Function: Difference between revisions

Every APL comes with [[primitive function]]s, and almost always [[system function]]s, which are defined by the language. [[Derived function]]s can also be created by applying [[operator]]s to [[operand]]s. For example <source lang=apl inline>+/</source>, [[Plus]] [[Reduce]], is a derived function that can be created in any APL. However, early APLs usually did not allow [[assign]]ing a name to such functions: the only ways to call them were to write the function out explicitly and to define an equivalent [[defined function]]. Function assignment became more common in the 1980s, and after the advent of [[tacit programming]] in the 90s it is present in all new APLs.

The traditional way of defining functions in APL, and the most common way to define functions currently, is to create a [[defined function]], which consists of a sequence of APL statements. This is the only way to create a [[niladic function]], although not all ways to define a function support creating niladic functions. There are three major families of function definition:

* [[Direct definition]], introduced by [[Ken Iverson]] in [[A Programming Language]] but rarely implemented. Direct definition might be considered a kind of APL [[wikipedia:pseudocode|pseudocode]].

* [[Tradfn]]s (often simply called "defined functions"), introduced by [[APL\360]]. In these functions, a [[function header|header]] line describes the way the function is called and the result is placed in a variable whose name is given in the header.

* [[dfn]]s, developed by [[John Scholes]] and introduced in [[Dyalog APL]]. In a dfn, the arguments are given the fixed names <source lang=apl inline>⍺</source> and <source lang=apl inline>⍵</source>, and the result of the first non-assignment statement is returned.