Derived function

A derived function is the result of applying an operator to its operands. Since it is a function, it can then be applied to argument arrays, resulting in other arrays.

Consider for example applying a Windowed Reduction to an array: In this statement the Reduce operator is applied to the Minimum function  resulting in the derived function. In most modern APLs this function can be manipulated like any other function, for example to assign it to a name: Here the value of  is a derived function. Many APLs display a derived function by placing its operands next to the operator, as it is written:

In APL a user-defined operator such as a tradop or dop creates a derived function from its arguments without executing any of the user's code. In J, the user's code is evaluated and must return a value, which could but doesn't have to be a derived function.