Scan

is a primitive monadic operator which takes a dyadic function operand and produces a monadic function which is equivalent to the reductions of the prefixes of the arguments. This operation is known as also known as cumulative reduction.

Explanation
Scan is similar to Reduce, however all the intermediate results will be retained. That being said, unlike Reduce, Scan does not reduce the rank of its argument, i.e. performing a scan over an N-dimensional array will produce an N-dimensional array.

Applications
Removing disconnected trailing 1s from a boolean mask:

Lessons

 * APL Cultivation

Documentation

 * Dyalog
 * APL
 * BQN