J3/06-165 Date: April 21, 2006 To: J3 From: Malcolm Cohen, Miles Ellis, David Muxworthy, Lawrie Schonfelder (in alphabetical order) Subject: Co-arrays to be optional This paper expresses the personal opinion of the authors and is not a formal UK position. Despite being initially in favour of extending Fortran to accommodate parallel architecture, we believe that there are significant problems with the current co-array proposals, for example: - the conceptual model is restricted to a small class of parallel machines - the I/O model is overly complicated - the synchronization model is overly complicated and too low-level - significant further technical work on the proposal is still needed so there is a serious risk of delaying the whole revision schedule - the co-array model is presented as the normal execution model with single processors as the exception; this is the wrong emphasis given that much of the new language is irrelevant to most users - the proposals as currently formulated are large and experimental and should be evaluated in practice before being standardized We believe the facts that co-arrays are of minority interest and that these proposals in detail are untested in practice point strongly to their being an optional, rather than a mandatory, part of the Fortran standard. We believe that the best way to do this would be for co-arrays to appear as a Technical Report (without necessarily guaranteeing inclusion in the next standard). This would allow those vendors whose primary markets are for single processors to avoid the burden of implementing features of little interest to their customers. We urge J3 to proceed on these lines and to so recommend to WG5.