Re: [Haskell-cafe] Laziness question

On Sunday 01 August 2010 10:52:48 am Felipe Lessa wrote:

On Sun, Aug 1, 2010 at 11:29 AM, Nicolas Pouillard

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