# Syntax: A Casual and Formal Look

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## A Formal Look

We now shall take a look at a more formal description of Puck's syntax. Syntax rules are described in extended Backus–Naur form (EBNF) - but most rules surrounding whitespace, and scope, and line breaks, are modified to how they would appear after a lexing step (whitespace is removed, line breaks are normalized, scope is switched to use `{` and `}`).

### Identifiers
```
IDENT  ::= LETTER (LETTER | DIGIT | '_')*
LETTER ::= 'A'..'Z' | 'a'..'z' | '\x80'..'\xff' # todo
DIGIT  ::= '0'..'9'
```

### Literals
```
INT_LIT ::= '-'? (DEC_LIT | HEX_LIT | OCT_LIT | BIN_LIT)
BIN_LIT ::= '0b' BIN_DIGIT ('_'? BIN_DIGIT)*
OCT_LIT ::= '0o' OCT_DIGIT ('_'? OCT_DIGIT)*
HEX_LIT ::= '0x' HEX_DIGIT ('_'? HEX_DIGIT)*
DEC_LIT ::= DIGIT ('_'? DIGIT)*
BIN_DIGIT ::= '0'..'1'
OCT_DIGIT ::= '0'..'7'
HEX_DIGIT ::= DIGIT | 'A'..'F' | 'a'..'f'
```

### Operators
```
OPERATOR ::= 'and' | 'or' | 'not' | 'xor' | 'shl' | 'shr' | # todo: more?
             'div' | 'mod' | 'rem' | 'is' | 'isnot' | OPR+
OPR ::= '=' | '+' | '-' | '*' | '/' | '<' | '>' | # todo: more?
        '@' | '$' | '~' | '&' | '%' | '|' |
        '!' | '?' | '^' | '.' | ':' | '\\'
```

### Chars, Strings, and Comments
```
CHAR    ::= '\'' (PRINT - '\'' | '\\\'')* '\''
STRING  ::= SINGLE_LINE_STRING | MULTI_LINE_STRING
COMMENT ::= SINGLE_LINE_COMMENT | MULTI_LINE_COMMENT | EXPRESSION_COMMENT
SINGLE_LINE_STRING  ::= '"' (PRINT - '"' | '\\"')* '"'
MULTI_LINE_STRING   ::= '"""' (PRINT | '\n' | '\r')* '"""'
SINGLE_LINE_COMMENT ::= '#' PRINT*
MULTI_LINE_COMMENT  ::= '#[' (PRINT | '\n' | '\r' | MULTI_LINE_COMMENT)* ']#'
EXPRESSION_COMMENT  ::= '#;' SINGLE_STMT
PRINT ::= LETTER | DIGIT | OPR |
          '"' | '#' | "'" | '(' | ')' | # notably the dual of OPR
          ',' | ';' | '[' | ']' | '_' |
          '`' | '{' | '}' | ' ' | '\t'
```

### Values
```
VALUE ::= INT_LIT | STRING | CHAR |
LIST_DECL   ::= '[' (EXPR (',' EXPR)*)? ']'
ARRAY_DECL  ::= '[' (EXPR (',' EXPR)*)? ']'
TUPLE_DECL  ::= '(' (IDENT '=')? EXPR (',' (IDENT '=')? EXPR)* ')'
STRUCT_DECL ::= '{' IDENT '=' EXPR (',' IDENT '=' EXPR)* '}'
# note: no union or enum. should struct exist? only in a structural system.
```

### Variables
```
DECL       ::= LET_DECL | VAR_DECL | CONST_DECL
LET_DECL   ::= 'let' GROUP (':' TYPE_DESC)? '=' EXPR
VAR_DECL   ::= 'var' GROUP (':' TYPE_DESC)? ('=' EXPR)?
CONST_DECL ::= 'pub'? 'const' GROUP (':' TYPE_DESC)? '=' EXPR
GROUP      ::= ('(' IDENT (',' IDENT)* ')') | IDENT
```

