ref.md

#BNF ##RULES RootNode(aka Program) = Statement | Statement* ;

Statement = Let_Statement | Expression_Statement | Block_Statement | Return_Statement ;

  Let_Statement = Token_let Identifier Token_Assignment Expression ;
    ex) let foo = 20;


  Expression_Statement = Expression (;) ;
    ex) 20; , 20 + 30; , call_fn(foo)


  Block_Statement = { Statement* } ;


  Return_Statement = return (Expression) ;


Expression = Identifier_Expression | Infix_Expression | Prefix_Expression 
                                   | Call_Expression | Index_Expression 
                                   | If_Expression | Literal_Expression  ;


  Identifier_Expression = Identifier ;


  Infix_Expression = Expression Infix_Operator Expression
    ex) 1 + 2, a - b, big >= small, ... 


  Prefix_Expression = Prefix_Operator Expression
    ex) -100, !true, !false

  
  Call_Expression = Identifier ( Expression* ) | Function_Literal ( Expression* ) 
    ex) some_fn(arg), another() , fn { return foo; }

  If_Expression = if ( Expression ) Block_Statement ( else Block_Statement )
    ex) if (flag == true) { return foo }
    ex) if (flag == true) { foo } else { return 20; }


  Index_Expression = Identifier [ Expression ] | Array_Literal  [ Expression ]


  Literal_Expression = Integer_Literal | Boolean_Literal | String_Literal | Function_Literal | Array_Literal

  
    Integer_Literal = (-) [ 0..=9 ] * ;


    String_Literal = "( [a-z|A-Z|0-9] * )" ;


    Boolean_Literal = true | false ;


    Function_Literal = fn (Identifier) ( (Identifier*) ) Block_Statement ;


    Array_Literal = [Expression (,Expression)* ]