add illustration samples

samples/fpsamples/sample3.hs

@@ -0,0 +1,12 @@
+-- This function uses a boolean guard and defines
+-- its value only for the remaining cases.
+fact :: Int -> Int
+fact x | x <= 1 = 1
+       | otherwise = x * (fact (x - 1))
+
+-- Alternatively, pattern matching can be used for an
+-- even more explicit declaration.
+fact' :: Int -> Int
+fact' 0 = 1
+fact' 1 = 1
+fact' x = x * (fact (x - 1))