[Haskell-cafe] Is it safe to use unsafePerformIO here?

[Cristiano Paris]

Hi Cafè,

I've the following problem: I have a (possibly very long) list of
files on disk. Each file contains some metadata at the beginning and
continues with a (possibly very large) chunk of data.

Now, the program I'm writing can be run in two modes: either read a
specific file from the disk and show the whole chunk of data on
screen, or read all the files' metadata, sort the file list based on
the metadata, and display a summary of those without reading the chunk
of data from each file. I've factored out the file access machinery in
a single module so as to use it indifferently under the two scenarios.

At first, I wrote a piece of code which, in spirit, works like the
following reduced case:

------
module Main where

import System.IO
import Control.Applicative
import Data.List
import Data.Ord

import Debug.Trace

data Bit = Bit { index :: Integer, body :: String }

readBit fn = withFile fn ReadMode $ \h -> Bit <$> (hGetLine h >>=
return . read) <*> readBody
             where readBody = withFile fn ReadMode $ \h -> do b <-
hGetContents h
                                                              seq b $
trace ("Read body from: " ++ fn) $ return b

main = do bl <- mapM readBit ["file1.txt","file2.txt"]
          mapM_ (putStrLn . show . index) $ sortBy (comparing index) bl
----

which is very expressive as it's written in applicative style.

Each file is like the following:

---- file1.txt ----
1
foo
----

I've created a separate IO action for reading the body in the hope
that it wouldn't get executed when the file list is sorted. But, to my
surprise, it didn't work as the trace message gets written for each
file before the summary is displayed.

Thinking about this, I came to the conclusion that the IO Monad is
enforcing proper IO ordering so that the IO action for file1's body
must be executed right before IO action for file2's index one.

If this is true, the only solution that came into my mind was to wrap
the IO action for reading the body in an unsafePerformIO call. I
actually ran the program with this modification and it works properly.

So, as using unsafePerformIO is a Great Evil (TM), I'm wondering if
there's a different way to do this which doesn't rely on retyping body
as an IO action returning a String, which would break my pure code
manipulating the files.

My opinion is that using unsafePerformIO here is like ensuring the
compiler that there're no observable side effects in running the IO
action for reading the body and that no other side effects would
impact this IO action.

Thank you for any thoughts.

Cristiano