At 2002-01-25 14:00, Hal Daume III wrote:
>> class D a where constMember :: Int
>> instance D Int where constMember = 8
>
>It seems ehre that there's no way to extract constMember for a
>/particular/ class, since you can't tell it what "a" is supposed to
>be. So, instead, I do:
>
>> class D a where constMember :: a -> Int -- should be independent of arg
>> instance D Int where constMember _ = 8
>
>(which brings me to my use of "undefined::a"). But is there a
>better/preferred way to do this?
Unfortunately this seems to be the preferred way to do this. For
instance, from the Prelude:
class (RealFrac a, Floating a) => RealFloat a where
floatRadix :: a -> Integer
floatDigits :: a -> Int
It's not the _best_ way to do it, in my opinion. I do this:
data Type a = Type
class D a where
constMember :: Type a -> Int
instance D Int where
constMember Type = 8
intConst = constMember (Type :: Type Int)
See
<http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/jvm-bridge/sourc
e/Haskell/Type.hs?rev=HEAD&content-type=text/plain>.
Why is this better?
1. It's absolutely clear to the user of the class that constMember
depends only on the type.
2. It prevents the user from putting in an inappropriate "back channel"
in the member for some instance of the class.
3. It avoids use of 'undefined', which is just plain ugly. After all,
intuitively everything is defined.
--
Ashley Yakeley, Seattle WA
Almost empty page: <http://semantic.org/>
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