[Haskell-cafe] operating on a hundred files at once

Duncan Coutts duncan.coutts at worc.ox.ac.uk
Mon Apr 9 18:50:59 EDT 2007

On Mon, 2007-04-09 at 14:40 -0400, Jefferson Heard wrote:
> Thanks for the advice.  I'm not so much interested in performance here,
> as this is just a one-off.  Disk thrashing or not, these files are only
> a few hundred K apiece, and I can't imagine that the whole computation
> will take more than a few minutes.  
> My question is more about how to deal with the IO monad "pollution" of
> all the data in a situation where you have N instances of IO [a] at step
> 1, and you have M computations to perform on those instances, which are
> all monad-free.

Perhaps you want one of these functions:

sequence :: Monad m => [m a] -> m [a]

sequence_ :: Monad m => [m a] -> m ()

for example in the case of IO it's:

sequence :: [IO a] -> IO [a]
sequence_ :: [IO a] -> IO ()

ie it takes a pure list of IO actions and sticks them together into one
IO action, or to put it another way, it performs all the actions in

Is this what you meant?


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