Add unfold

snippets/unfold.md

@@ -0,0 +1,25 @@
+---
+title: unfold
+tags: function,list,advanced
+---
+
+Builds a list, using an iterator function and an initial seed value.
+
+The iterator function accepts one argument (`seed`) and must always return a list with two elements ([`value`, `nextSeed`]) or `False` to terminate.
+Use a generator function, `fn_generator`, that uses a `while` loop to call the iterator function and `yield` the `value` until it returns `False`.
+Use list comprehension to return the list that is produced by the generator, using the iterator function.
+
+```py
+def unfold(fn, seed):
+  def fn_generator(val):
+    while True: 
+      val = fn(val[1])
+      if val == False: break
+      yield val[0]
+  return [i for i in fn_generator([None, seed])]
+```
+
+```py
+f = lambda n: False if n > 50 else [-n, n + 10]
+unfold(f, 10) # [-10, -20, -30, -40, -50]
+```