From the code, I think it is what I want. But still, I need some time
to understand it....
Anyway, thank you.
On Thu, Nov 17, 2011 at 4:02 PM, <o...@okmij.org> wrote:
>
> Multi-parameter type classes are more flexible. Here is how you can
> write your old code:
>
>> {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
>>
>> class (ClassA a, ClassB b) => ClassC a b where
>> from :: a -> [b]
>> to :: a -> [b]
>>
>> data H = H
>>
>> class ClassA a where toInt :: a -> Int
>> class ClassB b where fromInt :: Int -> b
>>
>> instance ClassB H where fromInt _ = H
>>
>> data Test = Test { m :: H }
>> instance ClassA Test where toInt _ = 0
>>
>> instance ClassC Test H where
>> from = (:[]) . m
>> to = (:[]) . m
>
>
> The constraints in the ClassC a b declaration specify that in all
> instances of ClassC, the type a must be in ClassA and the type b must
> be in ClassB. This is the case for the "ClassC Test H" instance.
>
> You can also specify that for some particular 'a' the function 'from'
> can produce the value of the type [b] for any b in ClassB. The caller
> will determine which b it wants. This is similar to your original
> intention, as I understand.
>
>> instance ClassA Int where toInt = id
>>
>> instance ClassB b => ClassC Int b where
>> from x = [fromInt x]
>>
>> t1:: [H]
>> t1 = from (5::Int)
>
>
>
--
竹密岂妨流水过
山高哪阻野云飞
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