Factorial Calculator with tail call optimization written in Hoon

fac-1.hoon

/+  mylib               ::  import
=,  mylib               ::  alias to the subject (don't have to type mylib anymore)
|=  a=@
^-  @
(new-fac a)

factorial.md

Factorial calculator with tail call optimization

Write a gate that takes some integer n and returns n! (i.e., the factorial of n).

0! = 1 1! = 1 2! = 2 x 1 3! = 3 x 2 x 1 4! = 4 x 3 x 2 x 1 and so on...

Make sure your gate is "tail call optimized". Feel free to look up TCO so that you have a strong understanding of it---this is an important concept for functional programming.

Having a TCO gate means, among other things, that any recursion you use shouldn't be embedded within a larger cell. E.g., don't recurse like this:

[one-result $(c new-c)]

mylib.hoon

|%

++  new-fac
  |=  a=@
  ^-  @
  %-  inner-fac  [a 1]

++  inner-fac
  |=  c=[a=@ b=@]                             :: sample (input mold)
  ^-  @                                               :: return mold

  ?:  (gte 0 a.c)
    1
    ?:  (gte 1 a.c)
      b.c
      %-  inner-fac  [(dec a.c) (mul b.c a.c)]

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