[developers] character-based discriminants

[Stephan Oepen]

many thanks for the quick follow-up, woodley!

in general, character-based discriminants feel attractive because the idea
promises increased robustness to variation over time in tokenization.  and
i am not sure yet i understand the difference in expressivity that you
suggest?  an input to parsing is segmented into a sequence of vertices (or
breaking points); whether to number these continuously (0, 1, 2, …) or
discontinuously according to e.g. corresponding character positions or time
stamps (into a speech signal)—i would think i can encode the same broad
range of lattices either way?

closer to home, i was in fact thinking that the conversion from an existing
set of discriminants to a character-based regime could in fact be more
mechanic than the retooling you sketch.  each current vertex should be
uniquely identified with a left and right character position, viz. the
+FROM and +TO values, respectively, on the underlying token feature
structures (i am assuming that all tokens in one cell share the same
values).  for the vast majority of discriminants, would it not just work to
replace their start and end vertices with these characters positions?

i am prepared to lose some discriminants, e.g. any choices on the
punctuation lexical rules that are being removed, but possibly also some
lexical choices that in the old universe end up anchored to a sub-string
including one or more punctuation marks.  in the 500-best treebanks, it
used to be the case that pervasive redundancy of discriminants meant one
could afford to lose a non-trivial number of discriminants during an update
and still arrive at a unique solution.  but maybe that works differently in
the full-forest universe?

finally, i had not yet considered the ‘twigs’ (as they are an FFTB-specific
innovation).  yes, it would seem unfortunate to just lose all twigs that
included one or more of the old punctuation rules!  so your candidate
strategy of cutting twigs into two parts (of which one might often come out
empty) at occurrences of these rules strikes me as a promising (still quite
mechanic) way of working around this problem.  formally, breaking up twigs
risks losing some information, but in this case i doubt this would be the
case in actuality.

thanks for tossing around this idea!  oe


On Sat, 23 Nov 2019 at 20:30 Woodley Packard <sweaglesw at sweaglesw.org>
wrote:

> Hi Stephan,
>
> My initial reaction to the notion of character-based discriminants is (1)
> it will not solve your immediate problem without a certain amount of custom
> tooling to convert old discriminants to new ones in a way that is sensitive
> to how the current punctuation rules work, i.e. a given chart vertex will
> have to be able to map to several different character positions depending
> on how much punctuation has been cliticized so far.  The twig-shaped
> discriminants used by FFTB will in some cases have to be bifurcated into
> two or more discriminants, as well. Also, (2) this approach loses the
> (theoretical if perhaps not recently used) ability to treebank a nonlinear
> lattice shaped input, e.g. from an ASR system.  I could imagine treebanking
> lattices from other sources as well — perhaps an image caption generator.
>
> Given the custom tooling required for updating the discriminants, I’m not
> sure switching to character-based anchoring would be less painful than
> having that tool compute the new chart vertex anchoring instead — though I
> could be wrong.  What other arguments can be made in favor of
> character-based discriminants?
>
> In terms of support from FFTB, I think there are relatively few places in
> the code that assume the discriminants’ from/to are interpretable beyond
> matching the from/to values of the `edge’ relation.  I think I would
> implement this by (optionally, I suppose, since presumably other grammars
> won’t want to do this at least for now) replacing the from/to on edges read
> from the profile with character positions and more or less pretend that
> there is a chart vertex for every character position.  Barring unforeseen
> complications, that wouldn’t be too hard.
>
> Woodley
>
> > On Nov 23, 2019, at 5:58 AM, Stephan Oepen <oe at ifi.uio.no> wrote:
> >
> > hi again, woodley,
> >
> > dan and i are currently exploring a 'makeover' of ERG input
> > processing, with the overall goal of increased compatibility with
> > mainstream assumptions about tokenization.
> >
> > among other things, we would like to move to the revised (i.e.
> > non-venerable) PTB (and OntoNotes and UD) tokenization conventions and
> > avoid subsequent re-arranging of segmentation in token mapping.  this
> > means we would have to move away from the pseudo-affixation treatment
> > of punctuation marks to a 'pseudo-clitization' approach, meaning that
> > punctuation marks are lexical entries in their own right and attach
> > via binary constructions (rather than as lexical rules).  the 'clitic'
> > metaphor, here, is intended to suggest that these lexical entries can
> > only attach at the bottom of the derivation, i.e. to non-clitic
> > lexical items immediately to their left (e.g. in the case of a comma)
> > or to their right (in the case of, say, an opening quote or
> > parenthesis).
> >
> > dan is currently visiting oslo, and we would like to use the
> > opportunity to estimate the cost of moving to such a revised universe.
> > treebank maintenance is a major concern here, as such a radical change
> > in the yields of virtually all derivations would render discriminants
> > invalid when updating to the new forests.  i believe a cute idea has
> > emerged that, we optimistically believe, might eliminate much of that
> > concern: character-based discriminant positions, instead of our
> > venerable way of counting chart vertices.
> >
> > for the ERG at least, we believe that leaf nodes in all derivations
> > are reliably annotated with character start and end positions (+FROM
> > and +TO, as well as the +ID lists on token feature structures).  these
> > sub-string indices will hardly be affected by the above change to
> > tokenization (except for cases where our current approach to splitting
> > at hyphens and slashes first in token mapping leads to overlapping
> > ranges).  hence if discriminants were anchored over character ranges
> > instead of chart cells ... i expect the vast majority of them might
> > just carry over?
> >
> > we would be grateful if you (and others too, of course) could give the
> > above idea some critical thought and look for possible obstacles that
> > dan and i may just be overlooking?  technically, i imagine one would
> > have to extend FFTB to (optionally) extract discriminant start and end
> > positions from the sub-string 'coverage' of each constituent, possibly
> > once convert existing treebanks to character-based indexing, and then
> > update into the new universe using character-based matching.  does
> > such an approach seem feasible to you in principle?
> >
> > cheers, oe
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.delph-in.net/archives/developers/attachments/20191125/25cda16c/attachment.html>