CVS update: gnucash/src/engine

[Linas Vepstas]

On Thu, Jun 12, 2003 at 11:04:49AM -0400, Derek Atkins was heard to remark:
> > 
> > Smells like 'relational' to me.   I think the predicates you need to
> > add are "has_a" and "is_a", but I'm not an expert .. I vaguely remember 
> > "uses_a", "child_of" and "parent_of" ... and some arguments about what 
> > the correct primitve predicates should really be. 
> 
> Well, currently my predicates are based on type-comparrisons, and each
> QueryCore type has its own predicates.  For example a string predicate
> has a two predicates, "matches" and "doesn't match".  Then there are two
> additional predicate flags: "match-case-insensitive" and "match-regex".

yeah, that's OK.

> There is not a generic "equals" predicate..  It's based on a data
> type.  

That's OK.  as long as the difference between equals and equivalent is
clear.

> So, as I said, I could build an "AccountGroup" predicate type,

Uhh, now you are making me expend brain cells.  I probably will
get this all wrong, but I think the textbook would say that you 
need an 'object' predicate, with the comparisons is_a and has_a.

So 
AccountGroup has_a GUID  ==> T
numeric has_a GUID ==> F

I'm not sure what happens next:

AccountGroup has_a GUID && GUID == val  ==> T if it has that GUID value ??

AccountGroup has_a Account ==> T whenever the group has subaccounts

AccountGroup has_a Account && Account has_a GUID && GUID == val ==> 
       T whenever the group has subaccounts, one of which has the given
       guid ??

I'm really not clear on this. 

Transaction has_a description ==> T always

Transaction has_a string ==> T always (in fact, trans has several
     strings, e.g. memo)

description is_a string ==> T always

Transaction has_a description  && description is_a string ==> T always 

Transaction has_a description  && description is_a string && string == val
 is true only if the transaction description matches the val.

Then there's joins:

Transaction has_a x && x is_a string && string == val

would return matches on description or memo or any string stored in the
transactions kvp tree ... !??

And finally there are lambda's or unbound vars:

Transaction has_a date && date == x

is a predicate that only acts on date values that are a part of 
transactions. 

You can use the lambda-thingy to build complex predicates (at run-time
instead of compile-time). I guess. Something like that.



> but it would be much better if I could base it on the existing "GUID"
> match....  However the GUID match requires you to match against a list
> of GUIDs.  I'd have to think about how to possibly extend this to
> provide parental searches.

I think it would be fruitful to skim the literature for ideas. Besides
(or instead of) is_a, has_a, there are other ways of combining things,
some supposedly better than others.  I think this dates back to the 
70's/80's when things like relational and object-relational were being
hotly debated.   I'd be happier if what you came up with was grounded 
in theory.

--linas

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pub  1024D/01045933 2001-02-01 Linas Vepstas (Labas!) <linas at linas.org>
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