’ | ‘~’ | ‘!’] SimpleExpr
SimpleExpr        ::=  SimpleRef
                    |  Literal
                    |  ‘_’
                    |  BlockExpr
                    |  ‘$’ ‘{’ Block ‘}’
                    |  Quoted
                    |  quoteId                                                  -- only inside splices
                    |  ‘new’ ConstrApp {‘with’ ConstrApp} [TemplateBody]
                    |  ‘new’ TemplateBody
                    |  ‘(’ ExprsInParens ‘)’
                    |  SimpleExpr ‘.’ id
                    |  SimpleExpr ‘.’ MatchClause
                    |  SimpleExpr TypeArgs
                    |  SimpleExpr ArgumentExprs
Quoted            ::=  ‘'’ ‘{’ Block ‘}’
                    |  ‘'’ ‘[’ Type ‘]’
ExprsInParens     ::=  ExprInParens {‘,’ ExprInParens}
ExprInParens      ::=  PostfixExpr ‘:’ Type
                    |  Expr
ParArgumentExprs  ::=  ‘(’ [‘using’] ExprsInParens ‘)’
                    |  ‘(’ [ExprsInParens ‘,’] PostfixExpr ‘*’ ‘)’
ArgumentExprs     ::=  ParArgumentExprs
                    |  BlockExpr
BlockExpr         ::=  <<< (CaseClauses | Block) >>>
Block             ::=  {BlockStat semi} [BlockResult]
BlockStat         ::=  Import
                    |  {Annotation {nl}} {LocalModifier} Def
                    |  Extension
                    |  Expr1
                    |  EndMarker

ForExpr           ::=  ‘for’ ‘(’ Enumerators0 ‘)’ {nl} [‘do‘ | ‘yield’] Expr
                    |  ‘for’ ‘{’ Enumerators0 ‘}’ {nl} [‘do‘ | ‘yield’] Expr
                    |  ‘for’     Enumerators0          (‘do‘ | ‘yield’) Expr
Enumerators0      ::=  {nl} Enumerators [semi]
Enumerators       ::=  Generator {semi Enumerator | Guard}
Enumerator        ::=  Generator
                    |  Guard
                    |  Pattern1 ‘=’ Expr
Generator         ::=  [‘case’] Pattern1 ‘<-’ Expr
Guard             ::=  ‘if’ PostfixExpr

CaseClauses       ::=  CaseClause { CaseClause }
CaseClause        ::=  ‘case’ Pattern [Guard] ‘=>’ Block
ExprCaseClause    ::=  ‘case’ Pattern [Guard] ‘=>’ Expr
TypeCaseClauses   ::=  TypeCaseClause { TypeCaseClause }
TypeCaseClause    ::=  ‘case’ InfixType ‘=>’ Type [nl]

Pattern           ::=  Pattern1 { ‘|’ Pattern1 }
Pattern1          ::=  Pattern2 [‘:’ RefinedType]
Pattern2          ::=  [id ‘@’] InfixPattern [‘*’]
InfixPattern      ::=  SimplePattern { id [nl] SimplePattern }
SimplePattern     ::=  PatVar
                    |  Literal
                    |  ‘(’ [Patterns] ‘)’
                    |  Quoted
                    |  SimplePattern1 [TypeArgs] [ArgumentPatterns]
                    |  ‘given’ RefinedType
SimplePattern1    ::=  SimpleRef
                    |  SimplePattern1 ‘.’ id
PatVar            ::=  varid
                    |  ‘_’
Patterns          ::=  Pattern {‘,’ Pattern}
ArgumentPatterns  ::=  ‘(’ [Patterns] ‘)’
                    |  ‘(’ [Patterns ‘,’] PatVar ‘*’ ‘)’

Type and Value Parameters

ClsTypeParamClause::=  ‘[’ ClsTypeParam {‘,’ ClsTypeParam} ‘]’
ClsTypeParam      ::=  {Annotation} [‘+’ | ‘-’] id [HkTypeParamClause] TypeParamBounds

