Re: [Haskell-cafe] Deduce problem.

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, 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

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? -- 竹密岂妨流水过 山高哪阻野云飞