IIUC in SQL this would simply be:
SELECT student, AVG(rating)
FROM scores
GROUP BY student
Apparently a DSL for querying tables can be handy. :)
The Racket equivalent for the special case of a 2-column table (a
hash-table) could be something like:
(define (sql-ish-aggregate-group-by f xs)
(for/hash ([(k v) (in-hash (for/fold ([ht (hash)]) ; a "gather"
([x (in-list xs)])
(match-define (list k v) x)
(hash-update ht k (λ (vs) (cons v vs)) '())))])
(cons k (f v))))
"Group by" seems to be a Rorschach phrase; overloaded meanings.
Clojure has a group-by that differs from SQL or unstable/list. I
don't know Clojure well enough to code golf this.
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