Magicloud Magiclouds wrote:
> So I think I got what you guys meant, I limited ClassB to only H.
> Then how to archive my requirement, that from and to only return items
> that instanced ClassB?
If you are willing to go beyond Haskell98 (or Haskell2010), you can use a
multi-parameter class. Enable
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, wrote:
>
> Multi-parameter type classes are more flexible. Here is how you can
> write your old code:
>
>> {-# LANGUAGE MultiParamTypeClasses, FlexibleIn
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 -> I
On Thu, Nov 17, 2011 at 1:17 PM, Brandon Allbery wrote:
> On Wed, Nov 16, 2011 at 23:54, Magicloud Magiclouds
> wrote:
>>
>> I think this is where I did not understand from the very beginning.
>> If the the declaration was correct, then why cannot b be H?
>> Referring to Data.List.genericLength,
On Wed, Nov 16, 2011 at 23:54, Magicloud Magiclouds <
magicloud.magiclo...@gmail.com> wrote:
> I think this is where I did not understand from the very beginning.
> If the the declaration was correct, then why cannot b be H?
> Referring to Data.List.genericLength, I was confused.
Because it does
Of course, b can be H. The important question is: why can't it be something
else? ClassC signature implies that b can be anything (of class ClassB) — not
just H.
Another error is that you declare from as returning a list, but you try to
implement it as returning a single value.
On 17 Nov 2011,
I think this is where I did not understand from the very beginning.
If the the declaration was correct, then why cannot b be H?
Referring to Data.List.genericLength, I was confused.
On Thu, Nov 17, 2011 at 12:34 PM, MigMit wrote:
> You've declared "from" as forall b. Test -> [b], but you're tryin
You've declared "from" as forall b. Test -> [b], but you're trying to implement
it as Test -> H.
On 17 Nov 2011, at 07:48, Magicloud Magiclouds wrote:
> Hi,
> Consider I have declarations like this:
> class (ClassA a) => ClassC a where
> from :: (ClassB b) => a -> [b]
> to :: (ClassB c) => a
On Thu, Nov 17, 2011 at 11:48 AM, Magicloud Magiclouds
wrote:
> Hi,
> Consider I have declarations like this:
> class (ClassA a) => ClassC a where
> from :: (ClassB b) => a -> [b]
> to :: (ClassB c) => a -> [c]
>
> data H = ...
>
> instance ClassB H where
> ...
>
> data Test = Test { m :: H }
Hi,
Consider I have declarations like this:
class (ClassA a) => ClassC a where
from :: (ClassB b) => a -> [b]
to :: (ClassB c) => a -> [c]
data H = ...
instance ClassB H where
...
data Test = Test { m :: H }
instance ClassA Test where
...
instance ClassC Test where
from = m
to = m
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