Defined function (traditional)

This page is about the function form typically written with the Del (∇) character. See Function styles for an overview of all forms; in particular dfns are considered a type of defined function in Dyalog APL and GNU APL.

A user-defined function (or tradfn, pronounced "trad fun", 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.

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, including Dels (∇) and line numbers, after having such a definition has been made. However, it should be noted that the ∇s are not part of the definition. Indeed, if using the Fix Definition (⎕FX) system function to define the function, Dels need not (or must not, depending on implementation) be included.

Beginning in the 2010s Dyalog-based APL dialects including ngn/apl, dzaima/APL, and APL\iv have removed function definition in favor of dfns. Wikipedia has a comparison of dfns versus tradfns.

In many dialects the function header syntax of defined functions is adapted to allow user-defined operators as well. A tradfn operator can also be called a tradop (pronounced "trad op"), parallel to how a "dfn" operator can be called a dop.

Examples

Basics

A simple dyadic function:

      ∇ r←l Tradfn r
        ⍝ ...do something
        r←l r
      ∇

Works in: Dyalog APL, APL2, GNU APL, NARS2000, APLX, and every older APL from APL\360

Semi-colons

Assignments are global by default. To keep a name local, it must be mentioned in the header, separated from the rest of the header by a semi-colon:

      ∇ r←l Tradfn r;intermediate
        intermediate←l r
        r←intermediate intermediate
      ∇

Works in: Dyalog APL, APL2, GNU APL, NARS2000, APLX, and every older APL from APL\360

Braces

An ambivalent function with an optional left argument, a conditional control structure, one local variable, and a shy result:

      ∇ {res}←{left} AddMult2 right;local
        :If 0=⎕NC'left'     ⍝ if variable "left" is not defined already
            left←0
        :EndIf
        local←left+right
        res←2×local
      ∇
      AddMult2 3     ⍝ result is "shy"
      ⎕←AddMult2 3   ⍝ coerce display of result
6
      1 AddMult2 3      ⍝ result is "shy"
      10×1 AddMult2 3   ⍝ use result anyway
80

Works in: Dyalog APL

Brackets

GNU APL allows functions and operators to accept an axis argument:

      ∇ Z←Average[X] B
        Z←(+/[X]B) ÷ (⍴B)[X]
      ∇
      Average[1] 5 5⍴⍳25
11 12 13 14 15
      Average[2] 5 5⍴⍳25
3 8 13 18 23

Works in: GNU APL

Operators

A monadic operator and a dyadic operator, both deriving monadic functions:

      ∇ res←(Function SELF) right
        res←right Function right
      ∇
      ×SELF 1 2 3 4 5
1 4 9 16 25

      ∇ res←(FunctionF HOOK FunctionG) right
        res←right FunctionF FunctionG right
      ∇
      ÷HOOK|2 0 ¯7
1 1 ¯1

Works in: Dyalog APL, APL2, GNU APL, NARS2000, APLX

A monadic operator and a dyadic operator, both deriving dyadic functions:

      ∇ res←left (Function SWAP) right
        res←right Function left
      ∇
      3 2 -SWAP 10
7 8

      ∇ res←left (FunctionF OVER FunctionG) right
        res←(FunctionG left) FunctionF (FunctionG right)
      ∇
      2 ¯7 2+OVER|¯3 1 4
5 8 6

Works in: Dyalog APL, APL2, GNU APL, NARS2000, APLX

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. Dyalog APL adds a handful of extensions this (see below).

Tradfns cannot be nested but can contain dfns. A tradfn can dynamically create another tradfn by "fixing" its source, and if the function thus created has a name which has been localised, the inner function will only exist the scope of the outer function. 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.

Dyalog APL extensions

Dyalog APL's tradfns are enhanced in various ways compared to most other dialects.

Dynamic localisation

Names can be localised dynamically, while a tradfn is running, using the Shadow Name (⎕SHADOW) system function.

      name←1 2 3
      ∇ res←Dummy
        ⎕SHADOW'name'
        name←42
        res←name
      ∇                      
      Dummy
42
      name
1 2 3

Works in: Dyalog APL

Locals lines

Additional local names can also be declared on separate lines following the header line:

      name←1 2 3
      ∇ res←Dummy
        ;name
        name←42
        res←name
      ∇                      
      Dummy
42
      name
1 2 3

Works in: Dyalog APL

Explicit ambivalence

The the header syntax can explicitly specify that a function is ambivalent by enclosing the left argument name in curly braces (for example result←{left} Function right). Others dialects treat all functions that declare a left argument name as ambivalent and leave it up to the programmer to check for the presence of a value.

Function results

A tradfn can return a function value as result. The returned function will replace the function and its arguments (if any) in the calling expression:

      ∇ Fn←Apply name       
        :If name≡'plus'     
            Fn←+            
        :ElseIf name≡'times'
            Fn←×            
        :EndIf              
      ∇                      
      3(Apply'plus')4
7
      3(Apply'times')4
12

Works in: Dyalog APL

Shyness

By default, functions in APL will print their result to the session log even without using ⎕← unless their results are shy. Shy results are declared by putting curly braces around the result name (for example {result}←left Function right). Assignment (name←array) exhibits the same behaviour as a shy function in that, by default, no result is printed when the function terminates, but attempting to use the result still succeeds.

Namelists

The right argument and the result can be a name list instead of single name. The interpreter will unpack the right argument when the function is called, and collect the result when the function returns.

      ∇ (c b a)←Rev(a b c)
      ∇
      Rev 1 2 3
3 2 1

Works in: Dyalog APL

A+ differences

A+ uses a reworked style 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 functions that do not return a result 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.

External links

Tutorials

• APL Cultivation: tradfns
• Bernard Legrand. Mastering Dyalog APL (page 176). Dyalog Ltd. November 2009.

Documentation

• Dyalog: Functions, Operators
• APLX: User-defined Functions, User-defined Operators