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Task/Hash-join/Racket/hash-join.rkt

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+#lang racket
+(struct A  (age name))
+(struct B  (name nemesis))
+(struct AB (name age nemesis) #:transparent)
+
+(define Ages-table
+  (list (A 27 "Jonah") (A 18 "Alan")
+        (A 28 "Glory") (A 18 "Popeye")
+        (A 28 "Alan")))
+
+(define Nemeses-table
+  (list
+   (B "Jonah" "Whales") (B "Jonah" "Spiders")
+   (B "Alan" "Ghosts") (B "Alan" "Zombies")
+   (B "Glory" "Buffy")))
+
+;; Hash phase
+(define name->ages#
+  (for/fold ((rv (hash)))
+    ((a (in-list Ages-table)))
+    (match-define (A age name) a)
+    (hash-update rv name (λ (ages) (append ages (list age))) null)))
+
+;; Join phase
+(for*/list
+    ((b (in-list Nemeses-table))
+     (key (in-value (B-name b)))
+     (age (in-list (hash-ref name->ages# key))))
+  (AB key age (B-nemesis b)))