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. 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!
This plays on the fact that
⍴X gives the shape (length of each axis; "dimension") of
⍴⍴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:
⎕←X←3 1 4∘.+2 7 1 8 5 10 4 11 3 8 2 9 6 11 5 12 ⍴X 3 4 ⍴⍴X 2 ⍴⍴⍴X 1
- Stallman, Richard M. "RMS Berättar".