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, o...@okmij.org wrote:
Multi-parameter type classes are more flexible. Here is how you can
write your old code:
{-# LANGUAGE MultiParamTypeClasses,
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
On Thu, Nov 17, 2011 at 11:48 AM, Magicloud Magiclouds
magicloud.magiclo...@gmail.com 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 =
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 - [c]
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 miguelim...@yandex.ru wrote:
You've declared from as forall b. Test -
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
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 doesn't
On Thu, Nov 17, 2011 at 1:17 PM, Brandon Allbery allber...@gmail.com wrote:
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?
