On Fri, 6 Nov 1998, Christian Sievers wrote:
| This gives one more example of what I always wanted to suggest:
| The documentation of a class should clearly state which the minimal
| sets of definitions are that one has to give, rather than let the
| reader figure this out from the code.
Indeed! Or better: let the compiler give a warning in this case.
| The default for (==) however seems odd to me. Someone could like to
| define (==) alone or both (==) and (/=), but it seems strange to me to
| only define (/=). Or do you just want to give all expressible
| relations?
No, look at the Num class:
class (Eq a, Show a, Eval a) => Num a where
(+), (-), (*) :: a -> a -> a
negate :: a -> a
abs, signum :: a -> a
fromInteger :: Integer -> a
fromInt :: Int -> a
x - y = x + negate y
fromInt = fromIntegral
Clearly, one would like to define the following default definition as
well:
negate x = 0 - x
Many classes suffer from this, this is just an example.
| And while you're still doing non-hurting class definition changes, let
| me repeat this: please let succ and pred be Enum class members.
I agree!
I would also like to argue for the following extension:
class Monad m where
...
join mm = mm >>= id
Or, even better (but I might step too many people on their toes by this):
class Functor m => Monad m where
...
m >>= k = join (map k m)
join mm = mm >>= id
Regards,
Koen.
--
Koen Claessen,
[EMAIL PROTECTED],
http://www.cs.chalmers.se/~koen,
Chalmers University of Technology.
