[Haskell-beginners] maybe this could be improved?

[Patrick LeBoutillier]

Michael,

Your code is interesting and I'd like to run it, but I'm not to
familiar with Maps and Monad transformers.
Could you provide a function to create a SampleMap and a way to test
it from ghci?

Thanks,

Patrick

On Sat, Nov 7, 2009 at 12:44 PM, Michael Mossey <mpm at alumni.caltech.edu> wrote:
> I've got some code which could be made simpler, I hope. the problem is
> this: I am
> implementing a software sampling synthesizer. For a given musical
> instrument, like piano, there are sound samples in memory. One purchases
> or creates sample sets. To
> save time money & resources, most sample sets are not complete---they have
> samples for only some of the pitches. Perhaps every third pitch has a
> sample. For the software to produce the sound for a non-included pitch,
> the software finds the closest included sample and plays it back slightly
> slower/faster to get the target pitch.
>
> That leads to the following code. Any ideas for improvement are welcome.
> The problem is that there are many cases to check: an empty map? the
> requested pitch less than all available pitches, greater than all
> available, or somewhere between? I am specifically writing this to run in
> O( log n) time. (It would be simpler as O(n).) This particular algorithm
> probably doesn't need to run in O(log n) time, but I want to do it as an
> educational experience---I will have other applications that need to use
> Map in O(log n) time.
>
> import Control.Monad.Identity
> import Control.Monad.Error
> import Control.Monad
> import qualified Data.Map as M
>
> type Pitch = Int
> type Sample = String
> type SampleMap = M.Map Pitch Sample
>
>
> -- Given a SampleMap and a Pitch, find the Pitch in the SampleMap
> -- which is closest to the supplied Pitch and return that. Also
> -- handle case of null map by throwing an error.
> findClosestPitch :: SampleMap -> Pitch -> ErrorT String Identity Pitch
> findClosestPitch samples inPitch = do
>  when (M.null samples) $ throwError "Was given empty sample table."
>  case M.splitLookup inPitch samples of
>    (_,Just _,_ ) -> return inPitch
>    (m1,_        ,m2) | (M.null m1) && not (M.null m2) -> case1
>                      | not (M.null m1) && (M.null m2) -> case2
>                      | otherwise                      -> case3
>      where case1 = return . fst . M.findMin $ m2
>            case2 = return . fst . M.findMax $ m1
>            case3 = return $ closest (fst . M.findMax $ m1)
>                                     (fst . M.findMin $ m2)
>            closest a b = if abs (a - inPitch) < abs (b - inPitch)
>                           then a
>                           else b
>
>
>
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-- 
=====================
Patrick LeBoutillier
Rosemère, Québec, Canada