September 2017 Update

Task/Haversine-formula/Haskell/haversine-formula.hs

@@ -1,28 +1,33 @@
 import Text.Printf
+import Control.Arrow ((***))
 
 -- The haversine of an angle.
-hsin t = let u = sin (t/2) in u*u
-
--- The distance between two points, given by latitude and longtitude, on a
--- circle.  The points are specified in radians.
-distRad radius (lat1, lng1) (lat2, lng2) =
-  let hlat = hsin (lat2 - lat1)
-      hlng = hsin (lng2 - lng1)
-      root = sqrt (hlat + cos lat1 * cos lat2 * hlng)
-  in 2 * radius * asin (min 1.0 root)
-
--- The distance between two points, given by latitude and longtitude, on a
--- circle.  The points are specified in degrees.
-distDeg radius p1 p2 = distRad radius (deg2rad p1) (deg2rad p2)
-  where deg2rad (t, u) = (d2r t, d2r u)
-        d2r t = t * pi / 180
+haversine :: Float -> Float
+haversine = (^ 2) . sin . (/ 2)
 
 -- The approximate distance, in kilometers, between two points on Earth.
 -- The latitude and longtitude are assumed to be in degrees.
-earthDist = distDeg 6372.8
+earthDist :: (Float, Float) -> (Float, Float) -> Float
+earthDist = distDeg 6371
+  where
+    distDeg radius p1 p2 = distRad radius (deg2rad p1) (deg2rad p2)
+    distRad radius (lat1, lng1) (lat2, lng2) =
+      (2 * radius) *
+      asin
+        (min
+           1.0
+           (sqrt $
+            haversine (lat2 - lat1) +
+            ((cos lat1 * cos lat2) * haversine (lng2 - lng1))))
+    deg2rad = d2r *** d2r
+      where
+        d2r = (/ 180) . (pi *)
 
-main = do
-  let bna = (36.12,  -86.67)
-      lax = (33.94, -118.40)
-      dst = earthDist bna lax :: Double
-  printf "The distance between BNA and LAX is about %0.f km.\n" dst
+main :: IO ()
+main =
+  printf
+    "The distance between BNA and LAX is about %0.f km.\n"
+    (earthDist bna lax)
+  where
+    bna = (36.12, -86.67)
+    lax = (33.94, -118.40)