[Haskell-beginners] Typeclasses vs. Data

[David Place]

Please disregard my previous message.  I didn't read your message carefully enough.
____________________
David Place   
Owner, Panpipes Ho! LLC
http://panpipesho.com
d at vidplace.com



On Jul 20, 2011, at 5:58 PM, Thomas wrote:

> Hello!
> 
> Trying to rewrite a program I ran into a type problem with typeclasses.
> 
> It's a mini-interpreter, the essential (extremely reduced) part is:
> 
> eval e k = case (car e) of
>     (Id "begin") ->  eval_begin (cdr e) k
> eval_begin e k = eval (car e) (if isNull (cdr e) then k else (BeginCont k (cdr e)))
> 
> Using "data" to define my data all is well:
> 
> data Continuation =
>     BeginCont Continuation Expression
> resume k e = case k of
>       BeginCont k' es -> eval_begin es k'
> 
> Unfortunately when trying to extend the program by other types of "Continuation" I must add to the data definition and add a matching clause to "resume". That is I must modify the core module.
> So I tried to decouple this using a typeclass like so:
> 
> class Continuation a where
>   resume :: a -> Expression -> Expression
> 
> data BeginCont a = BeginCont a Expression deriving (Show)
> instance (Continuation a) => Continuation (BeginCont a) where
>  resume (BeginCont k es) v = eval_begin es k
> 
> This, however, results in an "infinite type" error.
> 
> Is there a way to make the typeclass version typecheck?
> If not: How can one decouple this code in Haskell?
> 
> What also puzzles me are the differences in "infinite" types.
> The above data declaration for "Continuation" is essentially infinite, too. But it works. And I thought I had understood this part...
> 
> Any hints greatly appreciated!
> Thanks in advance,
> Thomas
> 
> 
> PS:
> The minimal "program" to get it type check is:
> 
> data Expression = Null | Num Int | Id String | List [Expression]
>     deriving (Eq, Show)
> 
> cdr :: Expression -> Expression
> cdr (List []) = error "cdr Null !"
> cdr (List (_:[])) = Null
> cdr (List (l:ls)) = List ls
> 
> car :: Expression -> Expression
> car (List l) = head l
> 
> isNull Null = True
> isNull _ = False
> 
> eval e k = case (car e) of
>     (Id "begin") ->  eval_begin (cdr e) k
> 
> eval_begin e k = eval (car e) (if isNull (cdr e) then k else (BeginCont k (cdr e)))
> 
> -- replace the following 5 lines...
> data Continuation =
>     BeginCont Continuation Expression
> 
> resume k e = case k of
>       BeginCont k' es -> eval_begin es k'
> 
> {- ... with these to see the error
> class Continuation a where
>   resume :: a -> Expression -> Expression
> 
> data BeginCont a = BeginCont a Expression deriving (Show)
> instance (Continuation a) => Continuation (BeginCont a) where
>  resume (BeginCont k es) v = eval_begin es k
> -]
> 
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