Bug: dl-eval-aggregate accepted non-variable agg-vars and non-
literal goals silently, producing weird/incorrect counts:
- `count(N, 5, p(X))` would compute count over the single
constant 5 (always 1), ignoring p entirely.
- `count(N, X, 42)` would crash with "unknown body-literal
shape" at saturation time rather than at rule-add time.
Fix: dl-eval-aggregate now validates up front that the second
arg is a variable (the value to aggregate) and the third arg is
a positive literal (the goal). Errors are descriptive and
include the offending argument.
2 new aggregate tests.
(findall L V Goal) — bind L to the distinct V values for which Goal
holds, or the empty list when none. One-line addition to
dl-do-aggregate that returns the unreduced list. Tests cover EDB,
derived relation, and empty cases.
Useful for "give me all the X such that ..." queries without
scalar reduction.
New lib/datalog/aggregates.sx: (count R V Goal), (sum R V Goal),
(min R V Goal), (max R V Goal). dl-eval-aggregate runs
dl-find-bindings on the goal under the outer subst, collects
distinct values of V, applies the operator, binds R. Empty input:
count/sum return 0; min/max produce no binding (rule fails).
Group-by emerges naturally from outer-subst substitution into the
goal — `popular(P) :- post(P), count(N, U, liked(U, P)), >=(N, 3).`
counts per-post.
Stratifier extended: dl-aggregate-dep-edge contributes a
negation-like edge so the aggregate's goal relation is fully
derived before the aggregate fires (non-monotonicity respected).
Safety relaxed for aggregates: goal-internal vars are existentials,
only the result var becomes bound.
