[Haskell-cafe] Deduce problem.

[Magicloud Magiclouds]

On Thu, Nov 17, 2011 at 1:17 PM, Brandon Allbery <allbery.b at gmail.com> wrote:
> On Wed, Nov 16, 2011 at 23:54, Magicloud Magiclouds
> <magicloud.magiclouds at 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 at gmail.com
> wandering unix systems administrator (available)     (412) 475-9364 vm/sms
>
>

Ah, list and single value is a typo problem.

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?

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