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From: George Neuner <gneuner2@comcast.net>
Newsgroups: comp.compilers
Subject: Re: Parsing partial sentences
Date: Mon, 10 Apr 2017 20:22:22 -0400
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On Fri, 7 Apr 2017 22:24:04 +0200, Hans-Peter Diettrich
<DrDiettrich1@netscape.net> wrote:

>Am 07.04.2017 um 18:59 schrieb George Neuner:
>
>> It might be easiest to just run the C preprocessor as a 1st pass and
>> then attempt to convert the result.
>
>That would obfuscate the source code widely :-(

That's true, but any C code you might encounter could, in itself, be
arbitrarily convoluted and hard to understand: e.g., deep pointer
chains, goto spaghetti, Duff devices, setjmp/longjmp, coroutines, etc.

I understand that you want to be able to preserve and relate the
original source to the translation, but I don't see any other real
choice other than to employ the C preprocessor.  As John said, a
#define body may not even be a complete expression - nevermind a legal
one.

Normally a parser would error on an illegal or incomplete expression.
Creating a parser that can return useful information about arbitrary
quasi-legal input I think would be extremely difficult.


>> Since a #define body is just text, it can be anything - people have
>> created whole DSLs using #define.  If you really need to figure them
>> out, I'm afraid you'll need (almost) the whole C language parser to do
>> it.
>
>I already wrote the parser, but it's LL. For code snippets a LR parser
>looks like the only solution?

You could do it in either LL or LR ... LR would be more runtime
efficient, but I think it would not be any easier to create the parser
in the first place.

To do what you want you'd need non-terminals not just for every legal
(sub)expression in C, but for every individual keyword, operator and
symbol, and also for any quasi-legal combination of them.  The parser
would be enormous.


Building on John's example, consider what you'd do with

# define FOO +
# define BAR + 42
# define BAZ + c   /* note 'c' is undefined */
int a, b;

    :
    a = a FOO b BAR BAZ;
    :


And then consider what you'd need to handle sh..stuff like this for
every keyword, operator, variable, etc.

George