A faster (^)-function.

This function performs better for me than the (^)-function in GHC. I seem to only be able to test it for the Integer type though and its only tested with ghc 6.6 (and ghc 6.6.1 by byorgey on #haskell). I'm not sure if you really need this or if it is correct, but after discussion on #haskell i was asked to make a bug report so here it is! Enjoy. :)

module Pow (pow) where
import Prelude hiding ((^))
pow = (^)

(^) :: (Integral b, Num a) => a -> b -> a
x ^ y | y < 0     = error "Negative exponent"
      | y == 0    = 1
      | y == 1    = x
      | odd y     = x * x^(y - 1)
      | otherwise = let x' = x^(y `div` 2) 
                    in x' * x'

Tests

-- TestData.hs
module TestData where
e = 10^8

-- mytest.hs
import Pow
import TestData
main = print $ (2 `pow` e) `mod` 2

-- ghctest.hs
import TestData
main = print $ (2 ^ e) `mod` 2

Test results (performance)

$ time ./ghctest
0

real    0m11.744s
user    0m11.449s
sys     0m0.104s

$ time ./mytest
0

real    0m6.794s
user    0m6.696s
sys     0m0.084s

-}

QuickCheck test

-- qc.hs
-- $ ./qc 
-- OK, passed 100 tests.

import Test.QuickCheck
import Pow

main = quickCheck prop 
prop x y = y >= 0 ==> x `pow` y == x^y

Trac metadata

Trac field	Value
Version	6.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown