Defined function (traditional)

A user-defined function (or tradfn, for "traditional function", in Dyalog APL) is a function defined using a header that includes the function's name. Introduced in APL\360, function definition was universally supported by APL dialects for much of the language's history, and is still commonly used in mainstream APLs. Since the 1990s other ways to describe functions have appeared, with J and K rejecting function definition in favor of anonymous function description. Beginning in the 2010s Dyalog-based APL dialects including ngn/apl, dzaima/APL, and APL\iv have removed function definition in favor of dfns.

In many dialects the function header syntax of defined functions is adapted to allow defined operators as well.

Representations
The canonical representation form is equivalent to what the user would type into the line editor to define the function. An alternative representation consists of the entire session log transcript after having such a definition has been made. A tradfn operator can also be called a tradop (pronounced "trad op").

Properties
Tradfns allow both functional and procedural programming, and are essential for object-oriented programming. They support a full set of keywords for flow control and object declarations.

One of the most noticeable differences from dfns is that tradfns must declare their locals, because assignments are global by default. The declaration is done in a header line which also determines the function's name and calling syntax. Tradfns cannot be nested (although a tradfn can dynamically create other tradfns by "fixing" their source, and if the function thus created has a name which has been localised, it will only exist that scope) but can contain dfns. However, nested functions are less necessary due to tradfns using dynamic scoping as opposed to the lexical scoping of dfns. In other words, a tradfn can "see" locals of its caller.

A tradfn can be niladic which causes it to behave syntactically like an array. However, every time its name is referenced, it will run to create a result (if any). Such methods are often used to return a cache or as an entry point for the user.

A+
A+ uses a reworked syle of function and operator definition than maintains the principle of a header that matches the way the function will be used, but differs in many details:
 * The result name is not included in the header; instead, the result of the last executed statement is returned (and so niladic functions cannot be defined).
 * The header is separated from the body with a colon, and the body of a multi-line function is enclosed in curly braces.
 * Functions have lexical scope. Variables assigned are local by default, and can be made global by enclosing their names in parentheses when assigning.

Comparison to dfns
Wikipedia has a comparison of dfns versus tradfns.

Tutorials

 * APL Cultivation: tradfns

Documentation

 * Dyalog
 * APLX