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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
                    |  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 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 ‘}’
                    |  ‘'’ ‘[’ 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          ::=  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        ::=  ‘@’ 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} 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
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         ::=  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
                    |  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