Hi,
Thank you very much for your helpful replies.
So, we define (or is it declare) a class with a polymorphic value(s
which will be used in defining function types or functions associated
with that class. The data types are then used for deriving instances
of that class specific to each data type. The function defined in the
general class can be redefined if necessary. It sort of reminds me
of specialisation in OO. Get thee behind me beast. :-)
Any comments?
Cheers,
Paul
At 20:05 07/04/2008, you wrote:
Paul,
> No, sorry. I'm not sure how this differs from my definition.
> Could you elaborate please?
Gladly. I haven't tested any of the code below, so pardon any typos
and thinkos.
It sounded like you were trying to say that Bool derives from Eq, Ord,
Show, and Read, and thus the keyword "from" would apply. Of course,
I may have misunderstood your intent.
I was countering by saying that the "deriving" keyword indicates something
different. I'll try to describe what I meant in a little more detail.
A type class definition defines properties of the type class. Eg.,
class Example a where
foo :: a
bar :: a -> a
The class is then related to specific types by giving them instance
declarations. The instance declarations can be explicit, as in
data Xyzzy = Xy | Zzy
instance Example Xyzzy where
foo = Xy
bar = const Zzy
Sometimes the instance declaration can be automatically derived instead,
which looks like:
data Xyzzy = Xy | Zzy
deriving (Example)
This lets the compiler derive the "obvious" class instance declaration.
But it won't work for just any class, because the compiler needs to know
how to derive "obvious" instances. A few classes have deriving
machinery "built-in": Eq, Ord, Enum, Bounded, Show, Read.
"deriving (Example)" won't work, since that machinery doesn't exist.
With effort, you could implement this "automatic derivation" machinery
for your own types, at least in GHC. I've never needed it, so I've
never learned exactly how to do it.
In short, so "data Foo [...] deriving (Eq)" means: "define a new type Foo,
and automatically derive its class instance for Eq".
Does this help?
Regards,
John
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