The reason I didn't want to open a bug report just yet is because I
wanted to discuss whether we really want to check alpha-equivalence
whenever a new scope link is added to the atomspace (and subsequently
consider both equal and return the previous one to the user).
Nil
On 11/17/2016 05:13 AM, Linas Vepstas wrote:
Look, if there is a bug, open a bug report. I'm nervous about these kind
of blanket statements about what's best and what is not best. The
atomspace is already complicated, adding more complexity to it is not a
solution.
--linas
On Wed, Nov 16, 2016 at 8:27 PM, Ben Goertzel <[email protected]
<mailto:[email protected]>> wrote:
Nil,
I just reviewed our dialogue on this from a year ago...
It seems what we provisionally concluded there was that the chainer
should do alpha-conversion on the fly in the course of pattern
matching ... but that the Atomspace shouldn't do alpha-conversion
"automatically" in any other sense [unless we want to add some Reduct
type engine on the Atomspace, which could do alpha-conversion along
with other normalizations, but that becomes a separate issue]
We also discussed a cog-new-var command that could be used to minimize
the complexities of alpha-conversions... (via reducing the incidence
of redundant variable names)
In this case, the alpha-conversion done by the chainer in the course
of doing its business, would need to handle LocalQuoteLink
correctly...
The choice of the chainer to do alpha-conversion but not (yet) more
general types of reduction, would be made because alpha-conversion is
cheaper and easier and of such broad utility. Later versions of the
chainer might do more general reductions as part of their ordinary
business, as well...
I may be missing something; a year ago when William and I were talking
about this, my head was fully immersed in the problem, but it's less
the case right now...
-- Ben
On Wed, Nov 16, 2016 at 8:34 PM, 'Nil Geisweiller' via opencog
<[email protected] <mailto:[email protected]>> wrote:
> I'm back to this issue.
>
> The notion of LocalQuote is indeed incompatible with systematic
> alpha-conversion.
>
> Consider this pattern
>
> (Get
> (VariableList
> (TypedVariable
> (Variable "$X")
> (Type "TypedVariableLink"))
> (TypedVariable
> (Variable "$P")
> (Type "PredicateNode"))
> (TypedVariable
> (Variable "$Q")
> (Type "PredicateNode"))
> (LocalQuote
> (ImplicationScope
> (Variable "$X")
> (Variable "$P")
> (Variable "$Q"))))
>
> This fetches ImplicationScope links.
>
> If the following
>
> (ImplicationScope
> (Variable "$X")
> (Variable "$P")
> (Variable "$Q"))
>
> happen to be alpha-equivalent to something with different variable
names it
> will render the Bind link invalid.
>
> Indeed alpha-conversion shouldn't be triggered in that case, the
right idea
> is that the ImplicationScope, when quoted corresponds to a
DIFFERENT atom
> than the one not being quoted. Also of course if we decide to not
perform
> systematic alpha-conversion then this problem doesn't arise.
>
> I'm re-iterating my question. Do we really want automatic
alpha-conversion
> to begin with?
>
> If the answer is yes then I suppose we need a way to tell that the
quoted
> version is different than then unquoted version.
>
> Nil
>
>
> On 10/22/2016 03:34 AM, Ben Goertzel wrote:
>>
>> Nil,
>>
>> Just brainstorming here, but perhaps the command for adding an Atom
>> should have an option that the user can set, which determines whether
>> the results would be alpha-converted or not
>>
>> The default would be to do the alpha-conversion (which would be
>> appropriate if the variable names are say randomly generated, and
thus
>> of no particular importance to the user -- the alpha conversion is
>> then just preventing odd collisions between randomly generated
>> variable names created by two different processes)
>>
>> However, if the user wants they can override this default and specify
>> "no alpha conversion", and then it is their responsibility to check
>> and be sure their chosen VariableNode names are not going to be used
>> in a way that creates some conflict ...
>>
>> This option would need to be added to Scheme, python, Haskell
>> bindings, but also to the core API for adding scoped links, I
guess...
>>
>> I am only about 83.456% sure I understand the problem here...
>>
>> -- Ben
>>
>>
>>
>> On Fri, Oct 21, 2016 at 11:55 PM, 'Nil Geisweiller' via opencog
>> <[email protected] <mailto:[email protected]>> wrote:
>>>
>>> Hi,
>>>
>>> I start to think that automatic alpha-conversion is evil.
>>>
>>> First let me recall what it does. Say you've added
>>>
>>> (Scope (VariableList (Variable "$X") (Variable "$Y"))
>>> (And (Variable "$X") (Variable "$Y")))
>>>
>>> and you subsequently add
>>>
>>> (Scope (And (Variable "$gold") (Variable "$silver")))
>>>
>>> then recalling the handle of that last addition, you'd get the first
>>> alpha-equivalent scope, which is
>>>
>>> (Scope (VariableList (Variable "$X") (Variable "$Y"))
>>> (And (Variable "$X") (Variable "$Y")))
>>>
>>> This is rather confusing to the user, but even worse the pattern
matcher
>>> behaves differently with the former or the latter. If you use
the former
>>> to
>>> match grounds containing variables "$X" and "$Y" it may not work
due to
>>> the
>>> pattern matcher discarding self-matches. The latter would match
UNLESS
>>> the
>>> former has been previously added, because the variables "$gold" and
>>> "$silver" would be silently replaced by "$X" and "$Y". This is
horribly
>>> confusing to the user!
>>>
>>> Second, it seems rather arbitrary to try to detect this kind of
>>> equivalence
>>> while there's an infinity of others. For instance
>>>
>>> (And (Variable "$X") (And (Variable "$Y"))
>>>
>>> is equivalent to
>>>
>>> (And (Variable "$X") (Variable "$Y"))
>>>
>>> For these reasons I think semantic equivalence detection
shouldn't be
>>> incorporated into the AtomSpace. The AtomSpace should take care
of the
>>> syntax only (OK, with the exception of unordered links), as it's
always
>>> been, and this task should differed to another process working
above the
>>> AtomSpace.
>>>
>>> It was suggested a while ago to have a normal form reduction
engine for
>>> the
>>> AtomSpace, similar to MOSES', and such an engine could be used
to reduce
>>> while adding atoms, if the user chooses so. This is a much
cleaner way to
>>> handle that. Also since semantic equivalence is undecidable,
there will
>>> always be a battle between completeness and performance. Another
reason
>>> to
>>> have this ever growing monster above the AtomSpace rather than
in it.
>>>
>>> OK, I don't know if I've convinced you, or even if I've
convinced myself,
>>> but it's really a discussion we need to have.
>>>
>>> Opinions welcome.
>>>
>>> Nil
>>>
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>>
>>
>>
>>
>
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--
Ben Goertzel, PhD
http://goertzel.org
“I tell my students, when you go to these meetings, see what direction
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