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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