PSPP goals

[Ben Pfaff]

Jason Stover and I met over lunch yesterday and talked over some
of the goals for PSPP.  I realized that I haven't ever done a
good job of expressing these on the list, although I've talked
them over with a few individuals at different times.  So I've
written up a statement of my long-term goals for PSPP, included
below.  I think I'd like to include this in the README for 0.4.0.
Comments are welcome--please give feedback.

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The long term goals for PSPP are ambitious.  We wish to provide the
following support to users:

        * All of the SPSS transformation language.  PSPP already
          supports a large subset of it.

        * All the statistical procedures that someone is willing to
          implement, whether they exist in SPSS or not.  Currently,
          statistical support is limited, but growing.

        * Compatibility with SPSS syntax, including compatibility with
          known bugs and warts, where it makes sense.  We also provide
          an "enhanced" mode in certain cases where PSPP can output
          better results that may surprise SPSS users.

        * Friendly textual and graphical interfaces.  PSPP does not do
          a good job of this yet.

        * Attractive output, including graphs, in a variety of human-
          and machine-readable formats.  PSPP currently produces
          output in ASCII, PostScript, and HTML formats.  We will
          enhance PSPP's output formatting in the future.

        * Good documentation.  Currently the PSPP manual describes its
          language completely, but we would like to add information on
          how to select statistical procedures and interpret their
          results.

        * Efficient support for very large data sets.  For procedures
          where it is practical, we wish to efficiently support data
          sets many times larger than physical memory.  The framework
          for this feature is already in place, but it has not been
          tuned or extensively tested.

Over the long term, we also wish to provide support to developers who
wish to extend PSPP with new statistical procedures, by supplying the
following:

        * Easy-to-use support for parsing language syntax.  Currently,
          parsing is done by writing "recursive descent" code by hand,
          with some support for automated parsing of the most common
          constructs.  We wish to improve the situation by supplying a
          more complete and flexible parser generator.

        * Easy-to-use support for producing attractive output.
          Currently, output is done by writing code to explicitly fill
          in table cells with data.  We should be able to supply a
          more convenient interface that also allows for providing
          machine-readable output.

        * Eventually, a plug-in interface for procedures.  Over the
          short term, the interface between the PSPP core and
          statistical procedures is evolving quickly enough that a
          plug-in model does not make sense.  Over the long term, it
          may make sense to introduce plug-ins.
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