## Types and Functions
```
FUNC_DECL ::= SIGNATURE '=' (EXPR | STMT)
SIGNATURE ::= 'pub'? ('pure' | 'yeet' | IDENT)? 'func' IDENT
              ('(' (PARAMETER (',' PARAMETER)?)? ')')?
              (':' TYPE_DESC)?
PARAMETER ::= (IDENT (':' ('var' | 'static')? TYPE_DESC)?
```

```
TYPE_DECL   ::= 'pub'? 'type' IDENT '=' 'ref'? 'distinct'? TYPE_DESC
TYPE_DESC   ::= TUPLE_TYPE | STRUCT_TYPE | UNION_TYPE | ENUM_TYPE | INTERFACE | IDENT
TUPLE_TYPE  ::= 'tuple' '[' (IDENT ':')? TYPE_DESC (',' (IDENT ':')? TYPE_DESC)* ']'
STRUCT_TYPE ::= 'struct' '[' 'pub'? IDENT ':' TYPE_DESC' (',' 'pub'? IDENT ':' 'TYPE_DESC')* ']'
# note: pub in structs conflicts w/ a structural system
UNION_TYPE  ::= 'union'  '[' IDENT ':' TYPE_DESC' (',' IDENT ':' 'TYPE_DESC')* ']'
ENUM_TYPE   ::= 'enum' '[' IDENT ('=' EXPR)? (',' IDENT ('=' EXPR)?)* ']'
FUNC_TYPE   ::= 'func' ('(' (PARAMETER (',' PARAMETER)?)? ')')? (':' TYPE_DESC)?
INTERFACE   ::= 'interface' '[' SIGNATURE (',' SIGNATURE)* ('for' TYPE_DESC)? ']'
```

## Control Flow
```
IF_EXPR    ::= 'if' EXPR ':' EXPR ('elif' EXPR ':' EXPR)* 'else' ':' EXPR
IF_STMT    ::= 'if' EXPR ':' STMT ('elif' EXPR ':' STMT)* ('else' ':' STMT)?
WHEN_EXPR  ::= 'when' EXPR ':' EXPR ('else' ':' EXPR)?
WHEN_STMT  ::= 'when' EXPR ':' STMT ('else' ':' STMT)?
BLOCK_EXPR ::= 'block' IDENT? ':' EXPR
BLOCK_STMT ::= 'block' IDENT? ':' STMT
MATCH_EXPR ::= 'match' EXPR ':'
               ('case' EXPR ('where' EXPR)? (',' EXPR ('where' EXPR)?)* ':' EXPR)+
MATCH_STMT ::= 'match' EXPR ':'
               ('case' EXPR ('where' EXPR)? (',' EXPR ('where' EXPR)?)* ':' STMT)+
LOOP_STMT  ::= 'loop' ':' STMT
WHILE_STMT ::= 'while' EXPR ':' STMT
FOR_STMT   ::= 'for' GROUP 'in' EXPR ':' STMT
```

## Modules
```
IMPORT_STMT ::= 'import' IDENT_AS?
                ('/' (IDENT_AS | '[' (IDENT_AS (',' IDENT_AS)*)? ']'))*
EXPORT_STMT ::= 'export' IDENT_AS?
                ('/' (IDENT_AS | '[' (IDENT_AS (',' IDENT_AS)*)? ']'))*
MODULE_STMT ::= 'module' IDENT ':' STMT
IDENT_AS    ::= IDENT ('as' IDENT)?
```

## Macros
```
MACRO_FUNC  ::= IDENT '(' EXPR ')'
MACRO_BLOCK ::= IDENT ':' EXPR # todo
```

## Calls, Statements, and Expressions
```
OPERATION   ::= EXPR OPERATOR EXPR
PREFIX      ::= OPERATOR EXPR
SUFFIX      ::= EXPR OPERATOR
APPLICATION ::= IDENT EXPR | IDENT PARAMS? | (IDENT | APPLICATION) '.' IDENT PARAMS?
PARAMS      ::= '(' ((IDENT '=')? EXPR (',' (IDENT '=')? EXPR)*)? ')'
```

```
STMT        ::= SINGLE_STMT | STMT+
SINGLE_STMT ::= IF_STMT | WHEN_STMT | BLOCK_STMT | MATCH_STMT |
                LOOP_STMT | WHILE_STMT | FOR_STMT |
                IMPORT_STMT | EXPORT_STMT | MODULE_STMT |
                TYPE_DECL | FUNC_DECL | (EXPR ';')
EXPR        ::= IF_EXPR | WHEN_EXPR | BLOCK_EXPR | MATCH_EXPR |
                MACRO_FUNC | MACRO_BLOCK |
                VALUE | APPLICATION | (STMT EXPR)
```