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

[Ovidiu D]

That makes perfect sense. Thanks for the detailed explanation.


On Sun, Mar 31, 2013 at 2:30 PM, Brent Yorgey <byorgey at seas.upenn.edu>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
>
> On Sun, Mar 31, 2013 at 01:48:23PM +0300, Ovidiu D wrote:
> > Right. I definitely want the lazy behavior.
> >
> > Thanks
> >
> >
> > On Sun, Mar 31, 2013 at 1:29 PM, Brent Yorgey <byorgey at seas.upenn.edu
> >wrote:
> >
> > > 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:
> > > > 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:
> > > > >I have the function f which reads lines form the stdin and looks
> like
> > > this:
> > > > >
> > > > >f :: [IO String]
> > > > >f = getLine : f
> > > > >
> > > > >What I don't like is the fact that the line processing I'm doing
> will
> > > > >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
> list
> > > > >to the processing function.
> > > > >
> > > > >Can I do this?
> > > > >
> > > > >Thanks
> > > > >
> > > > >
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