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 "$Q"))
happen to be alpha-equivalent to something with different variable names
it will render the Bind link invalid.
Indeed alpha-conversion shouldn't be triggered in that case, the right
idea is that the ImplicationScope, when quoted corresponds to a
DIFFERENT atom than the one not being quoted. Also of course if we
decide to not perform systematic alpha-conversion then this problem
doesn't arise.
I'm re-iterating my question. Do we really want automatic
alpha-conversion to begin with?
If the answer is yes then I suppose we need a way to tell that the
quoted version is different than then unquoted version.
Nil
On 10/22/2016 03:34 AM, Ben Goertzel wrote:
Nil,
Just brainstorming here, but perhaps the command for adding an Atom
should have an option that the user can set, which determines whether
the results would be alpha-converted or not
The default would be to do the alpha-conversion (which would be
appropriate if the variable names are say randomly generated, and thus
of no particular importance to the user -- the alpha conversion is
then just preventing odd collisions between randomly generated
variable names created by two different processes)
However, if the user wants they can override this default and specify
"no alpha conversion", and then it is their responsibility to check
and be sure their chosen VariableNode names are not going to be used
in a way that creates some conflict ...
This option would need to be added to Scheme, python, Haskell
bindings, but also to the core API for adding scoped links, I guess...
I am only about 83.456% sure I understand the problem here...
-- Ben
On Fri, Oct 21, 2016 at 11:55 PM, 'Nil Geisweiller' via opencog
<[email protected]> wrote:
Hi,
I start to think that automatic alpha-conversion is evil.
First let me recall what it does. Say you've added
(Scope (VariableList (Variable "$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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