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Task/Haversine-formula/Idris/haversine-formula.idris

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+module Main
+
+-- The haversine of an angle.
+hsin : Double -> Double
+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 : Double -> (Double, Double) -> (Double, Double) -> Double
+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 : Double -> (Double, Double) -> (Double, Double) -> Double
+distDeg radius p1 p2 = distRad radius (deg2rad p1) (deg2rad p2)
+  where
+        d2r : Double -> Double
+        d2r t = t * pi / 180
+        deg2rad (t, u) = (d2r t, d2r u)
+
+-- The approximate distance, in kilometers, between two points on Earth.
+-- The latitude and longtitude are assumed to be in degrees.
+earthDist : (Double, Double) -> (Double, Double) -> Double
+earthDist = distDeg 6372.8
+
+main : IO ()
+main = putStrLn $ "The distance between BNA and LAX is about " ++ show (floor dst) ++ " km."
+ where
+      bna : (Double, Double)
+      bna = (36.12,  -86.67)
+
+      lax : (Double, Double)
+      lax = (33.94, -118.40)
+
+      dst : Double
+      dst = earthDist bna lax