Re: Input Data Stream Uprgaded Grammar

[Hans Aberg]

At 17:12 +0300 1-01-27, Vladimir Rykov wrote:
>...I met a problem that seems unresolvable for  me. ...   Is it possible
>to >upgrade the compiled (binary)  Bison-made Grammar from the input data
>stream?

No.

>   Here is an example;   ...   a*b+c     - ordinary  data ...   (I want to
>>define new operation):   DEFINE ** AS n**2 = n*n    - or by  any other
>means   >... (Then I use this new operation in my input  stream)   d + f
>** 2   ...     >So - may I define new operation (let it be "square"  - **)
>using properly >keywords and other tools and so to upgrade my inner
>grammar?

A simple way to provide an extensible grammar, is to identify the different
possible grammar rules and let your lexer (by a table lookup) return
suitable token numbers.

In the languages Haskell and Prolog, one can define operators with a
precedence and left/right associativity. In Haskell, the precedence levels
are only ten, so you could write out the different combinations
expr:
    expr opl0 expr {...}
  ...
  | expr opl9 expr {...}
  ...
and then define the precedence levels correctly to Bison. The lexer would
then use the table lookup precedence level to return the correct opl0, ....

The Haskell interpreter Hugs http://haskell.org/hugs however skips the
precedence levels in the .y grammar, and makes that parsing in an
independent (rather simple) engine. In a Prolog reader you would have to
use this method, because the number of precedence levels are 1000.

In your case, you may not need this at all, if you only need macro
expansions and the like. Then you grammar might look like
  definition: DEFINE identifier AS expr
for the definition of new identifiers, and later in the use
  expr: expr identifier expr { $$ = (*rule($2))($1, $3) }
-- You then look-up the identifiers defined in the lookup table, and from
that can you see what to do with them. If the rule of the identifier `**'
is defined to a^b (in the example action above, a function pointer that the
function `rule' returns), that rule will be applied. (You must also figure
out a way to produce an error if an identifier is not defined.)

  Hans Aberg
                  * Email: Hans Aberg <mailto:address@hidden>
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