Performance question

[Hal Daume III]

If I'm understanding you correctly, you just want to be able to put F|Gs
into an unbooxed array.  You don't need to do this conversion stuff...you
can just define new instances.  Something like:

instance MArray IOUArray F IO where
    newArray (l,h) f = newArray (l,h) (f==F) >>= castIOUArray
    newArray_ (l,h)  =
       do (arr :: IOUArray ix Bool) <- newArray_ (l,h)
          castIOUArray arr
    unsafeRead arr i =
        do arr' <- castIOUArray arr
           b    <- unsafeRead arr' i
           return (if b then F else G)
    unsafeWrite arr i f = castIOUArray arr >>= \arr' ->
          unsafeWrite arr' i (f==F)

here we have represented Fs by their isomorphic type Bool, which already
has MArray instances.

HTH

 - Hal

--
Hal Daume III

 "Computer science is no more about computers    | hdaume@isi.edu
  than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

On 2 Oct 2002, Ketil Z. Malde wrote:

> 
> At the moment, I have a user defined data type on which I have defined
> some operations, instantiated (not derived) Eq and so on.  However,
> for efficiency I'm storing the actual data in UArrays of Word8.
> 
> In order for everything to work, I need functions 'toW8' and 'fromW8'
> to map between the data type and its representation in the array.
> 
> I gather it is impossible to hide the 'Eq' instance of Word8 and/or
> redefine it, but could this be circumvented by instead of
> 
>         data Foo = F | G
> 
>         instance Eq Foo where
>                 F == _ = True
>                 _ == _ = False
> 
> defining something like
> 
>         newtype Foo = F Word8
>         f, g, h :: Foo
>         f = F 0
>         g = F 1
> 
> and using the same instance declaration?  And will it still fit into a
> UArray? 
> 
> (It is of course possible that, while profiling indicates otherwise,
> the conversion functions have little impact in practice; i.e. the
> optimizer will do away with them.)
> 
> -kzm
> -- 
> If I haven't seen further, it is by standing in the footprints of giants
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