[Haskell-cafe] Monad transformers [Stacking monads]

David Menendez dave at zednenem.com
Tue Oct 7 17:38:04 EDT 2008

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?
>> 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.

Dave Menendez <dave at zednenem.com>

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