(bison.info)Semantic Tokens


Next: Lexical Tie-ins Up: Context Dependency

Semantic Info in Token Types
============================

   The C language has a context dependency: the way an identifier is
used depends on what its current meaning is.  For example, consider
this:

     foo (x);

   This looks like a function call statement, but if `foo' is a typedef
name, then this is actually a declaration of `x'.  How can a Bison
parser for C decide how to parse this input?

   The method used in GNU C is to have two different token types,
`IDENTIFIER' and `TYPENAME'.  When `yylex' finds an identifier, it
looks up the current declaration of the identifier in order to decide
which token type to return: `TYPENAME' if the identifier is declared as
a typedef, `IDENTIFIER' otherwise.

   The grammar rules can then express the context dependency by the
choice of token type to recognize.  `IDENTIFIER' is accepted as an
expression, but `TYPENAME' is not.  `TYPENAME' can start a declaration,
but `IDENTIFIER' cannot.  In contexts where the meaning of the
identifier is _not_ significant, such as in declarations that can
shadow a typedef name, either `TYPENAME' or `IDENTIFIER' is
accepted--there is one rule for each of the two token types.

   This technique is simple to use if the decision of which kinds of
identifiers to allow is made at a place close to where the identifier is
parsed.  But in C this is not always so: C allows a declaration to
redeclare a typedef name provided an explicit type has been specified
earlier:

     typedef int foo, bar, lose;
     static foo (bar);        /* redeclare `bar' as static variable */
     static int foo (lose);   /* redeclare `foo' as function */

   Unfortunately, the name being declared is separated from the
declaration construct itself by a complicated syntactic structure--the
"declarator".

   As a result, part of the Bison parser for C needs to be duplicated,
with all the nonterminal names changed: once for parsing a declaration
in which a typedef name can be redefined, and once for parsing a
declaration in which that can't be done.  Here is a part of the
duplication, with actions omitted for brevity:

     initdcl:
               declarator maybeasm '='
               init
             | declarator maybeasm
             ;
     
     notype_initdcl:
               notype_declarator maybeasm '='
               init
             | notype_declarator maybeasm
             ;

Here `initdcl' can redeclare a typedef name, but `notype_initdcl'
cannot.  The distinction between `declarator' and `notype_declarator'
is the same sort of thing.

   There is some similarity between this technique and a lexical tie-in
(described next), in that information which alters the lexical analysis
is changed during parsing by other parts of the program.  The
difference is here the information is global, and is used for other
purposes in the program.  A true lexical tie-in has a special-purpose
flag controlled by the syntactic context.


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Dirfile and infopages generated Sat Dec 3 02:07:54 2005