day one logic style

haskell/DayOne.hs

@@ -1,4 +1,5 @@
 module DayOne where
+import Control.Monad (guard)
 
 -- How many different ways can you find to write allEven?
 
@@ -100,3 +101,45 @@ maplayouts = [(al, mi, ga, tn, fl) |
 -- [(Red,Green,Green,Blue,Blue),(Red,Blue,Blue,Green,Green),(Green,Red,Red,Blue,Blue),(Green,Blue,Blue,Red,Red),(Blue,Red,Red,Green,Green),(Blue,Green,Green,Red,Red)]
 --
 
+-- Haskell Wiki: Logic programming
+-- https://wiki.haskell.org/Logic_programming_example
+
+
+data USMap = USMap {
+  alabama :: Color,
+  florida :: Color,
+  georgia :: Color,
+  mississippi :: Color,
+  tennessee :: Color
+} deriving Show
+
+maplayouts2 :: [USMap]
+maplayouts2 = do
+  al <- [Red, Blue, Green]
+  fl <- [Red, Blue, Green]
+  ga <- [Red, Blue, Green]
+  ms <- [Red, Blue, Green]
+  tn <- [Red, Blue, Green]
+
+  guard $ ms /= tn
+  guard $ ms /= al
+  guard $ al /= tn
+  guard $ al /= ms
+  guard $ al /= ga
+  guard $ al /= fl
+  guard $ ga /= fl
+  guard $ ga /= tn
+
+  return $ USMap {
+    alabama = al,
+    florida = fl,
+    georgia = ga,
+    mississippi = ms,
+    tennessee = tn
+  }
+
+-- >>> maplayouts2
+-- [USMap {alabama = Red, florida = Blue, georgia = Green, mississippi = Green, tennessee = Blue},USMap {alabama = Red, florida = Green, georgia = Blue, mississippi = Blue, tennessee = Green},USMap {alabama = Blue, florida = Red, georgia = Green, mississippi = Green, tennessee = Red},USMap {alabama = Blue, florida = Green, georgia = Red, mississippi = Red, tennessee = Green},USMap {alabama = Green, florida = Red, georgia = Blue, mississippi = Blue, tennessee = Red},USMap {alabama = Green, florida = Blue, georgia = Red, mississippi = Red, tennessee = Blue}]
+--
+  
+