[Haskell-beginners] typeclass confusion

[John Obbele]

On Tue, Aug 24, 2010 at 05:06:05AM +0200, Tobias Brandt wrote:
>    You don't need a type class, you can just define your functions with
>    pattern matching:
>    rad :: Angle a -> a
>    rad (Radians x) = x
>    rad (Degrees x) =*pi * (deg x) / 180
>    deg :: Angle a -> a
>    deg (Radians x) =*180 * (rad x) / pi
>    deg (Degrees x) = x

I may look like a nitpicker but the above function definitions
lose their type information. I'd have written it this way:

    data Angle a = Radians a | Degrees a deriving (Eq, Show)

    rad :: Floating a => Angle a -> Angle a
    rad (Radians x) = Radians x
    rad (Degrees x) = Radians (pi * x / 180)

    deg :: Floating a => Angle a -> Angle a
    deg (Radians x) = Degrees (180 * x / pi)
    deg (Degrees x) = Degrees x

On the other side, I am not particularly confident this approach
is the best. Other people may give you better solutions …

/‡ John