Parsing slightly ambigious grammar with LALR

First of all thanks for creating LARK. I have used bison in past life and to me it always felt challenging but it was an absolute joy when I first used Lark.

I have a slightly ambiguous grammar which works fine with early parser but fails with error when using LALR. The grammar is

parser = Lark(r"""
    expr: SIGNED_NUMBER
        | SIMPLE_STRING
        | name
        | FUNCTION_NAME "(" function_args ")"

    function_args : expr
                  | function_args "," expr
                  
    name: NAME

    NAME : /[a-zA-Z0-9_]+/
    SIMPLE_STRING : /'[^']*'/
    FUNCTION_NAME : /[a-zA-Z0-9_]+/
    
    %import common.SIGNED_NUMBER
    %import common.WS
    %ignore WS
""", start="expr", parser="lalr")

A token without any context can be either NAME or FUNCTION_NAME. While using the default parser, it resolves these ambiguities correctly

tree = parser.parse("A(B(x, y, 5))")
print(tree.pretty())

produces

expr
  A
  function_args
    expr
      B
      function_args
        function_args
          function_args
            expr
              name	x
          expr
            name	y
        expr	5

But with lalr it fails to parse with following error

UnexpectedCharacters: No terminal defined for ',' at line 1 col 6

A(B(x, y, 5))
     ^

Expecting: {'LPAR'}

Previous tokens: Token('FUNCTION_NAME', 'x')

I am sure its a very n00b question. So if its recommended to read something (books etc.) for me to understand this, I would love some suggestions. Thanks!

[erezsh]

As for a tip for next time, try to keep your terminals to a minimum, make sure there no collisions between regexps (regexp-string and string-string aren’t a problem, Lark can detect those).

We’re actually working a feature that would print a warning in this exact situation, but it’s not ready yet.

[ashish01]

Excellent! Thanks, adding a explicit priority solves the issue. For reference, here is how I changed the grammar

parser = Lark(r"""
    expr: SIGNED_NUMBER_R
        | SIMPLE_STRING
        | name
        | function_name "(" function_args ")"

    function_args : expr
                  | function_args "," expr
                  
    function_name: NAME
    name: NAME

    NAME.2 : /[a-zA-Z0-9_]+/
    SIMPLE_STRING : /'[^']*'/
    SIGNED_NUMBER_R.1: SIGNED_NUMBER
    
    %import common.SIGNED_NUMBER
    %import common.WS
    %ignore WS
""", start="expr", parser="lalr")
tree = parser.parse("A(B(x, 7y, 5))")
print(tree.pretty())

which produces

expr
  function_name	A
  function_args
    expr
      function_name	B
      function_args
        function_args
          function_args
            expr
              name	x
          expr
            name	7y
        expr
          name	5