A-M baby

Task/Fibonacci-sequence/Fexl/fibonacci-sequence.fexl

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+# fibonacci is the infinite list of all Fibonacci numbers.
+#
+# Note that this program uses the symbols "1" and "+".  You can specify
+# the definitions of those symbols however you like, allowing you to use
+# any system of arithmetic you need.  For example, you can use either the
+# built-in long arithmetic, or infinite precision arithmetic, by simply
+# defining "1" and "+" appropriately.
+
+\fibonacci =
+    (
+    \fibonacci == (\x\y
+        item x;
+        \z = (+ x y)
+        fibonacci y z
+        )
+
+    fibonacci 1 1
+    )
+
+# OK, so that's the list of *all* Fibonacci numbers.  If you want the nth number,
+# you can extract it with the fib function as follows.  By the way, this *does*
+# have the effect of caching, so once a particular point in the sequence is
+# calculated, it doesn't have to be calculated again.
+
+# (fib n) is the nth fibonacci number, starting with n==0, or 1 if n is negative.
+\fib = (\n list_at fibonacci n 1)