Modified PR

app/Main_abhiroop.hs

@@ -0,0 +1,71 @@
+module Main where
+
+-- How you are supposed to do this exercise ?
+-- 1) Recursion : This is a powerful thing.
+--    In haskell, you would see recursion at a lot of places.
+--    and it is easier to do as well..
+-- 2) writing sub-functions :
+
+-- See this example ;
+-- I have to write a function that give a number N
+--   gives me a list that has N "Hello" strings.
+-- Types says it all :)
+sayNHello :: Int -> [String]
+-- I will create a sub-function.. we use "where" for that..
+sayNHello n = sayNHello' 0 n []
+  where
+    -- this is how you write sub-function
+    -- we do not need to write types here as compiler derives it for us..
+    -- but just for sanity
+    sayNHello' :: Int -> Int -> [String] -> [String]
+    sayNHello' i n resultSoFar =
+      if i < n
+      -- keep appending to the list if we are not done yet..
+      then sayNHello' (i + 1) n ("Hello" : resultSoFar)
+      else resultSoFar -- otherwise we are done..
+
+toDigits :: Integer -> [Integer]
+toDigits 0 = []
+toDigits n = toDigits (n `div` 10) ++ [n `mod` 10]
+
+toDigitsRev :: Integer -> [Integer]
+toDigitsRev 0 = []
+toDigitsRev n = n `mod` 10 : toDigitsRev (n `div` 10) 
+
+-- You need to map over every element of the list..
+-- But you can use recursion for it as well..
+-- Otherwise you can write to the top of file after `module ..`
+-- import qualified Data.List as DL
+-- and use DL.map..
+-- but remember it is all about writing Recursion at this point..
+doubleEveryOther :: [Integer] -> [Integer]
+doubleEveryOther (x:y:xs) = x:(2 * y):doubleEveryOther xs
+doubleEveryOther x = x
+
+sumDigits :: [Integer] -> Integer
+sumDigits [x] = x
+sumDigits (x:xs) = x + sumDigits xs
+
+validate :: Integer -> Bool
+validate n = undefined
+
+main :: IO ()
+main = do
+  putStrLn "Hello world"
+  -- show takes a type and convert to string
+  putStrLn $ show 1
+  putStrLn $ show [1 , 2 , 3]
+  putStrLn $ show True
+  -- Exercise :
+  -- call all above functions here..
+  putStrLn $ show $ sayNHello 5
+  putStrLn $ show $ toDigits 12345
+  putStrLn $ show $ toDigitsRev 12345
+  putStrLn $ show $ doubleEveryOther [1 , 2 , 3 , 4 , 5]
+  putStrLn $ show $ sumDigits [1 , 2 , 3]
+
+
+-- syntax :
+ifSyntax num = if num > 0
+               then num - 1
+               else num + 1