Re: [Haskell-cafe] Re: Where do I put the seq?

On Thu, Aug 20, 2009 at 1:02 PM, Ketil Malde wrote:

David Leimbach writes:

...

I'm pretty certain that forcing a pattern match via case is what disallows the laziness to get out of hand.  The case statement, when evaluated, must choose a matched pattern branch, even if it's the only possibility, which ends up boiling down to "seq" anyway doesn't it?

   Prelude> case undefined of x -> ()    ()

So I think you are incorrect: the 'undefined' here isn't evaluated by the case.

It's doing a case on a pattern match that forces evaluation. Try:

case undefined of (x:xs) -> ()

Antoine