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File: X09_ExternalObjects.doc
By: J.M. Smeenk
Revision: 5/19/09 9:46 PM
-----------------------MORE ON EXTERNAL OBJECTS
------------------------9.
MORE ON EXTERNAL OBJECTS.
External names are names of objects or types (see section 12) outside the
active workspace. The name of the active workspace is @, and global names
within the active workspace are referred to as either @.Name, or more
conventionally, just Name.
A diagram of a typical system might be as follows:
Absolute Path
------------root
-->system_stuff1
-->system_stuff2
-->group1
-->user1
-->@
-->F
-->D
-->Y
-->@Data1
-->@Data2
-->@StoredWS1
-->X
-->D
-->Z
-->@StoredWS2
-->W
-->user2
-->@Data3
-->@StoredWS3
-->W
-->group2
-->user4
-->@
-->W
Pathname Relative to User1
-------------------------@@@@
-->@@@system_stuff1
-->@@@system_stuff2
-->@@@group1
-->@@user1
-->@
-->@.F or F
-->@.D or D
-->@.D.Y OR D.Y
-->@Data1
-->@Data2
-->@StoredWS1
-->@StoredWS1.X
-->@StoredWS1.D
-->@StoredWS1.D.Z
-->@StoredWS2
-->@StoredWS2.W
-->@@user2
-->@@user2.Data3
-->@@user2.StoredWS3
-->@@user2.StoredWS3.W
-->@@@group2
-->@@@group2.user4
-->@@@group2.user4.@
** INACCESSIBLE
-->@@@group2.user4.@.W ** INACCESSIBLE
The relative paths from user4's point of view are different. For all users,
absolute paths starting with root (e.g. root.group1.user1.@.F) are the same.
As mentioned in section 8, names beginning with @ are called external names,
with the exception of the workspace @ which is neither an external name nor an
internal name. External names are never local; local names are not permitted
to begin with @. External names may be manipulated and assigned from within
functions and in direct mode.
BPL disallows references to another user's active workspace, to avoid
concurrency problems. A typical BPL system has an administrator workspace that
places prohibitions on user accounts so that users cannot access each other's
workspace data.
9.1.
DITTO ASSIGNMENTS.
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When assigning lists of objects, the // token may be used to indicate "same
objects as the opposite side of assignment". The following table illustrates
ditto assignment:
Ditto Assignment
---------------Dir1.//: Dir2.X
@Dir1."X Y"== @Dir2.//
@@Dir1.Subdir1.//>< Dir2."X Y"
Meaning
------Dir1.X: Dir2.X
@Dir1."X Y"== @Dir2."X Y"
@@Dir1.Subdir1."X Y">< Dir2."X Y"
This is simply a convenience to the user to save typing.
9.2.
THE POWER OF EXTERNAL OBJECTS, DIRECTORIES, AND NAMELISTS.
The power of external objects, directories, and namelists may not be readily
apparent. The following table shows how they may be used to accomplish a
variety of tasks:
BPL Code
-------@: @WS1
//: @WS1.Dir1.""
//: @WS1."X Y Z"
//: @WS1.""
@WS1: @
@WS1.Dir1.//: "X Y Z"
@WS.Dir1."X Y": {}
"@": {}
@."": {}
@WS1.X: @WS2.Y
@WS1.X>< @WS2.Y
@.""
@@.""
Meaning
------load a saved workspace (including the values of
all its saved system objects) into the
workspace
copy all objects from an external directory
into the workspace under the same names
copy 3 specific objects from a save directory into
the workspace under the same names
copy all objects of a saved workspace into the
active workspace without resetting system
objects to their default values
save the workspace into an external directory
save 3 specific objects from the active workspace
into an external directory
erase 2 objects from an external directory
erase all user objects in the workspace and reset all
system objects to their default values
erase all objects in the workspace without
resetting all system variables to their
default values
reassign an object in one saved workspace to a
variable in another saved workspace
swap an object in one saved workspace with an
object in another saved workspace
list all objects in the active workspace
list all the user's workspaces
The user must be careful, since one typographical error with the active
workspace @ could wipe out a lot of work. BPL presumes that the user knows
what s/he is doing.
9.3.
FILES.
Files do not exist per se as BPL objects; files are treated as external
variables. Instead of reading/writing records from/to a file, portions of a
variable are accessed by selections. For example:
@FilelikeVar: 3@{}
@FilelikeVar[1]: {{"John Smith",555 1111,"Red"}}
@FilelikeVar[2]: {{"Jane Doe",555 2222,"Blue"}}
@FilelikeVar[3]: {{"Joe Jones",555 3333,"Green"}}
At all times, @FilelikeVar is stored externally to the workspace. A typical
"record" of this "file" is "read" using @FilelikeVar[2] and "written" using
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@FilelikeVar[2]: NewRecordlikeValue. Unlike files in many computer languages,
@FilelikeVar is treated as a BPL variable and any expression using @FilelikeVar
may be employed--regardless of the size of @FilelikeVar. Picked selection may
also be used with external variables. Treating files as large variables allows
far more flexibility than treating them as lists of records--but this may be an
implementation challenge.
9.4.
FOREIGN FILES.
A foreign file created using a non-BPL program may be imported/exported to/from
BPL by considering the foreign file's ASCII representation to be a literal
vector in BPL. For example:
MusicFile: @Song.MP3
@SongTwice.MP3: MusicFile~MusicFile
Some details of importing and exporting would probably depend on the interface
between BPL and the host operating system.