[Haskell-beginners] manipulating Map for music application

[Chaddaï Fouché]

This is a bit tricky but mapMaybeWithKey is clearly the tool for the job :

> mapMaybeWithKey :: (k -> a -> Maybe
<https://hackage.haskell.org/package/base-4.7.0.1/docs/Data-Maybe.html#t:Maybe>
b) -> Map
<https://hackage.haskell.org/package/containers-0.5.6.3/docs/Data-Map-Lazy.html#t:Map>
k a -> Map
<https://hackage.haskell.org/package/containers-0.5.6.3/docs/Data-Map-Lazy.html#t:Map>
k b

So :

mapNotesMaybe :: (Loc -> Chord -> Note -> Maybe Note) -> Composition ->
Composition
mapNotesMaybe f compo = mapMaybeWithKey go compo
  where
    go loc chords = mconcat . map (gogo loc) $ chords
    gogo loc chord = mconcat . map (fmap pure . f loc chord) $ chord

This should work.

-- 
Chaddaï


Le sam. 7 nov. 2015 à 07:07, Dennis Raddle <dennis.raddle at gmail.com> a
écrit :

> I have a Haskell program that computes music compositions. A composition
> consists of sounds that happen at locations. A location is data type Loc.
> At each location is a list of chords. A chord is data type Chord. Each
> chord contains some chord-specific data and a list of notes. A note is data
> type Note.
>
> So we have
>
> data Note = Note ...
> data Chord = Chord ChordSpecificData [Note]
> type Composition = Map Loc [Chord]
>
> I would like to write a few different functions that operate over all the
> notes.
>
> The following function breaks out all the notes, tupling them with the
> associated Locs and Chords:
> compositionToList :: Composition -> [(Loc,Chord,Note)]
>
> The following function transforms Notes, keeping only the Just results. If
> a Chord gets all its notes eliminated, that Chord is removed. If a Loc has
> all its Chords removed, that Loc is removed from the Map.
> mapNotesMaybe :: (Loc -> Chord -> Note -> Maybe Note) -> Composition
>    -> Composition
>
> Any advice for concise code appreciated.
> Dennis
>
>
> What's a concise way of doing this?
>
> Another useful function would be used for mapping
>
> Dennis
>
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