Proposal: Add Data.List.sortNub and sortNubBy

[Pepe Iborra]

On 13/03/2007, at 18:25, John Meacham wrote:

> once we have ordNub, we can create lots of RULES like the following
>
> {-# RULES "nub/ordNub" nub = ordNub :: [String] -> [String] #-}
> {-# RULES "nub/ordNub" nub = ordNub :: [Int] -> [Int] #-}
> {-# RULES "nub/ordNub" nub = ordNub :: [Integer] -> [Integer] #-}
>
> actually, we probably want one that uses IntMap for ints. it is
> signifigantly faster.

very nice idea!

> I don't suppose there is a way to match a RULE to a class  
> constraint? I
> don't see how it would be implemented in any easy way since classes  
> get
> desugared away pretty early... but it would be an interesting  
> feature..
>
> {-# RULES "nub/ordNub" forall (a::*) . Ord a => nub = ordNub :: [a]  
> -> [a] #-}
> or something...

I think yes, the right syntax would be something like:

> {-# RULES "nub/ordNub" forall (x::Ord a => a). nub x = ordNub x #-}

But hopefully someone else will confirm.

Also, what happens if the general Ord rule is added. Is it possible  
to convince GHC to use the more specific rule for Int before the  
general one?
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