From:	"Christopher Nelson" <paradox AT gye DOT satnet DOT net>
To:	<djgpp AT delorie DOT com>
Subject:	Re: Bison and [f]lexical tie-ins
Date:	Tue, 13 Jul 1999 17:53:39 -0600
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Reply-To:	djgpp AT delorie DOT com

>    The manner you described for your lexical tie-in, is not *exactly* the
>way it should work.  From your text, it sounds as if you're returning the
>type of the id? or perhaps a different token? depending on what you see the
>id as.  This is ambiguous, and creates the errors you described.
>
>    A better approach is to do the following:
>
>
>{ID}    yylvalue.id = install_id(yytext); return ID;
>
>
>    In this case, the lexical tie-in makes sure that the lexme that
>corresponds to this particular id is in the symbol table to start with -
but
>without a type.  Or more precisely, type_none.  If the lexme DOES match a
>symbol already in the table, that symbol is returned, otherwise the newly
>created symbol is returned.
>
>    Inside the parser you should check to see what the type of the symbol
is
>(pretending that $1 is your ID token):
>
>        if (lookup($1)==TYPE_NONE) addtype($1, the_type);
>       else /* this lexeme exists as a symbol of a different type, you'll
>have to create a
>                    new symbol */ { $1=duplicate_id($1); addtype($1,
>the_type); }
>
>    This follows C's most-closely nested rule by allowing you to get the
>last-declared version of a symbol first (assuming that your storage
>structure is a list of some sort) by starting at the head of the list and
>seeking towards the end.
>
>    You should, in addition, have some sort of prologue code on the same
>level as the parameter list nonterminal that sets a global-variable called
>type so that you can use it, or alternatively (and more flexible and
>powerful, though a bit more difficult), construct a small type-tree that
>contains a list or perhaps a stack of the parameters, then walk the tree,
or
>consider the stack until it's empty, applying the above logic to each of
>them.
>
>    In other words:
>
>parameter:    id    { $$=$1; }
>                ;
>
>parameter_list:     parameter                            { push($1,
>parmstack); }
>                        parameter_list ',' parameter      { push($1,
>parmstack); }
>                ;
>
>decl_prologue:    var_type        { $$=$1; }
>                ;
>
>declaration:    decl_prologue parameter_list ';' { symentry *sym;
>
>while((sym==pop(parmstack))
>
>{ /* consider each one as shown
>
>above, deciding what to do with
>
>it based on the type rules. */
>
> }
>                                                                      }
>
>
>    This has the added benefit of then allowing you to warn about
>shadow-variables, and finding scope rules and type sets, rather than just
>returning the first thing you see.
>
>        -={C}=-
>
>
>
>

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