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Snobol Like Matching in Python

From: "andrew cooke" <andrew@...>

Date: Tue, 30 Dec 2008 13:23:08 -0300 (CLST)

Found this while looking for info for work; need to go back when I have
more time and read it further.


Particularly like the array subset syntax for specifying repeats:
  * : [0:]
  + : [1:]
  ? : [0:1]


Rethinking Parsing

From: "andrew cooke" <andrew@...>

Date: Fri, 2 Jan 2009 21:56:30 -0300 (CLST)

I'm getting the urge to write another parsing library :o)

I like what was done in the work above and it seems like it could be
easily(?) made more general.  The existing package takes a list of
characters and generates labelled words.  It seems to me that could be
changed to generate a list of words.  Or even a list of arbitrary objects.
 And it also seems to me that the parsers could be changed to match not
just lists of characters, but lists of arbitrary objects.

If you do that then it can eat its own tail - you can have various levels
of parsers.  The way I see it, these constrict different levels in the

Does that make sense?  How is this different from normal?  There's a
slight difference in implementation - the source itself stores the new
stream, rather than it being returned by the parser.  I have no idea if
this is an improvement or not.  I need to read the code for the matcher
again.  Does it help with backtracking?  Is it just an implementation
detail?  What's wrong with returning the match?



From: "andrew cooke" <andrew@...>

Date: Sat, 3 Jan 2009 16:54:50 -0300 (CLST)

Prev comments were somewhat stupid.

With recursive descent the combinators construct a guided path down to the
matching against characters.  So there's no layered streams of characters
and tokens.

So what does the code in the paper give you?

First of all, it has some nice ideas about syntax.  Most of all - that by
using Python objects rather than just functions, you have  a lot more
freedom to play around.

Second, backtracking.

In addition, something that has always bugged me, and which I'd like to
improve: better diagnostics.


Slice Mechanics

From: "andrew cooke" <andrew@...>

Date: Sat, 3 Jan 2009 20:43:23 -0300 (CLST)

I just ran some simple tests.  If the appropriate __ methods are
implemented then:

foo[1] calls foo.__getitem__(1)
foo[1:2] calls foo.__getslice(1, 2)
foo[1:2:3] calls foo.__getitem(slice(1,2,3))

but if __getslice__ is missing then foo[1:2} calls
foo.__getitem(slice(1,2,None)) - that means that getitem handles all
calls, but the single arg case is passed a simple value.

The slice() object looks like an ordinary object with start, stop, step
and indices attributes.


More Ideas

From: "andrew cooke" <andrew@...>

Date: Sun, 4 Jan 2009 10:55:13 -0300 (CLST)

- Track deepest search via source for debugging

- Matching returns matched value or [value] or [(name, value)].  Latter
can be used to construct dictionaries.  Can return anything (via function)
but we support those via nice syntax.

- Node class that can be constructed from lists of (name, value). 
Implements iterator (all values) and attribute based (named values)
interface.  Several representations.  str returns matched string
(including unlabelled values).  Tree returns ascii format tree (with named
values only).


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