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 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]

data H = ...

instance ClassB H where  ...

data Test = Test { m :: H } instance ClassA Test where  ... instance ClassC Test where  from = m  to = m

 Well, I got "could not deduce" error here at "from = m" and "to = m". `c' is a rigid type variable bound by the type signature for to :: ClassB c => Test -> [c].  Referring to some similar questions on internet, I should remove the (ClassB c) thing. Is this the only solution? -- 竹密岂妨流水过 山高哪阻野云飞

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-- 竹密岂妨流水过 山高哪阻野云飞

On Wed, Nov 16, 2011 at 23:54, Magicloud Magiclouds < magicloud.magiclouds@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 mean that *you* get to decide what it is; it means *the caller* gets to decide, and you are obligated to honor the caller's wishes. Returning always an H violates this, because there is no way to prove that H is the only possible response. -- brandon s allbery allbery.b@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms

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 the extension: {-# LANGUAGE MultiParamTypeClasses #-} An then, instead of class (ClassA a) => ClassC a where from :: (ClassB b) => a -> [b] to :: (ClassB c) => a -> [c] you say class (ClassA a, ClassB b) => ClassC a b c where from :: c -> [b] to :: c -> [a] This means that for each triple of concrete types (a,b,c) that you wish to be an instance of ClassC, you must provide an instance declaration, e.g. instance ClassC Test H H where from = ...whatever... to = ...whatever... Now you have the fixed type H in the instance declaration and not a universally quantified type variable. Cheers Ben