Riddle 0 Answer
As several people guessed in the comments, the answer I had in mind was what I think of as “compressed” matrices—general lists where some entries are lists of the same length and others are atoms.
q)(1f;`a`b) 1f `a`b q)flip flip(1f;`a`b) 1 1 a b q)
This is, as far as I can tell, directly related to atomic extension (
x f'y behaves identically for vector/vector, atom/vector, and vector/atom) and similar concepts, such as the ability to use an atom for a constant column in a table literal (
(x:1 2 3;y:4)).
(My original intention was to post these more or less weekly; hopefully I’ll be able to stick a bit closer to that in future.)