如何在 antlr4 中为 function 创建令牌

Question

I need some help, because I know how to create a way to define a function on my language:

function: 
    ID ‘as’ DATATYPE ‘(‘ arguments ‘)’ instruction* EndFun;

My problem is, if I want to make 'void' functions, how can I prevent from user to input a 'return ID'

function: 
    ID (‘as’ DATATYPE)? ‘(‘ arguments ‘)’ instruction* EndFun;

ps: instructions token includes 'return ID'

Answer 1

Either check after parsing if a void function contains a return statement (which is what I'd do), or do something like this:

function
 : ID 'as' DATATYPE '(' arguments ')' instruction* EndFun
 | 'void' '(' arguments ')' instruction_no_return* EndFun
 ;

instruction
 : instruction_no_return
 | 'return' ID
 ;

instruction_no_return
 : instruction_a
 | instruction_b
 | instruction_c
 ;