Thu, 01 Mar 2012
User experiences with Marpa: some observations
When it comes to
user experiences with Marpa,
I confess to being a highly biased source.
I hope the following observations
will be useful nonetheless.
(Marpa, for those new to this blog,
is a new, powerful and fast parser and parsing algorithm.
To learn more,
check out its web page.)
Marpa does the job
If you've read user's accounts of work with BNF grammars over
(I have studied many),
you know they follow a familiar pattern.
The user has some BNF.
He then tries tool X (for X substitute
yacc, bison, PEG, recursive descent, etc.)
and finds that it almost works.
Almost, but not quite.
The rest of the account describes the user
beating up his grammar in an effort to make
it fit the tool.
Perhaps 50% of the time, he reports that his effort
The accounts from Marpa users are different,
because the problem the users of other tools spend
most of their time describing,
and spent most of their time solving,
does not exist with Marpa.
Once they have their grammar worked out,
Marpa parses it.
As a practical
matter, this difference is a big deal.
A methodology that, half the time,
takes you to a dead end,
is one that practitioners will avoid.
The reason that regular expressions became so dominant,
I am convinced,
is that they did the job.
If you can write your problem as a regular expression
(here I speak of regular expressions, not regexes)
you are guaranteed two things.
First that your regular expression engine *WILL* handle it.
Second, that it will do so in acceptable time.
Regular expressions did the job.
With Marpa, BNF does the job.
BNF as a lost art
An obstacle to learning Marpa seems to be
that writing BNF is something of a lost art.
BNF is not difficult.
By my accounting,
BNF is actually simpler than regular expressions.
BNF involves one idea: concatenation.
Regular expressions involve all of concatenation,
sequence and alternation.
Even if you don't agree with my way of calculating
I think you'll agree that
BNF is vastly simpler than Perl regexes.
When I learned programming (around 1970),
BNF was much better known than regular expressions.
BNF had been established for years
as the way of specifying languages.
on the other hand,
did not come in prominence
The first PDP11 assembler version of that came out in 1971,
the year I was taking the introduction to Computer Science
with Alan Perlis.
spent a lot of time on BNF
I cannot recall that he mentioned regular expressions.
Since then, at least
in the US, BNF and Markov machines seem to have been consigned to
nearly the same degree of oblivion.
It makes some sense that those with a pragmatic bent
might forget BNF.
Aside from reading standards documents,
there wasn't much you could do with it -- yacc
to the contrary notwithstanding.
in editing we came to use regular expressions so often
many of us can write short ones without thinking.
When I wrote Marpa,
I assumed that its users
would already know BNF.
A number of users seem to have done it
the opposite way --
they've learned Marpa and used
it to get the hang of BNF.
Now that I think of it,
I learned regular expressions,
not in the classroom,
but from the ed editor.
When there's a real use for it,
I have no doubt the lost art of BNF
will be once again found.
posted at: 07:49 |
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