Namespaces

Namespace is a (class 9) object in Dyalog APL. Namespaces are analogous to nested workspaces.

'Flat' APL Workspace      Workspace with Namespaces
.OLD-------------------.   .NEW-------------------.
|                      |   | FOO MAT VEC          |
| DISPLAY              |   | .Util----------.     |
|                      |   | |DISPLAY       |     |
|   FOO MAT VEC        |   | |...           |     |
|                      |   | '--------------'     |
|   WsDoc_Init         |   | .WsDoc-------------. |
|   WsDoc_Xref         |   | |Init .prt-..fmt--.| |
|   WsDoc_Tree         |   | |     |Init||line || |
|   WsDoc_prt_init     |   | |Tree |    ||     || |
|   WsDoc_current_page |   | |Xref |page||     || |
|   ...                |   | |     '----''-----'| |
|                      |   | '------------------' |
'----------------------'   '----------------------'
	

They provide the same sort of facility for workspaces as directories do for filesystems. The analogy, based on DOS, might prove helpful:

Operation	Windows	Namespace
Create	mkdir	)NS or ⎕NS
Change	cd	)CS or ⎕CS
Relative name	dir1\dir\file	NS1.NS2.OBJ
Absolute name	\file\file	#.NS.OBJ
Name separator	\	.
Top (root) object	\	#
Parent object	..	##

Namespaces bring a number of major benefits:

They provide lexical (as opposed to dynamic) local names. This means that a defined function can use local variables and functions which persist when it exits and which are available next time it is called.

Just as with the provision of directories in a filing system, namespaces allow us to organise the workspace in a tidy fashion. This helps to promote an object oriented programming style.

APL's traditional name-clash problem is ameliorated in several ways:

• Workspaces can be arranged so that there are many fewer names at each namespace level. This means that when copying objects from saved workspaces there is a much reduced chance of a clash with existing names.
• Utility functions in a saved workspace may be coded as a single namespace and therefore on being copied into the active workspace consume only a single name. This avoids the complexity and expense of a solution which is sometimes used in 'flat' workspaces, where such utilities dynamically fix local functions on each call.
• In flat APL, workspace administration functions such as WSDOC must share names with their subject namespace. This leads to techniques for trying to avoid name clashes such as using obscure name prefixes like '⍙⍙L1' This problem is now virtually eliminated because such a utility can operate exclusively in its own namespace.

The programming of GUI objects is considerably simplified.

• An object's callback functions may be localised in the namespace of the object itself.
• Static variables used by callback functions to maintain information between calls may be localised within the object.

This means that the object need use only a single name in its namespace.