[Haskell-beginners] Question about IO, particularly hGetContents

[Isaac Dupree]

On 04/01/10 21:06, Ken Overton wrote:
> Hello all, I was playing with some Haskell code that read in text files and processed them and often found myself writing empty result-files.  I've pared the problem down to the following small example.
>
> -- example program
> import IO
> main = do
>      rhdl<- openFile "test.in" ReadMode
>      content<- hGetContents rhdl
>      putStrLn content  -- if I cmt out this line, content will be empty
>      hClose rhdl
>      putStrLn "Content: "
>      putStrLn content      -- if the first 'putStrLn' call was commented, this will print a blank line
> -- end example
>
> For some reason, if I comment out the 'putStrLn content' between hGetContents and hClose, the data from the hGetContents call is not stored.  Can somebody verify this behavior and (if so) explain why it's happening?

Yes.  hGetContents retrieves the contents "lazily", only reading them 
when demanded by the program*.  When it's read all the contents, it 
automatically closes the file, so you shouldn't use hClose in 
combination with it (also it stops any future reading from happening, 
which is why your program broke).

*implemented using the somewhat controversial "unsafeInterleaveIO". Some 
people believe that lazy IO like this is not a good idea, for reasons 
similar to the one you encountered, but in circumstances that are harder 
to fix.

-Isaac