Obsession with lexical and syntactic issues

> Who >really< cares about syntax anyway?


Well, the customer, for one.


Syntax-related issues in compilers "in the real world" are *not*
trivial. For instance:


- Constant revisions/changes to the grammar


This is where tools that turn a grammar into a parser win big over
hand-coded parsers -- the language *is* going to go through 23
revisions before the compiler goes out the door. Sure, it started out
as "ANSI Standard", but the customer would like a couple of extra
features...


- Verifying that the new language features don't introduce ambiguities
into the language


Unless you design your language with an eye to making all
constructions *obviously* different, you will introduce ambiguities.
I've been involved with compilers for C, Algol 68, and a Cobol-like
business application language, and seen enough about Fortran and PL/1,
to know that this sort of problem is always biting you.


- Automatically producing good error recovery from syntax errors


This is clearly a major research area, and it is important in any
compiler that someone is actually going to use to write programs.
Even figuring out how error messages "ought" to be presented is still
unknown.


- Language designers steadfastly refuse to make LALR(1) languages


I've never yet seen a major programming language that was truly
LALR(1). And usually the points where they depart from LALR(1) are
seriously ugly -- consider the problem of tagging all the typedef
names in a C program in a truly ANSI Standard way.


- Building a lex/parse system that will Do What I Mean


Lexers and parsers specifications are *still* more complicated than
they "ought to be". Anything that can be done to put more wisdom in
the generator will provide immediate payoff.


Another reason that this is still an important area of study is that
we write many parsers, whereas, at least in theory, a good global
optimizer should be usable with little change in compilers for many
languages.


Dale Worley Compass, Inc. worley@compass.com

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