#14439: Remove redundant subtraction in (^) and stimes -------------------------------------+------------------------------------- Reporter: Bodigrim | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: | Version: 8.2.1 libraries/base | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Here is the source code of `Data.Real.^`: {{{#!hs (^) :: (Num a, Integral b) => a -> b -> a x0 ^ y0 | y0 < 0 = errorWithoutStackTrace "Negative exponent" | y0 == 0 = 1 | otherwise = f x0 y0 where -- f : x0 ^ y0 = x ^ y f x y | even y = f (x * x) (y `quot` 2) | y == 1 = x | otherwise = g (x * x) ((y - 1) `quot` 2) x -- g : x0 ^ y0 = (x ^ y) * z g x y z | even y = g (x * x) (y `quot` 2) z | y == 1 = x * z | otherwise = g (x * x) ((y - 1) `quot` 2) (x * z) }}} In both cases subtraction `y - 1` is redundant. The value of `y` is guaranteed to be positive and odd, so `(y - 1) `quot` 2 = y `quot` 2`. Same argument applies to `Data.Semigroup.Internal.stimes{Monoid,Default}`. For instance, {{{#!hs stimesMonoid :: (Integral b, Monoid a) => b -> a -> a stimesMonoid n x0 = case compare n 0 of LT -> errorWithoutStackTrace "stimesMonoid: negative multiplier" EQ -> mempty GT -> f x0 n where f x y | even y = f (x `mappend` x) (y `quot` 2) | y == 1 = x | otherwise = g (x `mappend` x) (pred y `quot` 2) x g x y z | even y = g (x `mappend` x) (y `quot` 2) z | y == 1 = x `mappend` z | otherwise = g (x `mappend` x) (pred y `quot` 2) (x `mappend` z) }}} Again, `pred y` is redundant; it is enough to divide `y` itself. My proposal is to replace `y - 1` and `pred y` by `y`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14439 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler