marvin at rectangular
Mar 1, 2008, 12:56 PM
Post #2 of 4
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On Mar 1, 2008, at 10:57 AM, Father Chrysostomos wrote:
> I just got this error message when testing my RegexpTermWeight class:
>
> Error in function kino_Weight_init_from_hash at ../c_src/KinoSearch/
> Search/Weight.c:24: Can't find 'similarity'
>
> Although this is not documented, Weight apparently requires a
> Similarity object to be passed to its constructor. Is this permanent
> (in which case I can write a doc patch)? or is this going to change?
It's permanent. I've committed a doc patch, and would appreciate
review:
http://xrl.us/bgzv6 (Link to www.rectangular.com)
Similarity objects are assigned via Schema, almost exactly like
Analyzers. The Schema itself has one primary Similarity; individual
FieldSpecs may override FieldSpec::similarity() to provide another.
The only difference is that Schema::analyzer() is an abstract method
that every subclass has to implement, while Schema::similarity()
returns a standard Similarity object by default.
If every Weight subclass was associated with a field, it would be
possible to automatically retrieve the correct similarity like so:
my $sim = $searchable->get_schema->fetch_sim($field);
However, some Weight subclasses don't have a field -- e.g.
BooleanWeight.
It's tempting to default to the Schema's primary similarity, but
that's not failsafe design. If a field-specific Weight subclass fails
to supply a value for "similarity", it should trigger an error rather
than the silently incorrect behavior of defaulting to the wrong object.
> Note that this also prevents the example
> in ::Cookbook::WildCardQuery from working.
I changed the Cookbook example in the commit as well. However, both
Weight and Cookbook::WildCardQuery now refer to Schema::fetch_sim,
which isn't currently exposed as a public method. We'll need to fix
that.
I think Nathan will argue that Schema::fetch_sim should be renamed to
Schema::fetch_similarity() before it goes public. If he does, he's
probably right. :)
Marvin Humphrey
Rectangular Research
http://www.rectangular.com/
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