Question

I just started learning Haskell. I think I’ve got the basics down, but I want to make sure I’m actually forcing myself to think functionally too.

data Dir = Right | Left | Front | Back | Up | Down deriving (Show, Eq, Enum)
inv Right = Left
inv Front = Back
inv Up = Down

Anyway, the jist of what I’m trying to do is to create a function to map between each “Dir” and its opposite/inv. I know I could easily continue this for another 3 lines, but I can’t help but wonder if there’s a better way. I tried adding:

inv a = b where inv b = a

but apparently you can’t do that. So my question is: Is there either a way to generate the rest of the inverses or an altogether better way to create this function?

Thanks much.

Answer 1

If the pairing between Up and Down and so on is an important feature then maybe this knowledge should be reflected in the type.

data Axis = UpDown | LeftRight | FrontBack
data Sign = Positive | Negative
data Dir = Dir Axis Sign

inv is now easy.