1 2Grammar of Starlark 3================== 4 5File = {Statement | newline} eof . 6 7Statement = DefStmt | IfStmt | ForStmt | WhileStmt | SimpleStmt . 8 9DefStmt = 'def' identifier '(' [Parameters [',']] ')' ':' Suite . 10 11Parameters = Parameter {',' Parameter}. 12 13Parameter = identifier | identifier '=' Test | '*' | '*' identifier | '**' identifier . 14 15IfStmt = 'if' Test ':' Suite {'elif' Test ':' Suite} ['else' ':' Suite] . 16 17ForStmt = 'for' LoopVariables 'in' Expression ':' Suite . 18 19WhileStmt = 'while' Test ':' Suite . 20 21Suite = [newline indent {Statement} outdent] | SimpleStmt . 22 23SimpleStmt = SmallStmt {';' SmallStmt} [';'] '\n' . 24# NOTE: '\n' optional at EOF 25 26SmallStmt = ReturnStmt 27 | BreakStmt | ContinueStmt | PassStmt 28 | AssignStmt 29 | ExprStmt 30 | LoadStmt 31 . 32 33ReturnStmt = 'return' [Expression] . 34BreakStmt = 'break' . 35ContinueStmt = 'continue' . 36PassStmt = 'pass' . 37AssignStmt = Expression ('=' | '+=' | '-=' | '*=' | '/=' | '//=' | '%=' | '&=' | '|=' | '^=' | '<<=' | '>>=') Expression . 38ExprStmt = Expression . 39 40LoadStmt = 'load' '(' string {',' [identifier '='] string} [','] ')' . 41 42Test = LambdaExpr 43 | IfExpr 44 | PrimaryExpr 45 | UnaryExpr 46 | BinaryExpr 47 . 48 49LambdaExpr = 'lambda' [Parameters] ':' Test . 50 51IfExpr = Test 'if' Test 'else' Test . 52 53PrimaryExpr = Operand 54 | PrimaryExpr DotSuffix 55 | PrimaryExpr CallSuffix 56 | PrimaryExpr SliceSuffix 57 . 58 59Operand = identifier 60 | int | float | string 61 | ListExpr | ListComp 62 | DictExpr | DictComp 63 | '(' [Expression [',']] ')' 64 | ('-' | '+') PrimaryExpr 65 . 66 67DotSuffix = '.' identifier . 68CallSuffix = '(' [Arguments [',']] ')' . 69SliceSuffix = '[' [Expression] [':' Test [':' Test]] ']' . 70 71Arguments = Argument {',' Argument} . 72Argument = Test | identifier '=' Test | '*' Test | '**' Test . 73 74ListExpr = '[' [Expression [',']] ']' . 75ListComp = '[' Test {CompClause} ']'. 76 77DictExpr = '{' [Entries [',']] '}' . 78DictComp = '{' Entry {CompClause} '}' . 79Entries = Entry {',' Entry} . 80Entry = Test ':' Test . 81 82CompClause = 'for' LoopVariables 'in' Test | 'if' Test . 83 84UnaryExpr = 'not' Test . 85 86BinaryExpr = Test {Binop Test} . 87 88Binop = 'or' 89 | 'and' 90 | '==' | '!=' | '<' | '>' | '<=' | '>=' | 'in' | 'not' 'in' 91 | '|' 92 | '^' 93 | '&' 94 | '-' | '+' 95 | '*' | '%' | '/' | '//' 96 . 97 98Expression = Test {',' Test} . 99# NOTE: trailing comma permitted only when within [...] or (...). 100 101LoopVariables = PrimaryExpr {',' PrimaryExpr} . 102 103 104# Notation (similar to Go spec): 105- lowercase and 'quoted' items are lexical tokens. 106- Capitalized names denote grammar productions. 107- (...) implies grouping 108- x | y means either x or y. 109- [x] means x is optional 110- {x} means x is repeated zero or more times 111- The end of each declaration is marked with a period. 112 113# Tokens 114- spaces: newline, eof, indent, outdent. 115- identifier. 116- literals: string, int, float. 117- plus all quoted tokens such as '+=', 'return'. 118 119# Notes: 120- Ambiguity is resolved using operator precedence. 121- The grammar does not enforce the legal order of params and args, 122 nor that the first compclause must be a 'for'. 123 124TODO: 125- explain how the lexer generates indent, outdent, and newline tokens. 126- why is unary NOT separated from unary - and +? 127- the grammar is (mostly) in LL(1) style so, for example, 128 dot expressions are formed suffixes, not complete expressions, 129 which makes the spec harder to read. Reorganize into non-LL(1) form? 130