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Groups > comp.lang.haskell > #516

Re: Get mapM to be lazy for IO

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On 2018-12-13 10:21:17 -05, Barry Fishman wrote:
> Oops, I was wrong.  If I replace the:
>
>    xs <- getWordList 1
>
> with:
>
>   xs <- take 3 `fmap` getWordList 1
>
> the values continue to be read.  So the IO can't be read lazily as far
> as I can tell.

All I can seem to do is produce a infinite lazy list of IO actions,
prompting for and reading the words:

--8<---------------cut here---------------start------------->8---
#! /usr/bin/env runghc

import System.IO (hFlush, stdout)

main :: IO ()
main = do
  putStrLn "Hello.  Please enter each word."
  xs <- sequence . take 3 $ map getWordIO [1..]
  mapM_ putStrLn xs
  where getWordIO :: Int -> IO String
        getWordIO n = do
          putStr $ "Input number " ++ show n ++ ": "
          hFlush stdout
          getLine
--8<---------------cut here---------------end--------------->8---

So the list that I 'take 3' from is a '[IO String]' and I sequence though
it to produce an 'IO [String]'.

--
Barry Fishman

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Thread

Get mapM to be lazy for IO kfjwheeler@gmail.com - 2018-12-11 05:19 -0800
  Re: Get mapM to be lazy for IO Mark Carroll <mtbc@bcs.org> - 2018-12-11 13:36 +0000
    Re: Get mapM to be lazy for IO Kaylen Wheeler <kfjwheeler@gmail.com> - 2018-12-12 17:52 -0800
      Re: Get mapM to be lazy for IO Paul Rubin <no.email@nospam.invalid> - 2018-12-12 19:29 -0800
    Re: Get mapM to be lazy for IO Kaylen Wheeler <kfjwheeler@gmail.com> - 2018-12-12 18:21 -0800
      Re: Get mapM to be lazy for IO Paul Rubin <no.email@nospam.invalid> - 2018-12-12 19:31 -0800
  Re: Get mapM to be lazy for IO Barry Fishman <barry@ecubist.org> - 2018-12-13 09:52 -0500
    Re: Get mapM to be lazy for IO Barry Fishman <barry@ecubist.org> - 2018-12-13 10:21 -0500
      Re: Get mapM to be lazy for IO Barry Fishman <barry@ecubist.org> - 2018-12-13 10:46 -0500

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