[Haskell-beginners] Re: how to read file with locking

[Ertugrul Soeylemez]

Joey Hess <joey at kitenet.net> wrote:

> The function below always returns "", rather than the file's contents.
> _Real World Haskell_ touches on how laziness can make this
> problimatic, without really giving a solution, other than throwing in
> a putStr to force evaluation, which can't be done here. How can I make
> hGetContents strict, to ensure the file contents are really read
> before it gets closed?

In general you don't want to read a file as a String non-lazily.  That
uses a lot of memory, because String is just a type alias for [Char],
which means a linked list of Char values.

To read an entire file eagerly the proper way is in most cases using
Data.ByteString or Data.ByteString.Char8, which have a strict interface.
A ByteString from those modules is always either non-evaluated or
completely evaluated, and it is a real byte array instead of a linked
list.  Both modules feature hGetContents:

  import qualified Data.ByteString.Char8 as B

  readFileLocked :: FilePath -> IO B.ByteString
  readFileLocked fn =
    withFile fn ReadMode $ \h -> do
      lockFile h  -- for a suitable function lockFile
      B.hGetContents h

It is, BTW, always preferable to use withFile over openFile, if you can.
This makes your code cleaner and also exception-safe.


Greets,
Ertugrul


-- 
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