Question

I’m building a parser for a language I’ve designed, in which type names start with an upper case letter and variable names start with a lower case letter, such that the lexer can tell the difference and provide different tokens. Also, the string ‘this’ is recognised by the lexer (it’s an OOP language) and passed as a separate token. Finally, data members can only be accessed on the ‘this’ object, so I built the grammar as so:

%token TYPENAME
%token VARNAME
%token THIS

%%

start:
    Expression
    ;

Expression:
    THIS
    | THIS '.' VARNAME
    | Expression '.' TYPENAME
    ;
%%

The first rule of Expression allows the user to pass ‘this’ around as a value (for example, returning it from a method or passing to a method call). The second is for accessing data on ‘this’. The third rule is for calling methods, however I’ve removed the brackets and parameters since they are irrelevant to the problem. The originally grammar was clearly much larger than this, however this is the smallest part that generates the same error (1 Shift/Reduce conflict) – I isolated it into its own parser file and verified this, so the error has nothing to do with any other symbols.

As far as I can see, the grammar given here is unambiguous and so should not produce any errors. If you remove any of the three rules or change the second rule to

Expression '.' VARNAME

there is no conflict. In any case, I probably need someone to state the obvious of why this conflict occurs and how to resolve it.

Answer 1

The problem is that the grammar can only look one ahead. Therefore when you see a THIS then a ., are you in line 2(Expression: THIS '.' VARNAME) or line 3 (Expression: Expression '.' TYPENAME, via a reduction according to line 1).

The grammar could reduce THIS. to Expression. and then look for a TYPENAME or shift it to THIS. and look for a VARNAME, but it has to decide when it gets to the ..