FW: lazy file reading in H98

Simon Peyton-Jones simonpj@microsoft.com
Tue, 3 Apr 2001 04:00:55 -0700

Here's a library issue.

The conclusion of this conversation was that H98 already specifies
option (1) below, and I will clarify that in revising the library
Nevertheless, the absence of a simple way to read-modify-write a file
is a pain in the neck.=20

Question: should one of our extended-IO libraries support a version of
openFile that guarantees option (2)?


-----Original Message-----
From: Manuel M. T. Chakravarty [mailto:chak@cse.unsw.edu.au]=20
Sent: 05 September 2000 02:10
To: haskell@haskell.org
Subject: lazy file reading in H98

In an assignment, in my class, we came across a lack of specification of
the behaviour of `Prelude.readFile' and `IO.hGetContents' and IMHO also
a lack of functionality.  As both operations read a file lazily,
subsequent writes to the same file are potentially disastrous.  In this
assignment, the file was used to make a Haskell data structure
persistent over multiple runs of the program - ie,=20

  readFile fname >>=3D return . read

at the start of the program and

  writeFile fname . show

at the end of the program.  For certain inputs, where the
data structure stored in the file was only partially used,
the file was overwritten before it was fully read.

H98 doesn't really specify what happens in this situation.
I think, there are two ways to solve that:

(1) At least, the definition should say that the behaviour
    is undefined if a program every writes to a file that it
    has read with `readFile' or `hGetContents' before.

(2) Alternatively, it could demand more sophistication from
    the implementation and require that upon opening of a
    file for writing that is currently semi-closed, the
    implementation has to make sure that the contents of the
    semi-closed file is not corrupted before it is fully

In the case that solution (1) is chosen, I think, we should also have
something like `strictReadFile' (and
`hStrictGetContents') which reads the whole file before proceeding to
the next IO action.  Otherwise, in situations like in the mentioned
assignment, you have to resort to reading the file character by
character, which seems very awkward.

So, overall, I think solution (2) is more elegant.


[1] On Unix-like (POSIX?) systems, unlinking the file and
    then opening the writable file would be sufficient.  On
    certain legacy OSes, the implementation would have to
    read the rest of the file into memory before creating
    a new file under the same name.