Scala 3 Syntax Summary
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 ::= FunArgTypes (‘=>’ | ‘?=>’) Type
| HKTypeParamClause '=>' Type
FunArgTypes ::= InfixType
| ‘(’ [ FunArgType {‘,’ FunArgType } ] ‘)’
| 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
ParamType ::= [‘=>’] ParamValueType
ParamValueType ::= Type [‘*’]
TypeArgs ::= ‘[’ Types ‘]’
Refinement ::= ‘{’ [RefineDcl] {semi [RefineDcl]} ‘}’
TypeBounds ::= [‘>:’ Type] [‘<:’ Type]
TypeParamBounds ::= TypeBounds {‘:’ Type}
Types ::= Type {‘,’ Type}
Expressions
Expr ::= FunParams (‘=>’ | ‘?=>’) Expr
| Expr1
BlockResult ::= FunParams (‘=>’ | ‘?=>’) 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
| HkTypeParamClause ‘=>’ Expr
| [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’ (‘(’ Enumerators ‘)’ | ‘{’ Enumerators ‘}’) {nl} [‘yield’] Expr
| ‘for’ Enumerators (‘do’ Expr | ‘yield’ Expr)
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 | Types) ‘)’
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 | Types) ‘)’
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] ‘(’ 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