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Postby Roger|Dyalog on Thu Apr 16, 2020 5:14 pm

The Socio-Technical Beginnings of APL, by Eugene McDonnell, APL Quote Quad, volume 10, number 2, 1979-12. Free download from ACM until 2020-06-30.
One of the most characteristic conveniences of the system is the cue chosen for the user to make a new entry. On other systems being developed at this time, this user cue varied from one kind of character sequence to another: on one system a question mark would be printed, on another, a rather long-winded time stamp, on another, the word “READY”. In keeping with its general policy of informing the user as briefly as possible, the signal APL uses to indicate that output display has finished, or that computer processing is over, is succinct indeed. No printing characters appear at all: six spaces are sent, moving the cursor or printing element to the right, and the keyboard is unlocked for a new input. To me, that six-space cue is one of the key items in APL’s elegance. Larry Breed recalls that it was Ken Iverson who suggested it.

Follow-on anecdote:
Early in the J implementation Ken suggested that the prompt should be 3 spaces instead of 6. There are suggestions and there are suggestions, and this suggestion was made in a tone which brooked no argument. So I made it so.

Later, k one-upped us by doing a one-space prompt.

Additional Eugene McDonnell Papers and Articles here; additional APL Quotations and Anecdotes here.
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