Re: LEX and YACC -- Are there better parser generators?

            Rex, Lalr and Ell - Compiler Construction Tools
            ===============================================


          Rex (Regular EXpression tool) is a scanner generator whose
specifications are based on regular expressions and arbitrary
semantic actions written in one of the target languages C or
Modula-2. As scanners sometimes have to consider the context to
unambiguously recognize a token the right context can be speci-
fied by an additional regular expression and the left context can
be handled by so-called start states. The generated scanners
automatically compute the line and column position of the tokens
and offer an efficient mechanism to normalize identifiers and
keywords to upper or lower case letters. The scanners are table-
driven and run at a speed of 180,000 to 195,000 lines per minute
on a MC 68020 processor.


          Lalr is a LALR(1) parser generator accepting grammars writ-
ten in extended BNF notation which may be augmented by semantic
actions expressed by statements of the target language. The gen-
erator provides a mechanism for S-attribution, that is syn-
thesized attributes can be computed during parsing. In case of
LR-conflicts unlike other tools Lalr provides not only informa-
tion about an internal state consisting of a set of items but it
prints a derivation tree which is much more useful to analyze the
problem. Conflicts can be resolved by specifying precedence and
associativity of operators and productions. The generated parsers
include automatic error recovery, error messages, and error
repair. The parsers are table-driven and run at a speed of
400,000 lines per minute. Currently parsers can be generated in
the target languages C and Modula-2.


          Ell is a LL(1) parser generator accepting the same specifi-
cation language as Lalr except that the grammars must obey the
LL(1) property. It is possible to evaluate an L-attribution
during parsing. The generated parsers include automatic error
recovery, error messages, and error repair like Lalr. The
parsers are implemented following the recursive descent method
and reach a speed of 750,000 lines per minute. The possible tar-
get languages are again C and Modula-2.


          All the tools are implemented in Modula-2.
A comparison of the above tools with the corresponding UNIX
tools shows that significant improvements in terms of error handling
as well as efficiency have been achieved:
Rex generated scanners are 4 times faster than those of LEX.
Lalr generated parsers are 2-3 times faster than those of YACC.
Ell generated parsers are 4 times faster than those of YACC.
The input languages of the tools are improvements of the LEX and YACC
inputs. The tools also understand LEX and YACC syntax with the help of
preprocessors.


The tools are publicly copyable. They have been developed in 1987.
They have been tested by generating scanners and parsers for
e. g. Pascal, Modula, Oberon, Ada and found stable.


The tools have been developed using our own Modula-2 compiler called
MOCKA on a MC 68020 based UNIX workstation. They have been ported
to the SUN workstation and been compiled successfully using the SUN
Modula-2 compiler. They also run on VAX/BSD UNIX and VAX/ULTRIX machines.
This should assure a reasonable level of portability.


I would be pleased to send the programs to everybody that is interested.
To minimize the costs (at least for me) I suggest to send a blank SUN
streamer tape. 1/2" magtapes are possible, too, but cause more work for me.
I will return the tape with the sources organized as a UNIX file tree
(tar format). The tape will contain user manuals for each tool.


Literature:


J. Grosch: "Generators for High-Speed Front-Ends",
LNCS 371, pp. 81-92, Berlin: Springer Verlag, 1989.


More papers about Rex and Lalr will be published soon.


PS: We have completed first versions of more tools:
Ast handles abstract syntax trees or attributed trees and graphs
(something like IDL).
Ag generates attribute evaluators for ordered attribute grammars (OAGs).




Josef Grosch
GMD - Forschungsstelle an der Universitaet Karlsruhe
Haid-und-Neu-Str. 7
D-7500 Karlsruhe 1
West Germany


Phone: +721-662226
Email: grosch@gmdka.UUCP

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