On Sun, 27 Jun 2010 19:55:21 +1000
Roman Leshchinskiy
On 25/06/2010, at 06:41, Alexey Khudyakov wrote:
Then constructor like one below arise naturally. And I don't know how to write them properly. It's possible to use fromList but then list could be allocated which is obviously wasteful.
vector2 :: Double -> Double -> Vec2D vector2 x y = ... -- Vec2D is some wrapper around Vector data type
Your best bet is probably singleton x ++ singleton y. Unfortunately, GHC doesn't seem to provide any real way of specifying array literals.
Thank you for advice. And what about using ST monad?
vector2 :: Double -> Double -> Vec2D vector2 x y = Vec2D $ create $ do a <- new 2 write a 0 x write a 1 y return a
Another question is there any specific problems with short vectors? They could be just 2 elements long. I mean performance problems
A data type like this one should be faster:
data Vec2D = Vec2D {-# UNPACK #-} !Double {-# UNPACK #-} !Double
Firstly, this needs one less indirection for accessing the components. Secondly, GHC's simplifier knows a lot more about algebraic data types than it does about arrays so the above definition will often lead to better optimisations. Whether or not the difference is significant probably depends on the program.
This is of course faster but what I really want is vectors with length
parametrized by type. This way I can write generic code. Uniform
representation is requirement for that.
--
Alexey Khudyakov