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Re: Parsing partial sentences

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On 2017-04-27, Walter Banks <walter@bytecraft.com> wrote:
> On 2017-04-03 3:57 AM, Hans-Peter Diettrich wrote:
>> Is there an easy way to parse e.g. C #defines into constants,
>> functions or other non-terminals, which are not the goal of the
>> entire grammar?
>
> In a word NO. #defines are always strings even when they look like
> constants, something I have found out the hard way. There have only been
> two ways that I have successfully dealt with #defines: a preprocessor
> pass or later and much faster pipeline the processing of C source and
> add the defined definition processing into part of the source fetch
> handling.

If we allow Pascal to be extended with a macro preprocessor,

I believe I could design a system for translating C to Pascal which
handles some macros, translating them to Pascal macros.  Even some
macros that "break" syntactic boundaries, such as "list_for_each (var,
list) { block }".

I don't believe such a project has any value beyond getting
a pat on the back from another developer; I wouldn't spend any
time on such a thing.  The end result might well be rejected by some
Pascal users, due to requiring the extended dialect, whether on
ideological grounds, or on practical issues with tooling (being able to
get the preprocessor running in a given Pascal development environment).

Here is a very high level sketch of the approach:

- We preprocess the C translation unit fully before parsing it.
  - However, we use our own specialized C preprocessor which is
    tightly integrated into our translator.
  - Our specialized C preprocessor carefully tracks, in detail
    the origin of every piece of syntax, to the macro which
    substituted that syntax, either as an argument or as body
    material.
  - As our parser is analyzing the code, it preserves this information
    in the abstract syntax tree: every tree fragment, if it
    was the result of a macro expansion, is tracked to the
    macro call. We also know that an entire node was the result
    of a macro call, and what that macro call looked like.
  - When we output the Pascal translation, if a tree node was the
    result of a macro call (a macro call that we we were successfully
    able to treat with our magic algoirthms) then rather than outputting
    the Pascal translation of that tree node, we output the macro
    call syntax (remembering that our Pascal dialect has a preprocessor
    to handle that).
  - We have a magic algorithm for reconstructing Pascal versions
    of macro bodies which works roughly like this:
    - When we translate a tree node from C to Pascal, if that tree node
      came from macrology, we keep track of which Pascal fragments
      correspond to C fragments.
    - We then reverse the macrology: we analyze the Pascal and see which
      fragments correspond to C material that came from a macro body,
      and which came from substitution of macro arguments and such.
      - From this we can reconstruct a Pascal macro body, using the
        corresponding Pascal fragments.
        - A Pascal fragment which coresponds to the insertion
          of some argument X, is just represented by the same X in the
          Pascal macro body.
        - A Pascal fragment which corresponds to the insertion of
          some body template material Y is replaced in the Pascal
          version of the macro by the corresponding Pascal piece.
    - Since a macro call often occurs in more than one place, we can
      somehow combine information from multiple sites to improve the
      translation, or at least validate that the same thing is
      happening.

  - The saving grace here what makes this even contemplatable is
    that C macros are dumb positional substitution: just pasting
    together of fragments. If C macros were like Lisp macros, doing
    arbitrary Turing computation, good luck with this approach, right?

  - I think, best forget C99 variadic macros and such cruft.

  - The system could have a mode whereby it gives up trying to translate
    a C macro to Pascal, but it at least reverses the syntax so that
    the overall Pascal fragment corresponding to the C code which came
    from that macro call is reversed back to a macro call. A diagnostic
    can be generated "missing macro needed", and the users themselves
    can use their human intelligence to finish the job (if possible).
    Find the C version of the macro and try to translate it.

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Thread

Parsing partial sentences Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2017-04-03 09:57 +0200
  Re: Parsing partial sentences Fernando <pronesto@gmail.com> - 2017-04-04 06:05 -0700
    Re: Parsing partial sentences Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2017-04-07 11:16 -0400
      Re: Parsing partial sentences George Neuner <gneuner2@comcast.net> - 2017-04-07 12:59 -0400
        Re: Parsing partial sentences Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2017-04-07 22:24 +0200
          Re: Parsing partial sentences George Neuner <gneuner2@comcast.net> - 2017-04-10 20:22 -0400
            Re: Parsing partial sentences Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2017-04-11 10:31 +0200
              Re: Parsing partial sentences Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2017-04-11 19:40 +0200
              Re: Parsing partial sentences George Neuner <gneuner2@comcast.net> - 2017-04-11 23:05 -0400
                Re: Parsing partial sentences Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2017-04-12 13:45 +0200
          Re: Parsing partial sentences Martin Ward <martin@gkc.org.uk> - 2017-04-11 10:30 +0100
  Re: Parsing partial sentences Walter Banks <walter@bytecraft.com> - 2017-04-27 10:58 -0400
    Re: Parsing partial sentences Kaz Kylheku <686-678-9105@kylheku.com> - 2017-04-27 19:08 +0000
      Re: Parsing partial sentences rugxulo@gmail.com - 2017-04-28 16:32 -0700
      Re: Parsing partial sentences Marco van de Voort <marcov@toad.stack.nl> - 2017-04-29 14:57 +0000
        Re: Parsing partial sentences Kaz Kylheku <686-678-9105@kylheku.com> - 2017-04-30 16:31 +0000
    Re: Parsing partial sentences Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2017-04-28 02:48 +0200

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