Michael Feathers mfeathers at mindspring.com
Mon Jul 7 08:21:15 EDT 2008

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

BTW, I only noticed the Complex type late.  I looked at it and noticed
that all I'd be using is the constructor and add.  Didn't seem worth the
change.

Michael

Derek Elkins wrote:
> To answer the question in your subject, yes!  We have a complex type.
> Not only does that make the code simpler and more obvious and idiomatic,
> but it's also more efficient because for this use you'd really prefer a
> strict pair type for "Point", and complex is strict in it's components.
>
> On Sun, 2008-07-06 at 21:02 -0400, Michael Feathers wrote:
>> Decided a while ago to write some code to calculate the Mandelbrot set
>> using the escape iterations algorithm.  Discovered after mulling it
>> about that I could just built it as an infinite list of infinite lists
>> and then extract any rectangle of values that I wanted:
>>
>> type Point = (Double, Double)
>
>> sq :: Double -> Double
>> sq x = x ^ 2
>>
>> translate :: Point -> Point -> Point
>> translate (r0, i0) (r1, i1) =
>>    (r0 + r1, i0 + i1)
>>
>> mandel :: Point -> Point
>> mandel (r, i) =
>>    (sq r + sq i, 2 * r * i)
>>
>> notEscaped :: Point -> Bool
>> notEscaped (r, i) =
>>    (sq r + sq i) <= 4.0
>>
>> trajectory :: (Point -> Point) -> [Point]
>> trajectory pointFunction =
>>    takeWhile notEscaped \$ iterate pointFunction seed
>>      where seed = (0.0, 0.0)
>>
>> escapeIterations :: (Point -> Point) -> Int
>> escapeIterations =
>>    length . tail . take 1024 . trajectory
>>
>> mandelbrot :: Double -> [[Int]]
>> mandelbrot incrementSize =
>>    [[ escapeIterations \$ translate (x, y) . mandel
>>      | x <- increments]
>>      | y <- increments] where
>>          increments = [0.0, incrementSize .. ]
>>
>> window :: (Int, Int) -> (Int, Int) -> [[a]] -> [[a]]
>> window (x0, y0) (x1, y1) = range x0 x1 . map (range y0 y1) where
>>    range m n = take (n - m) . drop m
>>
>>
>> _______________________________________________
>> Haskell-Cafe mailing list