[Haskell-cafe] Construction of short vectors

[Alexey Khudyakov]

On Sun, 27 Jun 2010 19:55:21 +1000
Roman Leshchinskiy <rl at cse.unsw.edu.au> wrote:

> 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 <alexey.skladnoy at gmail.com>