Solve 'Getting MAD' kata

Author: borisskert

src/GettingMAD.hs

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+module GettingMAD (gettingMAD) where
+
+-- https://www.codewars.com/kata/593a061b942a27ac940000a7/train/haskell
+
+import Data.List (sort)
+
+gettingMAD :: [Int] -> Int
+gettingMAD = minimum . map diff . tuples . sort
+
+tuples :: [a] -> [(a, a)]
+tuples [] = []
+tuples [_] = []
+tuples (x : y : xs) = (x, y) : tuples (y : xs)
+
+diff :: (Num a) => (a, a) -> a
+diff (x, y) = abs (x - y)

test/GettingMADSpec.hs

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+module GettingMADSpec (spec) where
+
+import GettingMAD (gettingMAD)
+import Test.Hspec
+
+spec :: Spec
+spec = do
+  it "example tests" $ do
+    gettingMAD [-10, 0, -3, 1] `shouldBe` 1
+    gettingMAD [0, 0, 0, 0] `shouldBe` 0
+    gettingMAD [-570, 542] `shouldBe` 1112
+    gettingMAD [-69, -808, 828, 57] `shouldBe` 126