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# Add alpha-unique-atom: Alpha-Equivalence Deduplication Function #150

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# Add alpha-unique-atom: Alpha-Equivalence Deduplication Function #150

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0845aae
test: add comprehensive alpha-safe test suite for alpha-unique-atom
aprilyab Mar 27, 2026
1764eb6
feat: add alpha-unique-atom for structural deduplication
aprilyab Mar 27, 2026
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59 changes: 59 additions & 0 deletions examples/test_alpha_unique_atom.metta
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Original file line number Diff line number Diff line change
@@ -0,0 +1,59 @@
; Strong test suite for alpha-unique-atom

; 1 Basic duplicates with different variables
!(test (=alpha (alpha-unique-atom ((link $x human) (link $y human) (link $z human)))
((link $a human)))
True)

!(test (=alpha (alpha-unique-atom ((parent $a human) (parent $b human) (child $c human)))
((parent $d human) (child $e human)))
True)

; 2 Different functors (should all remain)
!(test (=alpha (alpha-unique-atom ((parent $x human) (child $y human) (friend $z human)))
((parent $a human) (child $b human) (friend $c human)))
True)

!(test (=alpha (alpha-unique-atom ((likes $x) (hates $y) (knows $z)))
((likes $a) (hates $b) (knows $c)))
True)

; 3 Nested structures
!(test (=alpha (alpha-unique-atom ((link (foo $x) human) (link (foo $y) human) (link (bar $z) human)))
((link (foo $a) human) (link (bar $b) human)))
True)

!(test (=alpha (alpha-unique-atom ((parent (child $x) human) (parent (child $y) human) (parent (child $x) human)))
((parent (child $a) human)))
True)

; 4 Mix of unique and duplicates
!(test (=alpha (alpha-unique-atom ((link $x human) (parent $x human) (link $y human) (parent $z human) (link $x human)))
((parent $a human) (link $b human)))
True)

!(test (=alpha (alpha-unique-atom ((foo $x) (foo $y) (bar $x) (foo $x) (bar $y)))
((foo $a) (bar $b)))
True)

; 5 Numbers and atoms
!(test (=alpha (alpha-unique-atom (1 2 2 3 1 4 4 5))
(2 3 1 4 5))
True)

!(test (=alpha (alpha-unique-atom (a b a c b d e a))
(c b d e a))
True)
Comment on lines +40 to +46
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Copilot AI Mar 27, 2026

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The expected results here (e.g., (1 2 2 3 1 4 4 5) -> (2 3 1 4 5) and (a b a c b d e a) -> (c b d e a)) encode “keep last occurrence” semantics. If alpha-unique-atom is meant to mirror unique-atom and only change the equality notion (as described in the PR), these expectations should be updated to match first-occurrence preservation instead.

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; 6 Empty and single-element lists
!(test (=alpha (alpha-unique-atom ())
())
True)

!(test (=alpha (alpha-unique-atom (1))
(1))
True)

!(test (=alpha (alpha-unique-atom ((link $x human)))
((link $a human)))
True)
16 changes: 15 additions & 1 deletion src/metta.pl
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Expand Up @@ -112,6 +112,20 @@
first([A, _], A).
'second-from-pair'([_, A], A).
'unique-atom'(A, B) :- list_to_set(A, B).
'alpha-unique-atom'(A, B) :- alpha_list_to_set(A, B).
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Unlike unique-atom/2 (list_to_set/2), alpha-unique-atom/2 will simply fail if A is not a proper list (because alpha_list_to_set/2 only matches []/[H|T]) rather than raising a type error. If callers rely on the existing error behavior for invalid inputs, consider adding explicit list validation (e.g., must_be(list, A)) so alpha-unique-atom behaves consistently with unique-atom.

Suggested change
'alpha-unique-atom'(A, B) :- alpha_list_to_set(A, B).
'alpha-unique-atom'(A, B) :- must_be(list, A), alpha_list_to_set(A, B).

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% Helper: like list_to_set/2 but uses =@= (structural equality)
alpha_list_to_set([], []).
alpha_list_to_set([H|T], R) :-
( alpha_member_eq(H, T) ->
alpha_list_to_set(T, R)
;
alpha_list_to_set(T, RT),
Comment on lines +118 to +122
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alpha_list_to_set/2 currently removes an element if an alpha-equivalent one exists later in the list, which means it keeps the last occurrence of each equivalence class. This differs from unique-atom (list_to_set/2), which keeps the first occurrence, and it contradicts the PR description that “only the equality notion” changes. Consider rewriting alpha_list_to_set/2 to scan left-to-right while tracking already-seen alpha-variants so the first representative is preserved (or update the docs/tests if last-occurrence semantics are intended).

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R = [H|RT]
).
Comment on lines +117 to +124
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Copilot AI Mar 27, 2026

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alpha_list_to_set/2 performs an O(n^2) pairwise scan (alpha_member_eq/2 for each element). If alpha-unique-atom is expected to run on large atom lists (e.g., reasoning outputs), this could become a hot spot compared to the built-in list_to_set/2. Consider using a canonicalized representation for alpha-equivalence (e.g., copy_term + numbervars) and tracking seen keys in an assoc/dict to get closer to O(n).

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% Checks if X is structurally equal to any element in the list
alpha_member_eq(X, [H|_]) :- X =@= H, !.
alpha_member_eq(X, [_|T]) :- alpha_member_eq(X, T).
'sort-atom'(List, Sorted) :- msort(List, Sorted).
'size-atom'(List, Size) :- length(List, Size).
'car-atom'([H|_], H).
Expand Down Expand Up @@ -271,7 +285,7 @@
register_fun(N) :- (fun(N) -> true ; assertz(fun(N))).
:- maplist(register_fun, [superpose, empty, let, 'let*', '+','-','*','/', '%', min, max, 'change-state!', 'get-state', 'bind!',
'<','>','==', '!=', '=', '=?', '<=', '>=', and, or, xor, implies, not, sqrt, exp, log, cos, sin,
'first-from-pair', 'second-from-pair', 'car-atom', 'cdr-atom', 'unique-atom',
'first-from-pair', 'second-from-pair', 'car-atom', 'cdr-atom', 'unique-atom', 'alpha-unique-atom',
repr, repra, parse, 'println!', 'readln!', test, assert, 'mm2-exec', atom_concat, atom_chars, copy_term, term_hash,
foldl, first, last, append, length, 'size-atom', sort, msort, member, 'is-member', 'exclude-item', list_to_set, maplist, eval, reduce, 'import!',
'add-atom', 'remove-atom', 'get-atoms', match, 'is-var', 'is-expr', 'is-space', 'get-mettatype',
Expand Down
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