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.)