Grammar does not support field selectors / subscripts on general expressions

[DenSinH]

Currently, expressions like these are not parsed by the grammar:

s_fbMyFunctionBlock.m_Get(s_nIdx).p_bValue

even though they are perfectly valid. Just try here: https://www.lark-parser.org/ide/
with example code:

FUNCTION_BLOCK FB_Test

s_bTest := s_fbMyFunctionBlock.m_Get(s_nIdx).p_bValue;

END_FUNCTION_BLOCK

and the current iec.lark grammar. It will fail:

lark.exceptions.UnexpectedCharacters: No terminal matches ';' in the current parser context, at line 3 col 54

fbMyFunctionBlock.m_Get(s_nIdx).p_bValue;
                                        ^
Expected one of: 
	* DEREFERENCED
	* LPAR
	* LSQB
	* DOT

expecting a chained function call.

If you replace

field_selector: [ DEREFERENCED ] "." ( variable_name | INTEGER )

multi_element_variable: variable_name ( subscript_list | field_selector )+

with

field_selector: [ DEREFERENCED ] "." ( variable_name | INTEGER )

multi_element_variable: expression ( subscript_list | field_selector )+

the code does parse (which is expected), but this implies modifications in MultiElementVariable, which may have a lot of implications elsewhere.