Send a program, not a data structure

Alan Kay on ever expanding requirements

Footnote about “Ever expanding requirements at Parc” (prompted by Phillip Remaker’s comment and question)

When Gary Starkweather invented and got the first laser printer going very quickly, and at astounding speeds (a page per second, 500 pixels per inch), there was a push to get one of these on the networked Altos (for which the Ethernet had been invented). The idea was to use an Alto as a server that could set up and run a laser printer to rapidly print high quality documents.

Several of the best graphics people at Parc created an excellent “printing standard” for how a document was to be sent to the printer. This data structure was parsed at the printer side and followed to set up printing.

But just a few weeks after this, more document requirements surfaced and with them additional printing requirements.

This led to a “sad realization” that sending a data structure to a server is a terrible idea if the degrees of freedom needed on the sending side are large.

And eventually, this led to a “happy realization”, that sending a program to a server is a very good idea if the degrees of freedom needed on the sending side are large.

John Warnock and Martin Newell were experimenting with a simple flexible language that could express arbitrary resolution independent images — called “JAM” (for “John And Martin” — and it was realized that sending JAM programs to the printer was a much better idea than sending a data structure.

This is because a universal interpreter can both be quite small and also can have more degrees of freedom than any data structure (that is not a program). The program has to be run in a protected address space in the printer computer, but it can be granted access to a bit-buffer, and whatever it does to it can then be printed out “blindly”.

This provides a much better match up between a desktop publishing system (which will want to print on any of the printers available, and shouldn’t have to know about their resolutions and other properties), and a printer (which shouldn’t have to know anything about the app that made the document).

“JAM” eventually became Postscript (but that’s another story).

Key Point: “sending a program, not a data structure” is a very big idea (and also scales really well if some thought is put into just how the program is set up).

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