Re: [Haskell-cafe] Is there anything manifestly stupid about this code?

lrpalmer:

On Mon, Jul 7, 2008 at 2:21 PM, Michael Feathers wrote:

...

Thanks. Here's a newb question: what does strictness really get me in this code?

A bit of speed and memory improvements, I suspect. The type (Double,Double) has three boxes, one for the tuple and one for each double. The type Complex, which is defined as

data Complex a = !a :+ !a

has one box (after -funbox-strict-fields has done its work), the one for the type as a whole. So it will end up using less memory, and there will be fewer jumps to evaluate one (a jump is made for each box).

On a good day the two Double components will be unpacked into registers entirely. As here, a loop on Complex: {-# OPTIONS -funbox-strict-fields #-} module M where data Complex = !Double :+ !Double conjugate :: Complex -> Complex conjugate (x:+y) = x :+ (-y) realPart :: Complex -> Double realPart (x :+ _) = x go :: Complex -> Double go n | realPart n > pi = realPart n | otherwise = go (conjugate n) Note that notionally Complex has 3 indirections, the Complex constructor, and two for the Doubles. After optimisation however, there's only unboxed doubles in registers left: M.$wgo :: Double# -> Double# -> Double# M.$wgo = \ (ww_sjT :: Double#) (ww1_sjU :: Double#) -> case >## ww_sjT 3.141592653589793 of wild_Xs { False -> M.$wgo ww_sjT (negateDouble# ww1_sjU); True -> ww_sjT -- Don