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From: "Oliver Kellogg" <oliver.kellogg@t-online.de>
Newsgroups: comp.lang.ada
Subject: Re: GPS - A new kind of IDE?
Date: Tue, 11 Feb 2003 02:24:07 +0100
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Date: 2003-02-11T02:24:07+01:00
List-Id: <comp.lang.ada>

I've just bumped into an interesting data point on
parser contruction with ANTLR regarding parse speed.
ANTLR supports syntactic predicates such as

  (subprog_body) => subprog_body

where the rule subprog_body is applied on trial
and if it turns out not to apply then the parser backs
out and applies the next rule. Terence Parr, inventor
of ANTLR, calls this "infinite lookahead".

This feature makes for a prettier grammar because
it allows more liberal reuse of rules. In the above
example, we can write the subprog_body rule in a
place where also a subprog_spec could be expected;
the parser takes care of resolving the ambiguity.

However, that beauty has its price.

As a test, I submitted the source code of the
Ada Graphics Package (agp95_v0.1_x11/src) to parsing
(38560 LOC, 11851 executable semicolons.)

With usage of syntactic predicate for the subprog_spec/subprog_body
distinction ("pretty" soft-state grammar rule):

real    1m16.635s
user    1m15.930s
sys     0m0.300s

Without usage of syntactic predicate for the subprog_spec/subprog_body
distinction ("ugly" hard-coded state):

real    0m46.058s
user    0m45.110s
sys     0m0.370s

So for kdevelop adasupport I'm going with the hard state rule
for subprograms.

Oliver Kellogg