day one file

.gitignore

@@ -8,3 +8,4 @@ project/metals.sbt
 nashorn_code_cache
 out
 .nrepl-port
+dist-newstyle

haskell/DayOne.hs

@@ -0,0 +1,66 @@
+module DayOne where
+
+-- How many different ways can you find to write allEven?
+
+allEven :: [Integer] -> [Integer]
+allEven [] = []
+allEven (h:t) = if even h then h:allEven t else allEven t
+
+-- >>> allEven [1, 2, 3, 4, 5, 6]
+-- [2,4,6]
+--
+
+allEven2 :: [Integer] -> [Integer]
+allEven2 xs = [x | x <- xs, even x]
+
+-- >>> allEven2 [1, 2, 3, 4, 5, 6]
+-- [2,4,6]
+--
+
+allEven3 :: [Integer] -> [Integer]
+allEven3 = filter even
+
+-- >>> allEven3 [1, 2, 3, 4, 5, 6]
+-- [2,4,6]
+--
+
+
+-- Write a function that takes a list and returns the same list in reverse.
+
+backwards :: [a] -> [a]
+backwards = reverse
+
+-- >>> backwards [1, 2, 3, 4]
+-- [4,3,2,1]
+--
+
+backwards2 :: [a] -> [a]
+backwards2 [] = []
+backwards2 (x:xs) = backwards2 xs ++ [x]
+
+-- >>> backwards2 [1, 2, 3, 4]
+-- [4,3,2,1]
+--
+
+-- Write a function that builds two-tuples with all possible combinations of
+-- two of the colors black, white, blue, yellow, and red. Note that you should
+-- include only one of(black, blue)and(blue, black).
+
+data Color = Black
+             | White
+             | Blue
+             | Yellow
+             | Red
+             deriving (Enum, Ord, Eq, Show)
+
+-- https://stackoverflow.com/questions/4299319/getting-a-list-of-all-possible-data-type-values-in-haskell
+allColors :: (Enum a) => [a]
+allColors = [toEnum 0 ..]
+
+pairs :: [(Color, Color)]
+pairs = [(a, b) | a <- allColors, b <- allColors, a < b]
+
+-- >>> pairs
+-- [(Black,White),(Black,Blue),(Black,Yellow),(Black,Red),(White,Blue),(White,Yellow),(White,Red),(Blue,Yellow),(Blue,Red),(Yellow,Red)]
+--
+

haskell/Main.hs

@@ -1,4 +1,6 @@
 module Main where
 
+import DayOne
+
 main :: IO ()
 main = putStrLn "Hello, Haskell!"

haskell/all_even.hs

@@ -1,21 +0,0 @@
-module Main where
-  allEven :: [Integer] -> [Integer]
-  allEven [] = []
-  allEven (h:t) = if even h then h:allEven t else allEven t
-
-  allEven2 :: [Integer] -> [Integer]
-  allEven2 xs = [x | x <- xs, even x]
-
-  allEven3 :: [Integer] -> [Integer]
-  allEven3 = filter even
-
-  -- >>> allEven [1, 2, 3, 4, 5, 6]
-  -- [2,4,6]
-  -- >>> allEven2 [1, 2, 3, 4, 5, 6]
-  -- [2,4,6]
-  -- >>> allEven3 [1, 2, 3, 4, 5, 6]
-  -- [2,4,6]
-
-  main :: IO()
-  main =
-    print "yes"