Adding solutions for 12-a-fistful-of-monads.hs

12-a-fistful-of-monads.hs

@@ -5,27 +5,43 @@
  -   Success, which takes that parameter and
  -   Fail String, which represents a failure, and a reason for that failure
  -}
---data Validation a 
+data Validation a = Success a | Fail String deriving (Show)
 
 -- Make the Validation a Monad
+instance Monad Validation where 
+  (Success a) >>= f = f a
+  (Fail s) >>= _ = Fail s
 
+instance Applicative Validation where
+  pure a = Success a
+  Success f <*> Success a = Success (f a)
+  (Fail s) <*> _ = Fail s
+  _ <*> (Fail s) = Fail s
+
+instance Functor Validation where
+  fmap f (Success a) = Success (f a)
+  fmap _ (Fail s) = Fail s
 
 {-
  - Create a function, positiveCheck, which takes a number and returns a successful Validation if it's positive, 
  - and a failed Validation with a String message if not.
  -}
---positiveCheck :: (Num a, Ord a) => a -> Validation a
---positiveCheck x = undefined
+positiveCheck :: (Num a, Ord a) => a -> Validation a
+positiveCheck a = if (a > 0)
+                  then Success a
+                  else Fail "negative number"
 
 {-
  - Create a function, evenCheck, which returns a successful Validation if it's even,
  - and a failed Validation with a string message if it's odd
  -}
---evenCheck :: (Integral a)  =>  a -> Validation a
---evenCheck x = undefined
+evenCheck :: (Integral a)  =>  a -> Validation a
+evenCheck x = if (x `mod` 2 == 0)
+              then Success x
+              else Fail "odd number"
 
 {-
  - Write a function which uses positiveCheck and evenCheck to make sure a number is both positive and even
  -}
---positiveAndEvenCheck :: (Num a, Ord a, Integral a) => a -> Validation a
---positiveAndEvenCheck x = undefined
+positiveAndEvenCheck :: (Num a, Ord a, Integral a) => a -> Validation a
+positiveAndEvenCheck x = return x >>= positiveCheck >>= evenCheck