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

> mapMaybeWithKey :: (k -> a -> Maybe b) -> Map k a -> 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@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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