Scala 3 Syntax Summary
The following description of Scala tokens uses literal characters ‘c’ when referring to the ASCII fragment \u0000 – \u007F.
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’ | ‘$’ and any character in Unicode categories Lu, Lt or Nl,
and any character in Unicode categories Lo and Lm that doesn't have
contributory property Other_Lowercase
lower ::= ‘a’ | ... | ‘z’ | ‘_’ and any character in Unicode category Ll,
and any character in Unicode categories Lo or Lm that has contributory
property Other_Lowercase
letter ::= upper | lower
digit ::= ‘0’ | ... | ‘9’
paren ::= ‘(’ | ‘)’ | ‘[’ | ‘]’ | ‘{’ | ‘}’
delim ::= ‘`’ | ‘'’ | ‘"’ | ‘.’ | ‘;’ | ‘,’
opchar ::= ‘!’ | ‘#’ | ‘%’ | ‘&’ | ‘*’ | ‘+’ | ‘-’ | ‘/’ | ‘:’ |
‘<’ | ‘=’ | ‘>’ | ‘?’ | ‘@’ | ‘\’ | ‘^’ | ‘|’ | ‘~’
and any character in Unicode categories Sm or So
printableChar ::= all characters in [\u0020, \u007E] inclusive
UnicodeEscape ::= ‘\’ ‘u’ {‘u’} hexDigit hexDigit hexDigit hexDigit
hexDigit ::= ‘0’ | ... | ‘9’ | ‘A’ | ... | ‘F’ | ‘a’ | ... | ‘f’
charEscapeSeq ::= ‘\’ (‘b’ | ‘t’ | ‘n’ | ‘f’ | ‘r’ | ‘"’ | ‘'’ | ‘\’)
escapeSeq ::= UnicodeEscape | charEscapeSeq
op ::= opchar {opchar}
varid ::= lower idrest
boundvarid ::= varid
| ‘`’ varid ‘`’
plainid ::= alphaid
| op
id ::= plainid
| ‘`’ { charNoBackQuoteOrNewline | escapeSeq } ‘`’
idrest ::= {letter | digit} [‘_’ op]
quoteId ::= ‘'’ alphaid
spliceId ::= ‘$’ alphaid ;
integerLiteral ::= (decimalNumeral | hexNumeral) [‘L’ | ‘l’]
decimalNumeral ::= ‘0’ | digit [{digit | ‘_’} digit]
hexNumeral ::= ‘0’ (‘x’ | ‘X’) hexDigit [{hexDigit | ‘_’} hexDigit]
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 ::= ‘'’ (charNoQuoteOrNewline | escapeSeq) ‘'’
stringLiteral ::= ‘"’ {stringElement} ‘"’
| ‘"""’ multiLineChars ‘"""’
stringElement ::= charNoDoubleQuoteOrNewline
| escapeSeq
multiLineChars ::= {[‘"’] [‘"’] charNoDoubleQuote} {‘"’}
interpolatedString
::= alphaid ‘"’ {[‘\’] interpolatedStringPart | ‘\\’ | ‘\"’} ‘"’
| alphaid ‘"""’ {[‘"’] [‘"’] char \ (‘"’ | ‘\$’) | escape} {‘"’} ‘"""’
interpolatedStringPart
::= printableChar \ (‘"’ | ‘$’ | ‘\’) | escape
escape ::= ‘\$\$’
| ‘\$"’
| ‘\$’ alphaid
| ‘\$’ BlockExpr
alphaid ::= upper idrest
| varid
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 principle of optional braces is that any keyword that can be followed by { can also be followed by an indented block, without needing an intervening :. (Allowing an optional : would be counterproductive since it would introduce several ways to do the same thing.)
