Max Bolingbroke
This behaviour is part of the Haskell 98 specification (section 21.2.3, http://www.haskell.org/onlinereport/io.html):
Thanks for the explanation. Such sharing behavior should be mentioned in documentation: http://hackage.haskell.org/packages/archive/haskell98/latest/doc/html/IO.htm... What was the rationale behind such strict non-sharing policy? Anyway, where do I find an 'openFileShared' function? Packages unix/Win32 do not have obvious leads...
""" Implementations should enforce as far as possible, at least locally to the Haskell process, multiple-reader single-writer locking on files. That is, there may either be many handles on the same file which manage input, or just one handle on the file which manages output. If any open or semi-closed handle is managing a file for output, no new handle can be allocated for that file. """
I've been bitten by this before and don't like it. It would be possible for GHC to enforce Unix semantics instead (there are appropriate flags to CreateFile that get those semantics on Windows), which would support more use cases. This change would have to be carefully thought through, and the report would have to be amended.
Cheers, Max
-- Gracjan