Solve 'Geometry Basics: Distance between points in 2D' kata

src/Distance.hs

@@ -0,0 +1,11 @@
+module Distance (distance) where
+
+-- https://www.codewars.com/kata/58dced7b702b805b200000be/train/haskell
+
+import Preloaded (Point(..))
+
+distance :: Point -> Point -> Double
+distance p1 p2 = sqrt ((x p1 - x p2) ^ 2 + (y p1 - y p2) ^ 2)
+
+-- #againwhatlearned: destructure Point structure:
+-- distance (Point x1 y1) (Point x2 y2) = ...

src/Preloaded.hs

@@ -28,4 +28,6 @@ isWaveSorted xs
   where
     divvinger = divvy 3 2 xs
 
-data Tree = Node { val :: Int, left, right :: Maybe Tree }
+data Tree = Node {val :: Int, left, right :: Maybe Tree}
+
+data Point = Point {x :: Double, y :: Double} deriving (Show)

test/DistanceSpec.hs

@@ -0,0 +1,29 @@
+module DistanceSpec (spec) where
+
+import Preloaded (Point(..))
+import Distance (distance)
+import Test.Hspec
+import Control.Monad (unless)
+
+spec :: Spec
+spec = do
+  it "example tests" $ do
+    test Point { x = 0, y =   1 } Point { x =   0, y =  0 } 1.0
+    test Point { x = 0, y =   3 } Point { x =   4, y =  0 } 5
+    test Point { x = 1, y =  -1 } Point { x =   1, y = -1 } 0
+    test Point { x = 0, y =   0 } Point { x =   0, y =  0 } 0
+    test Point { x = 0, y = 0.3 } Point { x = 0.4, y =  0 } 0.5
+
+epsilon :: Double
+epsilon = 1e-6
+
+test :: Point -> Point -> Double -> Expectation
+test a b 0 = do
+  let actual = distance a b
+      message = "distance " ++ show a ++ " " ++ show b ++ ": expected actual value " ++ show actual ++ " to approximately equal expected value 0 ( accepted absolute error: " ++ show epsilon ++ " )"
+  unless (abs actual <= epsilon) $ expectationFailure message
+test a b expected = do
+  let actual = distance a b
+      message = "distance " ++ show a ++ " " ++ show b ++ ": expected actual value " ++ show actual ++ " to approximately equal expected value " ++ show expected ++ " ( accepted relative error: " ++ show epsilon ++ " )"
+  unless (abs (actual - expected) <= epsilon * abs expected) $ expectationFailure message
+