Re: [Haskell-beginners] [IO String] to IO [String]

That makes perfect sense. Thanks for the detailed explanation. On Sun, Mar 31, 2013 at 2:30 PM, Brent Yorgey wrote:

To elaborate a bit: you originally said "I don't like that the line processing I'm doing will have to be in the IO monad" -- but since you want the processing of the list of Strings to be interleaved with IO operations, you have no choice. Pure functions of type [String] -> [String] cannot have their evaluation interleaved with IO operations.

Note that functions using lazy I/O such as getContents, readFile, etc. can create exceptions to this -- but these use unsafeInterleaveIO under the hood and are widely regarded as problematic. So you could try using unsafeInterleaveIO, but it is unsafe for a reason. I do not actually know what is required to ensure you are using it safely; perhaps someone else could elaborate on this.

-Brent

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Right. I definitely want the lazy behavior.

Thanks

On Sun, Mar 31, 2013 at 1:29 PM, Brent Yorgey

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Unfortunately, with the definition

f = getLine : f

this will not work. 'sequence f' has to do *ALL* the IO before you can process even the first String in the resulting list. Since it is infinite, it will just sit there reading lines forever but never letting you process them.

I think in this case using [IO String] is actually a good solution.

-Brent

On Sun, Mar 31, 2013 at 12:24:33PM +0200, Nathan Hüsken wrote:

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Try

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

I thinkg

sequence f :: IO [String]

should be what you want.

On 03/31/2013 12:19 PM, Ovidiu D wrote:

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I have the function f which reads lines form the stdin and looks

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this:

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f :: [IO String] f = getLine : f

What I don't like is the fact that the line processing I'm doing

will

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have to be in the IO Monad

I would like to make this function to have the signature f : IO [String] ...such that I can get rid of the IO monad and pass the pure string

On Sun, Mar 31, 2013 at 01:48:23PM +0300, Ovidiu D wrote: like list

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to the processing function.

Can I do this?

Thanks

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