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Task/Flatten-a-list/EchoLisp/flatten-a-list.echolisp

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+(define (fflatten l)
+(cond
+[(null? l) null]
+[(not (list? l)) (list l)]
+[else (append (fflatten (first l)) (fflatten (rest l)))]))
+
+;;
+(define L' [[1] 2 [[3 4] 5] [[[]]] [[[6]]] 7 8 []])
+
+(fflatten L) ;; use custom function
+ → (1 2 3 4 5 6 7 8)
+(flatten L) ;; use built-in
+ → (1 2 3 4 5 6 7 8)
+
+;; Remarks
+;; null is the same as () - the empty list -
+(flatten '(null null null))
+   → null
+(flatten '[ () () () ])
+  → null
+(flatten null)
+❗ error: flatten : expected list : null
+
+;; The 'reverse' of flatten is group
+(group '( 4 5 5 5 6 6 7 8 7 7 7 9))
+    → ((4) (5 5 5) (6 6) (7) (8) (7 7 7) (9))