[Haskell-beginners] Typeclasses vs. Data

[Thomas]

Hi Felipe, David!

It works!!! :-)

What I do not really understand, however, is the difference between
  eval_begin n k = eval n (if (n<  0) then k else (BeginCont k (n - 1)))
and
  eval_begin n k = if (n<  0) then eval n k else eval n (BeginCont k (n 
- 1))

Anyway, it works and since I needed to use the "Rank2Types" and 
"RelaxedPolyRec" extensions I have to read (and hopefully understand ;-) 
these as well. Maybe they'll shed some light onto this.

I just wonder: Are typeclasses such an advanced feature that I'd better 
use alternatives wherever possible?
Yitzchak suggested this in this list a few days ago. But almost every 
(introductory) text explains them as a basic feature.

Thanks a lot to both of you!
Thomas


On 21.07.2011 05:27, Felipe Almeida Lessa wrote:
> On Wed, Jul 20, 2011 at 8:41 PM, Thomas<haskell at phirho.com>  wrote:
>> eval_begin :: Continuation a =>  Int ->  a ->  Int
>> eval_begin n k = eval n (if (n<  0) then k else (BeginCont k (n - 1)))
>
> Although 'eval n' is polymorphic, the if expression needs to have just
> one type, either 'a' or 'BeginCont a', and they can't be unified.  The
> solution is pretty simple, though, since eval's return type doesn't
> mention 'a' at all:
>
>    eval_begin :: Continuation a =>  Int ->  a ->  Int
>    eval_begin n k = if (n<  0) then eval n k else eval n (BeginCont k (n - 1))
>
> Note that 'eval n' is repeated.  If you don't to repeat it on your
> real world code you may give it an explicit name.  However you'll need
> to provide a type signature from GHC 7.0 onwards:
>
>    eval_begin :: Continuation a =>  Int ->  a ->  Int
>    eval_begin n k =
>      let eval' :: Continuation a =>  a ->  Int
>          eval' = eval n
>      in if (n<  0) then eval' k else eval' (BeginCont k (n - 1))
>
> HTH, =)
>