[Haskell-cafe] Monad transformers [Stacking monads]

[Daniel Fischer]

Am Dienstag, 7. Oktober 2008 23:38 schrieb David Menendez:
> On Tue, Oct 7, 2008 at 5:07 PM, Daniel Fischer <daniel.is.fischer at web.de> 
wrote:
> > Am Dienstag, 7. Oktober 2008 22:09 schrieb Andrew Coppin:
> >> Daniel Fischer wrote:
> >> > Maybe it is as simple as
> >> >
> >> > raw_bind (xs:xss) f = do
> >> >      rsYs <- mapM f xs
> >> >      ~rsZ <- raw_bind xss f
> >> >      return (foldr union (cost rsZ) rsYs)
> >> >
> >> > then rsZ should only be evaluated when it's needed
> >>
> >> Ooo... "lazy pattern matching"? Can somebody explain to me, _very
> >> slowy_, exactly what that means?
>
> <snip>
>
> >> If I'm not mistaken, the rsZ variable shouldn't be evaluated until
> >> needed *anyway*, so what is lazy pattern matching buying me here?
> >
> > That depends on how your Monad (and union) is implemented, it may or may
> > not make a difference. I must admit that I didn't really look at the code
> > you posted, so I don't know what would be the case here. It was just an
> > easy thing to try which *might* help.
>
> Unless you're pattern matching against a constructor, which rsZ is
> not, I think lazy pattern matching is no different from regular
> pattern matching.

I think you're right, I was being stupid. Oh, well, it's late here