FW: lazy file reading in H98

[Malcolm Wallace]

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

The Library Report is already stronger than this.  The behaviour is
fully defined: an error should be raised.  Here's what it says:

> Implementations should enforce as far as possible, locally to the Haskell
> process, multiple-reader single-writer locking on files.   ...   If any
> open or semi-closed handle is managing a file for input, new handles can
> only be allocated if they do not manage output.
>          ...
> Error reporting: the openFile computation may fail with isAlreadyInUseError
> if  the file is already open and cannot be reopened.

The only very slightly confusing aspect is that the handles used by
"readFile" and "writeFile" are internal, not written directly by
the programmer.  Perhaps the description of this behaviour should be
moved up from the 11.3.1 "Opening Files" subsection to the enclosing
11.3 section, because it is more generally applicable.

Subsection 11.2.1 "Semi-closed Handles" should mention "readFile"
in addition to "hGetContents".  It could also explicitly refer to
the multiple-reader single-writer restriction, which is not otherwise
mentioned here.

Regards,
    Malcolm