DefTypeParamClause::=  ‘[’ DefTypeParam {‘,’ DefTypeParam} ‘]’
DefTypeParam      ::=  {Annotation} id [HkTypeParamClause] TypeParamBounds

TypTypeParamClause::=  ‘[’ TypTypeParam {‘,’ TypTypeParam} ‘]’
TypTypeParam      ::=  {Annotation} id [HkTypeParamClause] TypeBounds

HkTypeParamClause ::=  ‘[’ HkTypeParam {‘,’ HkTypeParam} ‘]’
HkTypeParam       ::=  {Annotation} [‘+’ | ‘-’] (id [HkTypeParamClause] | ‘_’) TypeBounds

ClsParamClauses   ::=  {ClsParamClause} [[nl] ‘(’ [‘implicit’] ClsParams ‘)’]
ClsParamClause    ::=  [nl] ‘(’ ClsParams ‘)’
                    |  [nl] ‘(’ ‘using’ (ClsParams | FunArgTypes) ‘)’
ClsParams         ::=  ClsParam {‘,’ ClsParam}
ClsParam          ::=  {Annotation} [{Modifier} (‘val’ | ‘var’) | ‘inline’] Param
Param             ::=  id ‘:’ ParamType [‘=’ Expr]

DefParamClauses   ::=  {DefParamClause} [[nl] ‘(’ [‘implicit’] DefParams ‘)’]
DefParamClause    ::=  [nl] ‘(’ DefParams ‘)’ | UsingParamClause
UsingParamClause  ::=  [nl] ‘(’ ‘using’ (DefParams | FunArgTypes) ‘)’
DefParams         ::=  DefParam {‘,’ DefParam}
DefParam          ::=  {Annotation} [‘inline’] Param

Bindings and Imports

Bindings          ::=  ‘(’ [Binding {‘,’ Binding}] ‘)’
Binding           ::=  (id | ‘_’) [‘:’ Type]

Modifier          ::=  LocalModifier
                    |  AccessModifier
                    |  ‘override’
                    |  ‘opaque’
LocalModifier     ::=  ‘abstract’
                    |  ‘final’
                    |  ‘sealed’
                    |  ‘open’
                    |  ‘implicit’
                    |  ‘lazy’
                    |  ‘inline’
AccessModifier    ::=  (‘private’ | ‘protected’) [AccessQualifier]
AccessQualifier   ::=  ‘[’ id ‘]’

Annotation        ::=  ‘@’ SimpleType1 {ParArgumentExprs}

Import            ::=  ‘import’ ImportExpr {‘,’ ImportExpr}
Export            ::=  ‘export’ ImportExpr {‘,’ ImportExpr}
ImportExpr        ::=  SimpleRef {‘.’ id} ‘.’ ImportSpec
                    |  SimpleRef ‘as’ id
ImportSpec        ::=  NamedSelector
                    |  WildcardSelector
                    | ‘{’ ImportSelectors) ‘}’
NamedSelector     ::=  id [‘as’ (id | ‘_’)]
WildCardSelector  ::=  ‘*' | ‘given’ [InfixType]
ImportSelectors   ::=  NamedSelector [‘,’ ImportSelectors]
                    |  WildCardSelector {‘,’ WildCardSelector}

EndMarker         ::=  ‘end’ EndMarkerTag    -- when followed by EOL
EndMarkerTag      ::=  id | ‘if’ | ‘while’ | ‘for’ | ‘match’ | ‘try’
                    |  ‘new’ | ‘this’ | ‘given’ | ‘extension’ | ‘val’

