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 ::= ‘!’ | ‘#’ | ‘%’ | ‘&’ | ‘*’ | ‘+’ | ‘-’ | ‘/’ | ‘:’ |
‘<’ | ‘=’ | ‘>’ | ‘?’ | ‘@’ | ‘\’ | ‘^’ | ‘|’ | ‘~’
“… and Unicode categories Sm, So”
printableChar ::= “all characters in [\u0020, \u007E] 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
spliceId ::= ‘$’ alphaid ;
integerLiteral ::= (decimalNumeral | hexNumeral) [‘L’ | ‘l’]
decimalNumeral ::= ‘0’ | nonZeroDigit [{digit | ‘_’} digit]
hexNumeral ::= ‘0’ (‘x’ | ‘X’) hexDigit [{hexDigit | ‘_’} hexDigit]
nonZeroDigit ::= ‘1’ | … | ‘9’
floatingPointLiteral
::= [decimalNumeral] ‘.’ digit [{digit | ‘_’} digit] [exponentPart] [floatType]
| decimalNumeral exponentPart [floatType]
| decimalNumeral floatType
exponentPart ::= (‘E’ | ‘e’) [‘+’ | ‘-’] digit [{digit | ‘_’} digit]
floatType ::= ‘F’ | ‘f’ | ‘D’ | ‘d’
booleanLiteral ::= ‘true’ | ‘false’
characterLiteral ::= ‘'’ (printableChar | charEscapeSeq) ‘'’
stringLiteral ::= ‘"’ {stringElement} ‘"’
| ‘"""’ multiLineChars ‘"""’
stringElement ::= printableChar \ (‘"’ | ‘\’)
| UnicodeEscape
| charEscapeSeq
multiLineChars ::= {[‘"’] [‘"’] char \ ‘"’} {‘"’}
processedStringLiteral
::= alphaid ‘"’ {[‘\’] processedStringPart | ‘\\’ | ‘\"’} ‘"’
| alphaid ‘"""’ {[‘"’] [‘"’] char \ (‘"’ | ‘$’) | 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 colon token.
A colon token reads as the standard colon ":" but is generated instead of it where colon is legal according to the context free syntax, but only if the previous token is an alphanumeric identifier, a backticked identifier, or one of the tokens this, super, new, ")", and "]".
colon ::= ':' -- with side conditions explained above
<<< ts >>> ::= ‘{’ ts ‘}’
| indent ts outdent
:<<< ts >>> ::= [nl] ‘{’ ts ‘}’
| colon 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
| 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
| PrefixOperator SimpleExpr ‘=’ Expr
| SimpleExpr 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 ::= [PrefixOperator] SimpleExpr
PrefixOperator ::= ‘-’ | ‘+’ | ‘~’ | ‘!’ -- unless backquoted
SimpleExpr ::= SimpleRef
| Literal
| ‘_’
| BlockExpr
| ExprSplice
| Quoted
| quoteId -- only inside splices
| ‘new’ ConstrApp {‘with’ ConstrApp} [TemplateBody]
| ‘new’ TemplateBody
| ‘(’ ExprsInParens ‘)’
| SimpleExpr ‘.’ id
| SimpleExpr ‘.’ MatchClause
| SimpleExpr TypeArgs
| SimpleExpr ArgumentExprs
Quoted ::= ‘'’ ‘{’ Block ‘}’
| ‘'’ ‘[’ Type ‘]’
ExprSplice ::= spliceId -- if inside quoted block
| ‘$’ ‘{’ Block ‘}’ -- unless inside quoted pattern
| ‘$’ ‘{’ Pattern ‘}’ -- when inside quoted pattern
ExprsInParens ::= ExprInParens {‘,’ ExprInParens}
ExprInParens ::= PostfixExpr ‘:’ Type
| Expr
ParArgumentExprs ::= ‘(’ [ExprsInParens] ‘)’
| ‘(’ ‘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 {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 [semi]
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’ WithTemplateBody]
Extension ::= ‘extension’ [DefTypeParamClause] {UsingParamClause}
‘(’ DefParam ‘)’ {UsingParamClause} ExtMethods
ExtMethods ::= ExtMethod | [nl] <<< ExtMethod {semi ExtMethod} >>>
ExtMethod ::= {Annotation [nl]} {Modifier} ‘def’ DefDef
| Export
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}
WithTemplateBody ::= <<< [SelfType] TemplateStat {semi TemplateStat} >>>
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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