[Haskell-beginners] Re: data constructors

Heinrich Apfelmus apfelmus at quantentunnel.de
Mon Apr 20 03:10:35 EDT 2009

Michael Mossey wrote:
> A StaffItem can be one of several things. Let's say it can be a "chord"
> or a "control". I might like to define Chord and Control first:
> data Chord = Chord { duration :: Double, notes :: [Note] }
> data Control = DynamicMark Int
>              | TempoMark Int
> Okay, so now I want to express the idea a StaffItem can be a Chord or a
> Control.
> data StaffItem = StaffItemChord Chord
>                | StaffItemControl Control
> My problem is the awkward need for separately named constructors 
> "StaffItemChord" and "StaffItemControl". Is there a better way? (Is this
> even right?)

Why not simply

  data StaffItem = Chord { duration :: Double, notes :: [Note] }
                 | DynamicMark Int
                 | TempoMark Int

Unless you use  Control  and  Chords  in isolation, that's entirely fine.

It's basically a question of how much type safety you want. If you have
a function like

  chordName :: Chord -> String

that should only with proper  Chords  and not  Control  messages, giving
it the type signature

  chordName :: StaffItem -> String

is less safe; the compiler won't complain if you pass it a  Control
message. If you want the compiler to complain, then building "type
towers" as you did is the way to go.

You can use predefined building blocks like  Either

  type StaffLabel = Either Chord Control

to build your types. There are a few methods for making things less
clumsy available, like for example

  Wouter Swierstra. Data types à la carte.



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