[Haskell-beginners] constant set

[Ertugrul Söylemez]

Ovidiu Deac <ovidiudeac at gmail.com> wrote:

> If I want to define a constant set in Python I would do this:
> class Colors:
>   red = 1
>   blue=2
> ...
>
> and then I use them like this: Colors.red, Colors.blue...
>
> Can I do something similar in Haskell?

In Haskell this would usually be an abstract type:

    data Color = Red | Blue | ...

You could then use functions to convert between integers and colors.

    fromColor :: Color -> Int
    toColor   :: Int -> Maybe Color

You could also derive an Enum instance and get the conversion functions
'fromEnum' and 'toEnum' for free, although in that case the 'toEnum'
function is less safe (no Maybe):

    data Color = Red | Blue | ...
                 deriving Enum

    fromEnum :: Color -> Int
    toEnum   :: Int -> Color


Greets,
Ertugrul

-- 
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
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