Besides arguments and functions, APL has operators which take functions as operands, forming functions which are then free to act on arguments.
For example, the Commute operator exchanges the arguments when called with two of them (I pronounce this "swap") or duplicates the argument, placing it on the left and right, if there is only one ("self").
2 -⍨ 5
3
-⍨ 4
0
The derived function produced by ⍨ can be called either monadically or dyadically, but its operand is always called dyadically!