[rules-dev] More on Field binding
On 18/02/2013, Mark Proctor <mproctor(a)codehaus.org> wrote:
On 18 Feb 2013, at 18:38, Wolfgang Laun <wolfgang.laun(a)gmail.com> wrote:
I don't like nested bindings, I think it was there before due to backwards
compatibility.
5.1.1 permitted a binding to the field on which a constraint was based, e.g.,
Person( $a: age > 10 || $sa: sibling.age > 10 )
If we drop it for DRL6, we need to figure out a migration strategy. One
possible way is we allow it in the parser stiff, as a config option (off by
default, with warning of deprecated feature).
Also field bindings are hangover from the Lisp days. Maybe we don't need
property bindings anymore within patterns. Most use cases can use the
pattern binding accessor for property access.
I'd say that field bindings must have some attraction in addition to
their heritage. Note that Lisp-ish CLIPS can access fact properties
based on a binding to the fact and the field name, but field bindings
are there all the same. (And, just because something's been used in
another language doesn't mean it's bad and must be avoided like the
plague.)
Simple field bindings is the one feature that permits me to write
rules tersely, lets me introduce an abbreviation for lengthy
factref.nameOfTheField and - if used consistently - signals a
pattern's dependency on some fact field.
And what about the positional bindings? I guess they are here to stay.
If so, it would be bizarre to the extreme if you could write
Foo( $a, $b; )
but not
Foo( $a: a, $b: b )
Consider: a feature requiring annotations on the unordered fields Java
class is permitted, but a very similar feature NOT requiring
annotations is not...?
Summary: Retain simple forms for binding "on the fly" to a field.
IMHO, there's no need to go overboard to permit it anywhere in complex
expressions (Edson's worry).
-W
when
$p : Pattern()
then
println( $p.name );
end
Field bindings allow bindings to complex expression results, involving
nested accessors. We can use the "set" proposal I mentioned, and keep those
outside of patterns.
when
$p : Pattern()
$age : { $p.age * someGlobalIt } // $age will bind to the result of the
expression
then
println( $p.name + ":" + $age );
end