[Haskell-beginners] lazy mapM

[David McBride]

I know you have the best intentions in writing this, but there are
pitfalls.  Unexpected things happen when you interleave IO in this manner,
but nonetheless, here's how you would do it.

myGetLine = do
  x <- getLine
  if (x == "exit")
      then return []
      else do
        xs <- unsafeInterleaveIO myGetLine
        return (x:xs)

main = do
  x <- myGetLine
  print x

Just know that at some point you should learn to use conduits or pipes for
a much better approach to modeling things like this.



On Sun, Mar 31, 2013 at 7:26 PM, Ovidiu D <ovidiudeac at gmail.com> wrote:

> Hi again,
>
> Given the following code:
>
> g :: IO String -> IO String
>
> f :: [IO String] -> IO [ String ]
> f = mapM g
>
> The implementation of f is wrong because I would like to:
> 1. Make f behave lazy
> Its input list is made of lines read from stdin and I want it to process
> lines one by one as they are entered by the user.
>
> 2. Implement  f such that it stops consuming items from the input list
> when the input item meets some condition. For example:
> isExit item = ("exit" == item)
>
> I tried to implement my own custom iteration by recursion but I got stuck
> in the combination of IO and list monads.
>
> Any help is appreciated.
>
> Thanks!
>
>
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