A number of comments: Barry Jay writes:

1) Nico Verwer suggests using a general drawing package XY-pic. Drawings and diagrams differ fundamentally on the issue of who decides the position of picture elements. In a drawing package the user decides, in a diagram the program decides. ... Presumably our diagram tools should be designed in the same way. We need a diagram language based on general principles, and a convenience package built on top for the average user.

XY-pic (which I have used, and I agree much with Nico Verwer's praise) consists of core which is a general-purpose "drawing" package (entirely within TeX) in which almost everything within TeX's limited graphical capabilities may be done, and a diagram package (built on that) which makes possible specifying diagrams using the matrix analogy allowing for easy use.

To illustrate, Tex passed the music test: by adding a few macros,

Music has a well-established, linear and compositional notation. Diagrams may have a harder time.

Paul Taylor proposes allowing a good interface to any package. Perhaps by separating these issues, the need for many packages can be reduced. This is very true. The number of core macros and necessary font families should be low.

David Murphy writes:

I don't like Paul's syntax for either. My objection on the first count, and the reason I like Mike's proposal, is just taste; ... ... just how much of a graphics package do we want a commutative diagrams package to be?

This may be important. I don't know whether Michael Barr's initiation of the discussion was aimed at a general diagram drawing package, or just at "categorical" diagrams (then other kinds of digrams like syntax diagrams, flow charts, etc. being defined by other communities). Anyway, it is impossible to find a single (linear) notations suited for all diagram purposes, and meeting everybodys taste. I almost completely agree with Paul Taylors thoughts (though I have no strong meaning on his syntax compared to XY-pic). I would recommend: 1) The implementation of a general purpose core package based on pure TeX and compatible with Plain TeX, LaTeX and AmsTeX (at least). All packages have widespread use among mathematicians. The designers here should take a strong look at XY-pic with its possibilities of absolute, symbolic and relative adressings, uniform specifications of objects, arrows and arrow labels, easy definition of subdiagrams, and much more. 2) The addition of packages building on the core. These should at least include commutative diagrams (up to what is seen in basic category books like that of Barr and Wells, and perhaps some 2-cells and cubic drawings. I like best the notation philosophy of Paul Taylor and Kristoffer Rose, but one might include a number of common, special purpose macros a la Michael Barr). Other possibilities are syntax diagrams, flow charts, trees, and geometric drawings (note that at least here the paper-and-pencil approach is probably preferable). 3) For the core, and for each package, interfacing to the Plain, LaTeX and AmsTeX packages (possibly only a difference in the environment establishing macros; the problem is biggest for LaTeX who has tampered most with the basic TeX syntax). 4) One set of font families. Hans Dybkj{\ae}r +++++++++++++++++++++++++++++++++++++++++++++++++++++++++