[Haskell-cafe] Monad transformers [Stacking monads]
andrewcoppin at btinternet.com
Tue Oct 7 16:09:28 EDT 2008
Daniel Fischer wrote:
> Am Dienstag, 7. Oktober 2008 20:27 schrieb Andrew Coppin:
>> Basically, the core code is something like
>> raw_bind :: (Monad m) => [[x]] -> (x -> m (ResultSet y)) -> m
>> (ResultSet y)
>> raw_bind  f = return empty
>> raw_bind (xs:xss) f = do
>> rsYs <- mapM f xs
>> rsZ <- raw_bind xss f
>> return (foldr union (cost rsZ) rsYs)
>> As you can see, this generates all of rsZ before attempting to return
>> anything to the caller. And I'm really struggling to see any way to
>> avoid that.
> 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 doing this right, it seems that
rsZ <- raw_bind xss f
raw_bind xss f >>= \rsZ -> ...
If I'm not mistaken, the rsZ variable shouldn't be evaluated until
needed *anyway*, so what is lazy pattern matching buying me here?
Also, suppose I stack ResultSetT on top of IO. In that case, "f" is
allowed to perform externally-visible I/O operations. If there really
*is* a way to delay the execution of certain calls until the data is
needed... well that doesn't look right somehow. In fact, it looks like
what I'm trying to do *should* be impossible. :-/ Oh dear...
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