[Haskell-cafe] I hate Haskell's typeclasses

[Derek Elkins]

On Mon, 2008-04-21 at 22:58 -0700, Jonathan Cast wrote:

> class Forceable alpha where
>    seq :: alpha -> beta -> beta
> 
> Instances derived automatically by the compiler, when possible, for  
> every type (like Typeable should be).  We can omit functions if  
> desired (I don't remember why I thought this was a good idea).  When  
> you say
> 
> f :: alpha -> beta
> 
> or
> 
> f :: C alpha => alpha -> beta
> 
> The compiler adds implicit Forceable constraints on alpha and beta.   
> But, if you say
> 
> f :: !Forceable alpha => alpha -> beta
> 
> The compiler leaves the Forceable alpha constraint off.  Then you can  
> say
> 
> build :: (forall c. !Forceable c => (a -> c -> c) -> c -> c) -> [a]
> 
> And the foldr/build law is still sound.

Why do you want types that lie, plus some crazy ad-hoc special case?
Why not just let f :: a -> b mean what you write as f :: !Forceable a =>
a -> b exactly as it would if seq were moved (back) into a class?  Then
the free theorems would hold for the types as stated.