[Haskell-cafe] Newbie question

Arthur Baars arthurb at cs.uu.nl
Fri Aug 12 09:04:52 EDT 2005

Hi André,

The problem is that hGetContents does lazy reading of the handle.

I you do:
readDataFromFile "test.txt" >>= print

the handle is closed by hClose (in readDataFromFile) before "print" 
demands the contents from the handle.

Just don't close the Handle explicitly.
This code works:
readDataFromFile filename =
  do h <-openFile filename ReadMode
     contents <- hGetContents h
     return (lines contents)

You do not really need to close it, because hGetContents "semi-closes" 
the handle. Read more:
http://www.haskell.org/onlinereport/io.html (Section 21.2.2  
Semi-Closed Handles)



On 12-aug-05, at 14:17, André Vargas Abs da Cruz wrote:

> Hi everyone,
>    I think this is a totally newbie question as i am a complete novice 
> to Haskell. I am trying to write down a few programs using GHC in 
> order to get used with the language. I am having some problems with a 
> piece of code (that is supposed to return a list of lines from a text 
> file) which I transcribe below:
> module Test where
> import IO
> readDataFromFile filename = do
>    bracket (openFile filename ReadMode) hClose
>            (\h -> do contents <- hGetContents h
>                      return (lines contents))
>    The question is: if i try to run this, it returns me nothing (just 
> an empty list). Why does this happen ? When i remove the "return" and 
> put a "print" instead, it prints everything as i expected.
>    Thanks in advance
>    André
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> Haskell-Cafe at haskell.org
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