The lexical analyzer 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
| interpolatedStringLiteral
| 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
| SimpleType TypeArgs
| SimpleType ‘#’ 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 id ColonArgument
| 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
| SimpleExpr ColonArgument
ColonArgument ::= colon [LambdaStart]
indent (CaseClauses | Block) outdent
LambdaStart ::= FunParams (‘=>’ | ‘?=>’)
| HkTypeParamClause ‘=>’
Quoted ::= ‘'’ ‘{’ Block ‘}’
| ‘'’ ‘[’ TypeBlock ‘]’
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
TypeBlock ::= {TypeBlockStat semi} Type
TypeBlockStat ::= ‘type’ {nl} TypeDef
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 ::= PatVar ‘:’ RefinedType
| [‘-’] integerLiteral ‘:’ RefinedType
| [‘-’] floatingPointLiteral ‘:’ RefinedType
| Pattern2
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
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’)] Param
DefParamClauses ::= DefParamClause { DefParamClause } -- and two DefTypeParamClause cannot be adjacent
DefParamClause ::= DefTypeParamClause
| DefTermParamClause
| UsingParamClause
TypelessClauses ::= TypelessClause {TypelessClause}
TypelessClause ::= DefTermParamClause
| UsingParamClause
DefTypeParamClause::= [nl] ‘[’ DefTypeParam {‘,’ DefTypeParam} ‘]’
DefTypeParam ::= {Annotation} id [HkTypeParamClause] TypeParamBounds
DefTermParamClause::= [nl] ‘(’ [DefTermParams] ‘)’
UsingParamClause ::= [nl] ‘(’ ‘using’ (DefTermParams | FunArgTypes) ‘)’
DefImplicitClause ::= [nl] ‘(’ ‘implicit’ DefTermParams ‘)’
DefTermParams ::= DefTermParam {‘,’ DefTermParam}
DefTermParam ::= {Annotation} [‘inline’] Param
Param ::= id ‘:’ ParamType [‘=’ Expr]
Bindings and Imports
Bindings ::= ‘(’ [Binding {‘,’ Binding}] ‘)’
Binding ::= (id | ‘_’) [‘:’ Type]
Modifier ::= LocalModifier
| AccessModifier
| ‘override’
| ‘opaque’
LocalModifier ::= ‘abstract’
| ‘final’
| ‘sealed’
| ‘open’
| ‘implicit’
| ‘lazy’
| ‘inline’
| ‘transparent’
| ‘infix’
AccessModifier ::= (‘private’ | ‘protected’) [AccessQualifier]
AccessQualifier ::= ‘[’ id ‘]’
Annotation ::= ‘@’ SimpleType {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
| ‘var’ ValDcl
| ‘type’ {nl} TypeDef
ValDcl ::= ids ‘:’ Type
DefDcl ::= DefSig ‘:’ Type
Def ::= ‘val’ PatDef
| ‘var’ PatDef
| ‘def’ DefDef
| ‘type’ {nl} TypeDef
| TmplDef
PatDef ::= ids [‘:’ Type] [‘=’ Expr]
| Pattern2 [‘:’ Type] [‘=’ Expr] PatDef(_, pats, tpe?, expr)
DefDef ::= DefSig [‘:’ Type] [‘=’ Expr] DefDef(_, name, paramss, tpe, expr)
| ‘this’ TypelessClauses [DefImplicitClause] ‘=’ ConstrExpr DefDef(_, <init>, vparamss, EmptyTree, expr | Block)
DefSig ::= id [DefParamClauses] [DefImplicitClause]
TypeDef ::= id [TypeParamClause] {FunParamClause} TypeBounds TypeDefTree(_, name, tparams, bound
[‘=’ Type]
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`, `DefTypeParamClause`, `UsingParamClause` must be present
GivenType ::= AnnotType {id [nl] AnnotType}
StructuralInstance ::= ConstrApp {‘with’ ConstrApp} [‘with’ WithTemplateBody]
Extension ::= ‘extension’ [DefTypeParamClause] {UsingParamClause}
‘(’ DefTermParam ‘)’ {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 ::= SimpleType {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
| 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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