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+; P07 Flatten a nested array structure.
+; Transform an array, possibly holding arrays as elements into a `flat'
+; list by replacing each array with its elements (recursively).
+
+(def a (list "a" (list "b" (list "c" "d") "e")))
+
+; this belongs in stdlib
+(func extend (lst1 lst2)
+    (reduce append lst2 lst1))
+
+; can overload functions of different arities
+(func flatten-iter (item)
+    (flatten-iter item (list)))
+
+; iterative solution
+(func flatten-iter (item acc)
+    (branch
+        ((eq? (typeof item) :[:any])
+            (if (empty? item)
+                    acc
+                (flatten-iter (rest item) (extend acc (flatten-iter (first item))))))
+        (#true
+            (append acc item))))
+
+; we can also use the type system/dynamic dispatch to split up the work
+(func flatten-typ (item)
+    (flatten-typ item (list)))
+
+; non-empty lists
+(func flatten-typ (lst :{:any} acc)
+    (flatten-typ (rest lst) (extend acc (flatten-typ (first lst)))))
+
+; empty lists
+(func flatten-typ (lst :nil acc)
+    acc)
+
+; other things
+; note: we would need to know what type(s) is inside the list to avoid an
+; ambiguous definition
+;(func flatten-typ (item :string acc)
+;    (append acc item))
+
+; this might be clearer/more flexible
+; maybe "not this type" should be a thing, like :![:any]
+(func not-a-list? (item)
+    (not (eq? (typeof item) :[:any])))
+
+(type :not-a-list :any not-a-list?) ; should be able to pass a lambda
+
+; this handles anything that's not a list
+(func flatten-typ (item :not-a-list acc)
+    (append acc item))
+
+(print (->string a))
+(print (->string (flatten-iter a)))
+(print (->string a))
+(print (->string (flatten-typ a)))