Re: Compiler Books?

VBDis <vbdis@aol.com> wrote:
>IMO top-down parsers are easier to understand than bottom-up
>parsers. Even if both types can be used for C and Pascal, bottom-up
>parsers are commonly described and used for C, and top-down parsers
>for Pascal and other "Wirthian" languages.
>...if your language has to be somewhat compatible with C,
>you have to go the harder way.


Sorry, not so. C was designed for top-down parsing; the first C
compiler, Dennis Ritchie's own, used recursive descent with some
flourishes.


Even ANSI C is not that hard to parse top-down. It requires some work
and thought, not just a mechanical transcription of the standard's
grammar. There are a few places where it helps to peek ahead one
extra token, to decide which way to go in a timely way. And of course
you need some interaction with the symbol table to get typedefs right.
Some of this is annoying but none of it is prohibitive; I've done it.


Incidentally, Bjarne Stroustrup is on record as saying that letting
Aho and Johnson talk him out of writing a recursive-descent parser for
the early C++ was a major mistake. C++ is extremely difficult to
parse using pure bottom-up techniques, and needs a lot of kludges,
while doing it top-down is fairly straightforward and is common in
production compilers.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. | henry@spsystems.net

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