[Haskell-beginners] Re: data constructors

[Heinrich Apfelmus]

Michael Mossey wrote:
> I want to write a function that converts the first way of organizing the
> information into the second. I tried writing types like
> 
> type Chord = ...
> type Time = Double
> type Name = String
> 
> type TimedChord = (Time,Chord)
> type Staff = [(Time,Chord)]
> type NamedStaff = (Name,Staff)
> type NamedChord = (Name,Chord)
> type Vertical = [NamedChord]
> type TimedVertical = (Time,Vertical)
> 
> The function I want is
> 
> convert :: [NamedStaff] -> [TimedVertical]
> 
> As you can imagine, this is a confusing mess, with all these variants on
> named and timed things. I thought it might help to create functors
> called Named and Timed, which might help abstracting operations on named
> and timed things. For example,
> 
> datatype Named a = Named { namedName :: Name, namedData :: a }
> 
> instance Functor Named =
>    name a :: Name
>    name a = namedName a
>    x `fmap` f = Named { namedName = namedName x, namedData = f $
> namedData x }
> 
> Any other suggestions?

Functors sounds good to me.

   data Named a = N Name a
   data Timed a = T Time a

   instance Functor Named where ...
   instance Functor Timed where ...

   convert :: Named [Timed Chord] -> Timed [Named Chord]


But you can also use plain type synonyms

   type Named a = (Name,a)
   type Timed a = (Time,a)

and write your own record selectors by hand

   name :: Named a -> Name
   name = fst

   time :: Timed a -> Time
   time = fst

   value :: (b,a) -> a
   value = snd



Regards,
apfelmus

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