From: Michael Orlitzky <michael@orlitzky.com>
Date: Tue, 25 Oct 2011 04:00:19 +0000 (-0400)
Subject: Add bang patterns to the Misc module and use them to write a faster
X-Git-Tag: 0.0.1~85
X-Git-Url: http://gitweb.michael.orlitzky.com/?p=spline3.git;a=commitdiff_plain;h=40a354da29b99d773fc18c4c4ad0d136a93d7264

Add bang patterns to the Misc module and use them to write a faster
factorial function (Ben Lippmeier).
---

diff --git a/src/Misc.hs b/src/Misc.hs
index b1cb1af..4b285e4 100644
--- a/src/Misc.hs
+++ b/src/Misc.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE BangPatterns #-}
 -- | The Misc module contains helper functions that seem out of place
 --   anywhere else.
 module Misc
@@ -24,11 +25,12 @@ import Test.QuickCheck
 --   24
 --
 factorial :: Int -> Int
-factorial n
-    | n <= 1 = 1
-    | n > 20 = error "integer overflow in factorial function"
-    | otherwise = product [1..n]
-
+factorial !n
+    | n > 20    = error "integer overflow in factorial function"
+    | otherwise = go 1 n
+    where go !acc !i
+                | i <= 1    = acc
+                | otherwise = go (acc * i) (i - 1)
 
 -- | Takes a three-dimensional list, and flattens it into a
 --   one-dimensional one.