[Haskell-cafe] Laziness question

[Dan Doel]

On Sunday 01 August 2010 10:52:48 am Felipe Lessa wrote:
> On Sun, Aug 1, 2010 at 11:29 AM, Nicolas Pouillard
> 
> <nicolas.pouillard at gmail.com> wrote:
> > Finally maybe we can simply forbidden the forcing of function (as we do
> > with Eq). The few cases where it does matter will rescue to
> > unsafeSeqFunction.
> 
> What's the problem with
> 
>   class Eval a where
>     seq :: a -> t -> t
> 
>   instance Eval b => Eval (a -> b) where
>     seq f = seq (f undefined)
> 
> It would reduce at least to WHNF as unsafeSeq would.  Does it compute
> more than WHNF?
> 
> Hmmm, I see, if we had
> 
>   f :: Int -> Int
>   f _ = undefined
> 
> Then my seq above would diverge while unsafeSeq wouldn't.  Perhaps
> that instance would be a good compromise if we insist in not using
> 'unsafeSeq' to define it.

It also diverges for every strict function f.

  seq id x = seq (id undefined) x = seq undefined x = undefined

-- Dan