Declarations and Definitions

RefineDcl         ::=  ‘val’ ValDcl
                    |  ‘def’ DefDcl
                    |  ‘type’ {nl} TypeDcl
Dcl               ::=  RefineDcl
                    |  ‘var’ VarDcl
ValDcl            ::=  ids ‘:’ Type
VarDcl            ::=  ids ‘:’ Type
DefDcl            ::=  DefSig ‘:’ Type
DefSig            ::=  id [DefTypeParamClause] DefParamClauses
TypeDcl           ::=  id [TypeParamClause] {FunParamClause} TypeBounds [‘=’ Type]

Def               ::=  ‘val’ PatDef
                    |  ‘var’ PatDef
                    |  ‘def’ DefDef
                    |  ‘type’ {nl} TypeDcl
                    |  TmplDef
PatDef            ::=  ids [‘:’ Type] ‘=’ Expr
                    |  Pattern2 [‘:’ Type] ‘=’ Expr
DefDef            ::=  DefSig [‘:’ Type] ‘=’ Expr
                    |  ‘this’ DefParamClause DefParamClauses ‘=’ ConstrExpr

TmplDef           ::=  ([‘case’] ‘class’ | ‘trait’) ClassDef
                    |  [‘case’] ‘object’ ObjectDef
                    |  ‘enum’ EnumDef
                    |  ‘given’ GivenDef
ClassDef          ::=  id ClassConstr [Template]
ClassConstr       ::=  [ClsTypeParamClause] [ConstrMods] ClsParamClauses
ConstrMods        ::=  {Annotation} [AccessModifier]
ObjectDef         ::=  id [Template]
EnumDef           ::=  id ClassConstr InheritClauses EnumBody
GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | StructuralInstance)
GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClause} ‘:’         -- one of `id`, `DefParamClause`, `UsingParamClause` must be present
StructuralInstance ::=  ConstrApp {‘with’ ConstrApp} [‘with’ TemplateBody]
Extension         ::=  ‘extension’ [DefTypeParamClause] {UsingParamClause}
                       ‘(’ DefParam ‘)’ {UsingParamClause} ExtMethods
ExtMethods        ::=  ExtMethod | [nl] <<< ExtMethod {semi ExtMethod} >>>
ExtMethod         ::=  {Annotation [nl]} {Modifier} ‘def’ DefDef
Template          ::=  InheritClauses [TemplateBody]
InheritClauses    ::=  [‘extends’ ConstrApps] [‘derives’ QualId {‘,’ QualId}]
ConstrApps        ::=  ConstrApp ({‘,’ ConstrApp} | {‘with’ ConstrApp})
ConstrApp         ::=  SimpleType1 {Annotation} {ParArgumentExprs}
ConstrExpr        ::=  SelfInvocation
                    |  <<< SelfInvocation {semi BlockStat} >>>
SelfInvocation    ::=  ‘this’ ArgumentExprs {ArgumentExprs}

TemplateBody      ::=  :<<< [SelfType] TemplateStat {semi TemplateStat} >>>
TemplateStat      ::=  Import
                    |  Export
                    |  {Annotation [nl]} {Modifier} Def
                    |  {Annotation [nl]} {Modifier} Dcl
                    |  Extension
                    |  Expr1
                    |  EndMarker
                    |
SelfType          ::=  id [‘:’ InfixType] ‘=>’
                    |  ‘this’ ‘:’ InfixType ‘=>’

EnumBody          ::=  :<<< [SelfType] EnumStat {semi EnumStat} >>>
EnumStat          ::=  TemplateStat
                    |  {Annotation [nl]} {Modifier} EnumCase
EnumCase          ::=  ‘case’ (id ClassConstr [‘extends’ ConstrApps]] | ids)

TopStats          ::=  TopStat {semi TopStat}
TopStat           ::=  Import
                    |  Export
                    |  {Annotation [nl]} {Modifier} Def
                    |  Extension
                    |  Packaging
                    |  PackageObject
                    |  EndMarker
                    |
Packaging         ::=  ‘package’ QualId :<<< TopStats >>>
PackageObject     ::=  ‘package’ ‘object’ ObjectDef

CompilationUnit   ::=  {‘package’ QualId semi} TopStats

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