@@ -423,6 +423,26 @@

                       (reverse (cdr it))))

               (alexandria:hash-table-alist groups)))))

+(defun hash-join (probe join-fn &key (test 'eql) (key 'car))

+  (let* ((lookup (make-hash-table :test test :size (length probe)))

+         (lookup-fn (functionalize lookup)))

+    (map nil

+         (lambda (it)

+           (setf (gethash (funcall key it)

+                          lookup)

+                 it))

+         probe)

+    (lambda (collection)

+      (map (etypecase collection

+             (list 'list)

+             (vector 'vector)

+             (sequence 'list))

+           (lambda (it)

+             (let* ((key-value (funcall key it))

+                    (matching-probe (funcall lookup-fn key-value)))

+               (funcall join-fn it matching-probe)))

+           collection))))

+

 #+nil

 (defmacro <> (arity &rest funs)

   (let ((arg-syms (loop repeat arity collect (gensym))))

@@ -22,6 +22,7 @@

            #:∘ #:suffixp #:functionalize #:inc #:group-by #:keys

            #:conj #:disj #:delay #:of-type #:transform #:calling*

            #:calling

+           #:hash-join

            #:tap))

 (defpackage :data-lens.transducers.internals