Re: [Haskell-cafe] Monad transformers [Stacking monads]

Am Dienstag, 7. Oktober 2008 23:38 schrieb David Menendez:

On Tue, Oct 7, 2008 at 5:07 PM, Daniel Fischer wrote:

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Am Dienstag, 7. Oktober 2008 22:09 schrieb Andrew Coppin:

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Daniel Fischer wrote:

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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?

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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