Atop (operator)

is a primitive dyadic operator which takes two function operands and produces a derived function which uses the left operand monadically to post-processes the result of the ambivalent right operand.

Explanation
When the resulting function is used monadically, it has the same behaviour as if the Atop 2-train or any of the Beside or Over operators had been used: When the resulting function is used dyadically, the result is post-processed:

Close composition
In SHARP APL and J, Atop is implemented as a close composition, meaning that (using SHARP syntax)  has the overall function rank of. J uses  for the close form and   for the rankless form that appears in modern APLs.

Lessons

 * APL Cultivation

Documentation

 * Dyalog
 * J Dictionary, NuVoc