module Plutarch.Docs.Recursion (pfac) where 
import Plutarch.Prelude

Recursion

To emulate recursion in UPLC (Untyped Plutus Core), you need to use the Y combinator. Plutarch provides the Y combinator with the name pfix:

pfix :: Term s (((a :--> b) :--> (a :--> b)) :--> (a :--> b))

It works as you would expect, though the type is scary. Think of it as the Haskell type:

fix :: ((a -> b) -> (a -> b)) -> (a -> b)

The first argument is “self”, or the function you want to recurse with.

The below example implements a Plutarch-level factorial function:

pfac :: Term s (PInteger :--> PInteger)
pfac = pfix #$ plam f
  where
    f :: Term s (PInteger :--> PInteger) -> Term s PInteger -> Term s PInteger
    f self n = pif (n #== 1) n $ n * (self #$ n - 1)
-- (ignore the existence of non positives :D)

Note how f takes in a self and just recurses on it. All you have to do, is create a Plutarch level function by using plam on f and pfix the result - and that self argument will be taken care of for you.