Oh, come on. It's not that bad. Just start every module with
import Prelude()
import MyPrelude
that's what I do. There's nothing sacred about the Prelude (except a
few things used for special syntax). It just happens to be in scope.
-- Lennart
On Dec 8, 2006, at 15:04 , Dan Weston wrote:
The following observations are not new, insightful, or gracious,
but I was lusting after the innocent +,-,* operators for my own
evil ends and was mildly curious why...
Num is such a fat and greedy class. If you want to marry
Cinderella, you have to take her ugly stepsisters too.
1) Groups may only want to define addition. Why can't they use +
(instead of <+>, >++++?&**+>, or other such perversion)?
2) Affine spaces have a (-) but no (+). Worse, the signature might be
(-) :: Point -> Point -> Vector, which doesn't unify with (a -> a -
> a).
Wouldn't the following be more useful/general?
class Subtraction a b | a -> b where
(-) :: a -> a -> b
Or would this require needless type annotation for the common
subset of (a -> a -> a) instances?
3) Quaternions have no signum, unit quaternions have (*), (/) but
no (+) or (-), abs would have a different signatures (Quaternion -
> Double) which doesn't unify with (a -> a), and fields cannot be
scaled with (*) as in (*) :: (Field f) => Double -> f -> f
Would it not make sense to put each of these operators (division
too) into their own individual superclasses that Num inherits? My
(obviously naive) philosophy about type classes is that operations
should be bundled only when they are mutually recursive (i.e. there
is more than one useful minimal definition). If there is just one
minimal set of operations, they can be in their own parent class too.
Then again, I should get over my lust and stick with my own
operators <+++++++++>, <---------------------->, and <*******>. Not
too pretty, but they have a wonderful personality all their own!
Dan
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