Program      ::= Statement+

Statement    ::= (AssignmentStatement | FunctionCallStatement) SEMICOLON

AssignmentStatement ::= "let" IDENTIFIER EQUALS Expression

FunctionCallStatement ::= IDENTIFIER LPAREN FunctionArguments? RPAREN

FunctionArguments ::= Expression ("," Expression)*

Expression   ::= AdditiveExpression | FunctionDefinition

AdditiveExpression ::= PrimaryExpression ("+" AdditiveExpression)

PrimaryExpression ::= NUMBER | IDENTIFIER | FunctionCallStatement

FunctionDefinition ::= LPAREN FunctionParamters? RPAREN ARROW LBRACE Statement* ReturnStatement RBRACE

FunctionParameters ::= IDENTIFIER ("," IDENTIFIER)*

ReturnStatement ::= RETURN Expression SEMICOLON --TODO: I dont like this