diff --git a/src/NetPresentValue.hs b/src/NetPresentValue.hs
index af73ac8c4961f693f73a9c38ef6312c9909d4397..735ed016b7b22e648f06a63a6cfc98f4ab172008 100644
--- a/src/NetPresentValue.hs
+++ b/src/NetPresentValue.hs
@@ -29,25 +29,29 @@ calculateNetPresentValue :: InputNetPresentValue -> Double
 calculateNetPresentValue inputNetPresentValue = netPresentValue inputNetPresentValue
 
 -- high order function
+netPresentValue :: InputNetPresentValue -> Double
 netPresentValue inputNetPresentValue = foldl (+) 0 pv
     where
         pv = presentValue inputNetPresentValue
 
 -- high order function
+presentValue :: InputNetPresentValue -> [Double]
 presentValue inputNetPresentValue = zipWith (*) (netcashflow income outcome) (factorpresentvalue (project_interest inputNetPresentValue))
     where
         income = [income_1 inputNetPresentValue, income_2 inputNetPresentValue, income_3 inputNetPresentValue, income_4 inputNetPresentValue, income_5 inputNetPresentValue]
         outcome = [outcome_1 inputNetPresentValue, outcome_2 inputNetPresentValue, outcome_3 inputNetPresentValue, outcome_4 inputNetPresentValue, outcome_5 inputNetPresentValue]
 
 -- currying
+-- partial evaluation
+netcashflow :: [Double] -> [Double] -> [Double]
 netcashflow = zipWith (-) 
 
--- recursion
-power n 1 = n
-power n m = n * (power n (m-1))
-
--- recursion
-powervalue interest n = 1 / (power (1 + interest) n)
+-- pure function
+powervalue :: Floating a => a -> a -> a
+powervalue interest n = 1 / ((1 + interest)**n)
 
 -- high order function
+-- anonymous function
+-- polymorphic
+factorpresentvalue :: (Floating b, Enum b) => b -> [b]
 factorpresentvalue interest = map (\x -> powervalue interest x) [1..5]