Richard Stallman

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This article is about Stallman's relationship with APL. For more complete information about Stallman, see Wikipedia.

Richard Matthew Stallman, founder of the GNU Project, was an early adopter of APL, and used it to write a text editor as a high school student in the summer of 1969[1]. The same year, he also wrote a now famous rhyme:

Rho, rho, rho of X
Always equals 1
Rho is dimension, rho rho rank.
APL is fun![2]

This plays on the fact that X gives the shape (length of each axis; "dimension") of X while ⍴⍴X gives the rank (number of axes/dimensions) and since the number of axes is always a count, it is a single number, which has the shape 1 if a vector, and always returns a vector:

      X3 1 4∘.+2 7 1 8
5 10 4 11
3  8 2  9
6 11 5 12
3 4