This document covers the following topics:
Some languages, for example Arabic and Hebrew, are written from right-to-left (RTL), whereas the majority of the languages, for example English and German, are written from left-to-right (LTR). Text which contains both left-to-right and right-to-left characters is called bidirectional text.
Support for bidirectional languages is not activated
automatically; the user always has to specify all required parameters (for
example, PM=I) as described below.
The output of Natural programs can be controlled using the profile
parameter PM, the
terminal command %V, and the session
parameter PM.
The profile parameter
DO (Display Order) is
additionally used to support applications that have been originally written for
terminals which support inverse (right-to-left) print mode, but no
bidirectional data. These applications create the display order of
bidirectional data in the application code. With the parameter
DO, these applications are enabled to run compatibly
also with I/O devices that support bidirectional data. This is for instance the
case if an application runs in a browser with the Natural Web I/O
Interface.
The profile parameter PM defines the default
screen direction. When PM is set to R
(reset), the default screen direction is left-to-right. When
PM is set to I (inverse), the default
screen direction is right-to-left. All non-alphanumeric fields, system variables and PF key lines are automatically
inverted by Natural so
that they are displayed correctly from right-to-left if the screen direction is
right-to-left.
The terminal command %V can be used to
change the screen direction. If the screen direction is right-to-left, the
layout of the current window is mirrored, which means that the origin of all
window components or fields is the upper right corner. The screen direction is
changed to right-to-left using %VON and is reverted
to left-to-right using %VOFF.
The session parameter PM reverses the
direction of a field. The effect of "reversing the direction of a
field" depends on the statement in which the PM
parameter is used and the platform. If the PM parameter
is used in a MOVE statement, the content of the field is simply
reversed (that is, the first character will become the last character, and so
on); the result does not depend on the characters of the field. Trailing blanks
are removed before the field is reversed.
For example, the following program
DEFINE DATA LOCAL 1 TEST1 (A10) 1 TEST2 (A10) END-DEFINE TEST1 := 'program' MOVE TEST1 (PM=I) TO TEST2 INPUT TEST1 (AD=O) TEST2 (AD=O) END
produces the following output:
TEST1 program TEST2 margorp
where "margorp" is the reversed version of "program".
When the PM parameter is used for IO
statements such as INPUT or DISPLAY, its effect is
even more complex. In this case, the field direction is based on the screen
direction:
If the screen direction is left-to-right and PM=I
is applied to a field, the field direction changes to right-to-left.
If the screen direction is right-to-left and PM=I
is applied to a field, the field direction changes to left-to-right.
On browser terminals (Natural Web I/O Interface), "reversing the field direction" does not mean that the characters of the field are simply reversed. Instead, the complex bidirectional algorithm is applied. On character-oriented terminals, however, the characters of a field are not resorted; they are simply reversed.
In the following example, the characters assigned to the variable
TEST have been entered in the following sequence:
If the characters are entered in the sequence as described above, the program is displayed in the following way, because the characters are simply displayed in the keying sequence.
DEFINE DATA LOCAL 1 TEST (A20) END-DEFINE TEST := 'abc123' SET CONTROL 'voff' INPUT TEST (AD=O) / TEST (AD=O PM=I) SET CONTROL 'von' INPUT TEST (AD=O) / TEST (AD=O PM=I) END
This program produces two identical screens because the statements
SET CONTROL 'voff' and SET CONTROL 'von' do not apply
to alphanumeric fields. Both screens look as follows:
TEST abccba
In Arabic text, all characters of a string are normally connected with each other. For this reason, Arabic characters have up to 4 presentation forms: the isolated, the final, the initial and the medial form. The form that will be used depends on the position of the character in the string. For example, the Arabic character "MEEM" has the following forms in Unicode:
| U+0645 |  |
ARABIC LETTER MEEM |
| U+FEE1 | |
ARABIC LETTER MEEM ISOLATED FORM |
| U+FEE2 | |
ARABIC LETTER MEEM FINAL FORM |
| U+FEE3 |  |
ARABIC LETTER MEEM INITIAL FORM |
| U+FEE4 | |
ARABIC LETTER MEEM MEDIAL FORM |
Moreover, some characters are combined to a new form if they appear consecutively in a string. This is called a "ligature". For example, the characters
| U+0644 |  |
ARABIC LETTER LAM |
| U+0627 |  |
ARABIC LETTER ALEF |
have the following combined form:
| U+FEFB |  |
ARABIC LIGATURE LAM WITH ALEF ISOLATED FORM |
Unicode strings should include only the Arabic characters in the Arabic block (U+0600 through U+06FF) or the Arabic Supplement block (U+0750 through U+077F); it is not recommended to use the presentation forms in regular Arabic text. It is up to the user interface to display the correct shapes of the characters.
"Shaped" means that every Arabic base character is converted to the appropriate Arabic presentation form. The string may contain each of the four presentation forms of a character. For example, if U+0645 (ARABIC LETTER MEEM) is used as the last character of a string, it is converted to U+FEE2 (ARABIC LETTER MEEM FINAL FORM).
"Unshaped" means that each character is represented only by its basic form. For example, instead of U+FEE2 (ARABIC LETTER MEEM FINAL FORM), U+0645 (ARABIC LETTER MEEM) is used. The conversion to the correct presentation form is performed by the rendering engine of the output device.
Natural strings are internally represented as unshaped alpha or
Unicode strings. If strings are displayed with a browser using the Natural Web
I/O Interface client or the PROCESS PAGE statement, no
transformation is required since the rendering engine of the browser takes care
of the correct presentation. Incoming strings from such devices are already
unshaped and can be directly passed to Natural. If a string is displayed on a
terminal such as 3279 or a terminal emulator such as IBM Personal
Communications, it must be converted into the shaped form since the terminal
itself does not take care of the correct presentation. Accordingly, incoming
strings are in the shaped form and must be transformed into the unshaped form
to be processed correctly by Natural. The most popular code page for Arabic
terminals on the mainframe is IBM420. Compared to Unicode, the number of
characters is reduced and not each form of a character is contained. The
conversion of strings into IBM420 substitutes unavailable forms of a character
by a similar presentation form. For example, the medial form of the Arabic
letter MEEM (U+FEE4) is substituted by the initial form (U+FEE3) of the
character.
In the Arabic EBCDIC code page IBM420, the Arabic character "MEEM" is represented by the following presentation forms:
| H’BA’ | |
ARABIC LETTER MEEM |
| H’BB’ | |
ARABIC LETTER MEEM INITIAL FORM |
The Arabic characters SEEN (U+0633), SHEEN (U+0634), SAD (U+0635)
and DAD (U+0636) (Seen Family) are displayed on terminals as two bytes if they
appear in the final form. Code page IBM420 contains a so-called
"Arabic tail fragment" that completes the final form
of a Seen Family character on terminals or terminal emulators. Of course, the
Arabic tail fragment needs an additional position on the screen. The Arabic
tail fragment is not required by the browsers. If a string with the final form
of a Seen Family character is entered in a browser (Natural Web I/O Interface
client or PROCESS PAGE statement) and subsequently displayed on a
terminal, the Arabic tail fragment is appended to the string with the
consequence that the length of the string increases. If a string with the final
form of a Seen Family character is entered via a terminal or terminal emulator
and subsequently displayed in a browser, the Arabic tail fragment is removed
from the string.