Note: this page describes ideas under discussion, and may change at any time. It is currently mainly based on from the developes list.
Elementary Predicate cfrom/cto
The semantics of the cfrom/cto on the EP are clearly defined. They correspond to the (character) span between the start of the first token and the end of the last token spanned by the chart edge which introduced the predicate (or, more simply, the text span to which the EP corresponds).
Global cfrom/cto
Global cfrom/cto corresponds to the character span of the input segment ("sentence") which was given to the parser. This would normally be what the sentence splitter returns.
The input segment might have certain characters at the end (or start) stripped by the preprocessor, or the final token might be semantically vacuous (oe's insight). Such semantics also fit neatly with the text interface spec, which contains a global cfrom/cto corresponding to exactly the span of the input segment.
Example
Suppose we have a bit of a document:
<p> <b>Carp</b>: a fish. Sometimes eaten.<br>But not oftenNevertheless it is tasty.
and this gets processed by a sentence splitter to give:
"<b>Carp</b>: a fish. " "Sometimes eaten." "But not often." "Nevertheless it is tasty."
cto of sentence 1 is cfrom of sentence 2, but cto of sentence 2 is less than cfrom of sentence 3 because the <br> has been removed. Sentence 3 has an additional . which wasn't in the original (assume the sentence splitter is doing some regularisation - I can give a more convincing example if necessary but I really hope nobody is going to argue that we should disallow this in principle), so the `often' has a cto which is the same as the period's cfrom and cto.
Issues
- We have to be more formal about specifying what the sentence splitter does, not because we expect all sentence splitters to behave in the same way (we don't) but because we want the cfrom and cto to be consistent with different implementations of the same sentence splitting algorithm.
- there are issues with the tokeniser - e.g., you have
a string abc d ef' - the preprocessor tokenises to ab' `x' - what is the cfrom/cto for those tokens?
What do cfrom/cto represent?
Two different axes giving four possibilities
- positions vs points
- token vs character
It would be nice to converge on one.
Positions vs Points
The currently agreed semantics of CFROM/CTO is that they refer to character positions (2005). Eg. given the text 'abcd' the range CFROM=0 to CTO=2 refers to the "abc" substring.
abcd 0123 = character positions
I would like to suggest we use character _points_ (the points between characters) instead of the above -- more expressive and allows the specification of empty ranges. Eg. given the text 'abcd' the range CFROM=0 to CTO=2 would refer to the "ab" substring, whilst the range CFROM=0 to CTO=3 would refer to the "abc" substring
.a.b.c.d. 0 1 2 3 4 = character points
The conversion from character positions to character points is simple: CFROM values are the same, to convert a CTO character position to character point you must add 1.