He Johann, Functors have the type * -> *, so your MusicDoc type doesn't match. But if you fix that it is easy to create a functor, especially when using DeriveFunctor, if you are a lazy person: {-# LANGUAGE DeriveFunctor #-} import qualified Data.Map as M data PartInfo = Void deriving Show type PartId = Int newtype MusDoc a = MusDoc { docParts :: M.Map PartId (PartInfo, a) } deriving (Functor,Show) singleton a = MusDoc $ M.singleton 1 (Void, a) Test: *Main> fmap (+1) $ singleton 1 Loading package array-0.3.0.0 ... linking ... done. Loading package containers-0.3.0.0 ... linking ... done. MusDoc {docParts = fromList [(1,(Void,2))]} *Main> Greets, Edgar On Fri, Jul 23, 2010 at 12:04 PM, Johann Bach wrote:

I'm wondering if the following problem might reveal a use for Functor or Applicative, or just generally something cooler than I'm doing now.

I have a music document as type MusDoc. It has [Part]. Part has [Note]. I have defined them using field names because I am deliberating allowing for this to get more complicated later.

The question is about mapping functions over the individual parts. I often find myself wanting to alter or filter all the parts. In the following example I define filterDoc with a couple of helper functions. What I would like to know: is there a cooler way to do this using instances of Functor or something? Defining operators?

import Map(Map) import qualified Map as M

type PartId = String -- identification of a part in a music document data PartInfo = .. -- ancillary information about a part

-- the notes, markings, etc in a part -- It is conceivable other fields could be added later. data Part = Part { partNotes :: [Note] }

-- Main music document, consisting of parts. data MusDoc = MusDoc { docParts :: Map PartId (PartInfo,Part) }

-- Helper function to map over parts mapParts :: (Part -> Part) -> MusDoc -> MusDoc mapParts g (MusDoc parts) = MusDoc $ M.map (second g) parts

-- Filter notes that meet a predicate filterDoc :: (Note -> Bool) -> MusDoc -> MusDoc filterDoc pred doc = mapParts (filterPart pred) doc

-- Helper function to filterDoc. filterPart :: (Note -> Bool) -> Part -> Part filterPart pred (Part notes) = Part (filter pred notes) _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

Johann Bach wrote:

I'm wondering if the following problem might reveal a use for Functor or Applicative, or just generally something cooler than I'm doing now.

I have a music document as type MusDoc. It has [Part]. Part has [Note]. I have defined them using field names because I am deliberating allowing for this to get more complicated later.

The question is about mapping functions over the individual parts. I often find myself wanting to alter or filter all the parts. In the following example I define filterDoc with a couple of helper functions. What I would like to know: is there a cooler way to do this using instances of Functor or something? Defining operators?

You could make the types polymorphic to be able to make a functor instance, but it's not necessarily a good idea. In any case, you may want to have a look at functional references http://article.gmane.org/gmane.comp.lang.haskell.cafe/28094 One implementation of functional references is http://hackage.haskell.org/package/data-accessor Then, you can for instance write parts :: Accessor MusDoc [Part] notes :: Accessor Part [Notes] example :: MusDoc -> MusDoc example = modify parts . map . modify notes $ filter (== CSharp) where notes and parts are two appropriate functional references, which are best generated automatically. Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com