How do I attach optional attributes to values?

问题:

I want to store a list of "things" which can have some optional extra attributes attached to them. Each thing can have one or more attributes. And different attributes are of different types.

I want to be able to create literal lists of these things concisely in code. But I'm having trouble seeing how to get this past the type system because tuples allow mixtures of types but are fixed length, while lists are variable length but one type.

This is a toy example of what I want to be able to do :

things = [
   Thing 1 RED,
   Thing 2 RED LARGE,
   Thing 3 BLUE SMALL,
   Thing 4 SMALL,
   Thing 5 BLUE DOTTED
]

etc.

What's the right way to do this?

回答1:

Basically, rather than storing the attributes as given, you should store the resultant properties of a chord with these attributes. One simple (but not really nice, musically) solution would be, storing only the final pitches:

newtype Pitch = Pitch {midiNote :: Int}

a, as, bb, b, bs, c, cs, db, d, ds, eb, e, es, f, fs, gb, g, gs, ab :: Pitch
[ a, as,bb, b,bs, c,cs,db, d,ds,eb, e,es, f,fs,gb, g,gs,ab] = map Pitch
 [55,56,56,57,58,58,59,59,60,61,61,62,63,63,64,64,65,66,66]

type Chord = [Pitch]

minor :: Pitch -> Chord
minor (Pitch fund) = map (Pitch . (fund+)) [0, 3, 7]

seventh :: Pitch -> Chord
seventh (Pitch fund) = map (Pitch . (fund+)) [0, 4, 7, 10]

spread :: Chord -> Chord
spread = sort
 . zipWith (\octShift (Pitch note) -> Pitch $ note + 12 * octShift) $ cycle [0,1]

To be used as e.g.

chords :: [Chord]
chords = [ minor e, seventh d, minor e, minor a, seventh b, spread $ minor e ]

A more sophisticated approach might actually store the information about a chord in a more musically meaningful way:

data Chord = Chord { fundamental :: Pitch
                   , gender :: Maybe ChordGender
                   , ExtraNotes :: [AddNote]
                   , OctaveShifts :: [Int]
                   }

data ChordGender = Major | Minor
data AddNote = AddNote { roughInterval :: Int, intervalIsMajor :: Bool }

major :: Pitch -> Chord
major fund = Chord fund (Just Major) [] []

sus4 :: Pitch -> Chord
sus4 fund = Chord fund Nothing [AddNote 4 False] []

spread :: Chord -> Chord
spread ch@(Chord _ _ _ shifts)
  = ch{shifts = cycle [0,1]}

This can be used in much the same way, but is more versatile.

If you don't like giving the attributes as prefix functions, you can do as the diagrams package, with

infixl 8 #
(#) :: a -> (a -> b) -> b
(#) = flip ($)

to write

chords = [ c # major
         , g # sus4
         , g # major
         , a # minor
         , f # major # spread
         , g # sus4  # spread
         , g # major # spread
         , c # major # spread
         ]

回答2:

Let's say Chords are collections of Notes.

data Note = A | Bb | B | C | ...

But also with optional sets of Annotations

data Ann = Seventh | Sus2 | Split | ...

We can model Chords thus as

data Chord = Chord { notes :: [Note]
                   , anns :: [Ann]
                   }

We can build an entire vocabulary this way

maj :: Note -> Chord
ann :: Ann -> Chord -> Chord
transpose :: Int -> Note -> Note
transposeChord :: Int -> Chord -> Chord

And then build our list like so

chords = [
  ann Seventh (maj C)
, ann Split (ann Sus2 (maj A))
]

回答3:

There are various ways to implement heterogeneous collections in Haskell, but using a list of arbitrary values of arbitrary types is probably more flexibility than you need, and more trouble than it's worth. You're probably better off creating one type with an expressive set of possible values, and using a homogeneous collection of that.

Your example attributes appear to fall into a set of known categories: color (e.g. RED, BLUE), size (e.g. LARGE, SMALL), and what I'll call "texture" (e.g. DOTTED). A Thing doesn't necessarily have an attribute in each category, but I'll assume that it shouldn't have more than one attribute in the same category — it doesn't make sense for a single Thing to be both LARGE and SMALL.

You could represent these categories as algebraic data types:

data Color = Red
           | Blue

data Size = Large
          | Small

data Texture = Dotted

and combine them with a data structure:

data ThingAttributes = ThingAttributes {
  thingColor :: Maybe Color,
  thingSize :: Maybe Size,
  thingTexture :: Maybe Texture
}

Now you can just associate a single ThingAttributes value with each Thing.

If you do want to allow multiple attributes in the same category (e.g. having a Thing that's both LARGE and SMALL), you can use another algebraic data type to bring all the category types together:

data ThingAttribute = ColorAttribute Color
                    | SizeAttribute Size
                    | TextureAttribute Texture

and then associate a Set ThingAttribute — or simply a [ThingAttribute] if you don't mind duplicates of the same attribute — with each Thing.