[Haskell-beginners] uses for Functor or Applicative?

[edgar klerks]

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 <johann.bach1127 at gmail.com>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)
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