Question

I have a type

data Phase = PhaseOne
           | PhaseTwo
           | PhaseThree deriving Enum

and five operations to do on each Phase

• read
• write
• validate
• evalStatus
• update

I began trying to create a type class. The problem is, they are all the same type.
I’d like to be able to do something like

instance MyClass PhaseThree where
  read a = ...

Also, I need to overload the return type.

I know type classes aren’t what I want. But I’m not sure how to do what I want. I thought
of a GADT but that’s not quite right as I need to be able to have each instance in a separate file.

I’d like some advice on which mechanisms I need to investigate? Have I given enough information?

Answer 1

I recommend inverting things a bit.

data Phase = Phase {
    read :: String -> Foo,
    write :: Foo -> IO (),
    validate :: Foo -> Bool,
    evalStatus :: IO (),
    update :: Foo -> Foo
}

phaseOne, phaseTwo, phaseThree :: Phase

(or some similar rejiggering of classes into explicit records).