import Data.Function factF f = \n -> if n == 0 then 1 else n * f(n-1) fact0 = factF undefined fact1 = factF . factF $ undefined fact2 = factF . factF . factF $ undefined fact3 = factF . factF . factF . factF $ undefined fact = fix factF
$ghci Prelude>:load fact.hs *Main> fact0 0 1 *Main> fact0 1 *** Exception: Prelude.undefined *Main> fact1 0 1 *Main> fact1 1 1 *Main> fact1 2 *** Exception: Prelude.undefined *Main> fact 100 93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000