[Haskell-cafe] Takusen and strictness

Bayley, Alistair Alistair_Bayley at invescoperpetual.co.uk
Fri Mar 2 09:48:17 EST 2007

> From: Paul Moore [mailto:p.f.moore at gmail.com] 
> > If you don't need the entire list at once, then push your processing
> > into the iteratee.
> Hmm, that's what I was trying to avoid. The article I mentioned made a
> strong point that laziness allows you to factor out processing from IO
> - so you can write (for example)
> main = do
>   s <- getContents
>   let r = map processIt (lines s)
>   putStr (unlines r)
> and laziness means that IO is performed "on demand", so that the above
> code never has to read the whole input into memory. I was hoping to do
> something similar for database access, with runSql taking the place of
> getContents. Having to incorporate "processIt" into the database
> access code breaks this idiom.

There's a big difference between getContents and Takusen: getContents
has a non-trivial implementation (using unsafeInterleaveIO) that allows
it to return data lazily. Takusen has no such implementation. I'm not
sure if it would be possible. I don't really understand how getContents
works; is there any advice or guidelines as to how to use (or abuse)
unsafeInterleaveIO? Some googling has found:

> That's what my earlier code looked like, and I found it harder to
> understand than the getContents/process/put approach. I'm trying to
> explore ways of factoring data manipulation code out of database
> access functions, but maybe that's not the right way of doing it.

I don't think it's possible to pursue this style of programming with
Takusen. If you do, you'll have to process the entire result-set into a
data structure and then process it, which has obvious memory

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