How to get rid of the following multiple alternatives warnings in my ANTLR3 grammar?

问题

[11:45:19] warning(200): mygrammar.g:14:57: Decision can match input such as "','" using multiple alternatives: 1, 2
As a result, alternative(s) 2 were disabled for that input
[11:45:19] warning(200): C:\Users\Jarrod Roberson\mygrammar.g:14:57: Decision can match input such as "','" using multiple alternatives: 1, 2
As a result, alternative(s) 2 were disabled for that input

I want to be able to nest functions inside other functions.

myfunction(x) ->
  sqr(a) -> a * a,
  y -> sqr(x).

here is the line it is complaining about

function : ID '(' args ')' '->' statement (',' statement)* ;

and here is what it is considering the alternative

statement : ATOM
          | expression
          | assignment
          | function
          ;

I am using . as my statement end rule

program : (statement'.')*;

Here is what the synatx diagram looks like in ANTLRWorks

_{(source: vertigrated.com)}

I really like things to compile/work without any warnings. How do I resolve this warning condition?

回答1:

Jarrod Roberson wrote:

I really like things to compile/work without any warnings. How do I resolve this warning condition?

Your parser can parse the following input:

f(x)-> g(y)-> y*y, x=y

in two different parse trees:

and:

You can fix this by forcing the parser to look ahead and make sure there is ',' statement ahead before actually matching these rules. You can do that by using a syntactic predicate (the (...)=> part) with said rule inside:

function
  :  ID '(' args ')' '->' statement ((',' statement)=> ',' statement)* 
  ;

However, you don't need the predicate if your function rule has some sort of an "end" token, which you haven't defined. From your earlier questions, and your example:

myfunction(x) ->
  sqr(a) -> a * a,
  y = sqr(x).

it seems you're using the '.' as the end of a function. If you add that to your function rule:

function
  :  ID '(' args ')' '->' statement (',' statement)* '.'
  ;

you don't need a predicate at all.

来源：https://stackoverflow.com/questions/8125141/how-to-get-rid-of-the-following-multiple-alternatives-warnings-in-my-antlr3-gram

标签

antlr

antlr3

antlrworks