September Morn Update

Task/Gamma-function/Haskell/gamma-function-2.hs

@@ -1,7 +1,22 @@
-procedure main()
-    every write(left(i := !10/10.0,5),gamma(.i))
-end
+import Control.Applicative
 
-procedure gamma(z)	# Stirling's approximation
-    return (2*&pi/z)^0.5 * (z/&e)^z
-end
+cof :: [Double]
+cof =
+  [ 76.18009172947146
+  , -86.50532032941677
+  , 24.01409824083091
+  , -1.231739572450155
+  , 0.001208650973866179
+  , -0.000005395239384953
+  ]
+
+gammaln :: Double -> Double
+gammaln =
+  ((+) . negate . (((-) . (5.5 +)) <*> (((*) . (0.5 +)) <*> (log . (5.5 +))))) <*>
+  (log .
+   ((/) =<<
+    (2.5066282746310007 *) .
+    (1.000000000190015 +) . sum . zipWith (/) cof . enumFrom . (1 +)))
+
+main :: IO ()
+main = mapM_ print $ gammaln <$> [0.1,0.2 .. 1.0]